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SC441A
POWER MANAGEMENT Features

High Efficiency Integrated Driver for 4-Strings of 150mA LEDs
Description
The SC44A is a high-efficiency multiple string WLED driver with an integrated boost converter. It operates over a wide input range from 4.5V to 2V with a maximum output voltage of 36V and a 2.5A internal power switch. It can drive up to forty WLEDs in 4 strings with current up to 50mA per string. The string-to-string current matching is 2% typical, 3% maximum. The overall efficiency is greater than 90% due to the low current sense voltage and a low-impedance internal power switch. The wide PWM dimming range boasts a ratio of 500: . The 700KHz switching frequency enables the user to optimize the external component sizes for efficiency. When there are fewer than 0 LEDs in each string, users can adjust the output voltage protection yielding an allowable reduction in associated costs, size and voltage ratings of the output capacitor. The SC44A also features comprehensive open and shortcircuit LED protection functions. It disables the corresponding strings with LED open or LED short conditions while maintaining normal operation of other, unaffected LED strings. This feature allows LCD panels to remain viewable even under LED failure, wire disconnect, or short-circuit conditions. The internal thermal shutdown protects the IC from overheating at abnormal conditions. The SC44A is available in a thermally-enhanced TSSOP20 EDP package.
Wide input voltage range from 4.5V to 2V 36V maximum output voltage Drives up to 40 WLEDs in 4 strings Programmable LED current for up to 150mA per string +/- 2% string-to-string current matching Up to 90% efficiency Wide 0.2% to 00% PWM dimming range Possible analog dimming Integrated 2.5A power switch 700KHz switching frequency for small size Adjustable OVP for cost-effective output cap selection Open/short LED protection Thermal protection with auto-recovery Thermally enhanced TSSOP-20 EDP package Fully WEEE and RoHS compliant
Applications
Medium-sized LCD panel Notebook Display Automotive Car Navigation Display Sub-Notebook and Tablet Computer Displays Portable Media Players

Typical Application Circuit
Vin (4.5V -21V) 1 2 VIN EN FFLAG PWM OVPRTN SW SW VOUT OVPIN OVPIN 4 Strings
SC441A
COMP
IO1-4 IO1- 4 IOSET IOSET AGND AGND
4
10
SS EDP PGND PGND
IOGND PGND
July 23, 2009
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SC441A
Pin Configuration
5 4 3 2 1
Ordering Information
Device Package TSSOP-20 EDP Evaluation Board SC44ATETRT (1,2)
D
D
IO2 IO1 IOGND AGND
1
20
IO3 IO4 PGND SW SW PGND OVPIN OVPRTN VIN
SC44AEVB
C
SS COMP EN IOSET
Notes: () Available in tape and reel only. A reel contains 2,500 devices. (2) Available in lead-free package only. Device is WEEE and RoHS C compliant.
B
PWM VOUT 10 11
B
FFLAG
JA = 39 C/W
A
(TSSOP-20 EDP)
5 4 3 2 1
A
Marking Information
SC44A : Part Number yyww = Date (Example: 0952) xxxxxx = Semtech Lot# (Example: A94A0)
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SC441A
Absolute Maximum Ratings
VIN Pin: Supply Voltage ................................. -0.3 to 25V Maximum Output Power .................................... 22W SW, OVPIN, OVPRTN, VOUT, IO~IO4 Voltage ...... -0.3 to 40V IOSET Voltage ............................................. -0.3 to 2V SS, COMP Voltage ....................................... PGND to AGND ............................................. ESD Protection Level ....................................
(2)
Recommended Operating Conditions
Input Voltage Range .................................... 4.5V~2V Output Voltage .......................................... Up to 36V LED Current .......................................... Up to 50mA
-0.3 to 4V 0.3V 2.5kV
Thermal Information
Junction to Ambient() .................................... 39C/W Maximum Junction Temperature ........................... 50C Storage Temperature .............................. -65 to +50C Lead Temperature (Soldering) 0 sec ..................... 260C
EN, PWM, FFLAG Voltage ........................ -0.3 to VIN +0.3V Peak IR Reflow Temperature ................................. 260C
Exceeding the above specifications may result in permanent damage to the device or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not recommended. NOTES() Calculated from package in still air, mounted to 3" x 4.5", 4 layer FR4 PCB with thermal vias under the exposed pad per JESD5 standards. (2) Tested according to JEDEC standard JESD22-A4-B.
Electrical Characteristics
Unless otherwise specified: VIN =2V, -40C < TA = TJ < 05C, RIOSET=.74kW.
Parameter Input Supply Under-Voltage Lockout Threshold UVLO Hysteresis VIN Quiescent Supply Current VIN Supply Current in Shutdown Oscillator Switching Frequency Internal Power Switcher Switch Current Limit Switch Saturation Voltage Switch Leakage Current Maximum Duty Cycle Minimum Duty Cycle Minimum On-Time Compensation Sourcing Current Sinking Current
(c) 2009 Semtech Corporation
()
Symbol
Conditions
Min
Typ
Max
Units
UVLO-TH UVLO-H IIN_Q IIN_S
VIN rising No switching EN / PWM = low
4.3 250 3
4.45
V mV mA
A
FS
0.56
0.7
0.84
MHz
ISW VSAT IS_LEAK DMAX DMIN TON_MIN ISW = A
2.5
3.32 200 0. 350 0 00
A mV A % % ns
88
93
IEA_SOURCE IEA_SINK
VCOMP = 0.5V VCOMP = 2V
5 6
A A
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SC441A
Electrical Characteristics (continued)
Parameter Control Signals EN, PWM High Voltage EN, PWM Low Voltage EN, PWM Leakage Current PWM Dimming Frequency
()
Symbol
Conditions
Min
Typ
Max
Units
VEN_H, VPWM_H VEN_L, VPWM_L IEN, IPWM FDimming DMIN_Dimming
(2)
2 0.4 VEN, VPWM = 5V 50 FDimming = 200Hz 0.2 5 200 IFFLAG = 2 mA VSS = 0V VSS = 2V at OVP or OTP VIN = 2V, TJ = 25 C VIN = 2V OVPIN - AGND VOVPIN = 20V IOVPRTN = 00 A VOVPRTN = 20V VOUT = VIN + 3V VOUT = 40V TJ = 25 C TJ = 25 C 40 200 TJ = 25 C, VIO ~ VIO4 EN = 0, VIO ~ VIO4 = VIN TJ = 25 C, VIO ~ VIO4 0.963 0.9095 2.2 2.35 0. .07 .07 00 50 30 2.55 .77 .2305 .43 0.5 0.25 4.5 0.7 2.5 .52 0. 60 0. 0.9 0. 50 +/- 2 60 +/-3 .58 0.85 300 0. 50k
V V A Hz % s ns V A V V V A mV A mA A mA % mA V A V V mV C C
PWM Dimming Minimum Duty Cycle PWM Dimming Minimum Pulse-Width PWM Dimming Minimum Off Time FFLAG Voltage SS Source Current SS Sink Current SS Switching Threshold SS End Value Over-Voltage Protection OVPIN Threshold OVPIN Leakage Current OVPRTN Saturation Voltage OVPRTN Leakage Current VOUT Internal Pull-down Current Source VOUT Leakage Current Current Source (IO1 ~ IO4) Current Accuracy Current Matching Maximum LED Current LED Short-Circuit Protection Leakage Current
Overshoot Protection Threshold Overshoot Protection Hysteresis
TMIN_Dimming TMIN_Off VFFLAG ISS_SOURCE ISS_SINK VSS_Switching VSS_END VOVPIN_TH IOVPIN VOVPRTN IOVPRTN IOVP IVOUT_Leak IO~IO4 IOMAX VIO_SCP IIO_LEAK VIO~VIO4 Any of IO~IO4 TOTP TOTP_H
Over-Temperature Protection Thermal Shutdown Temperature Thermal Shutdown Hysteresis
Notes: () Guaranteed by design. (2) For achievable PWM dimming minimum pulse-width in applications, see the corresponding curves in Typical Characteristics. (c) 2009 Semtech Corporation www.semtech.com 4
SC441A
Typical Characteristics
UVLO Threshold vs. Temperature
4.36 4.35 4.34
UVLO Hysteresis (mV)
UVLO Hysteresis vs. Temperature
247
VIN Quiescent Supply Current vs. Temperature
3.20 3.10 3.00 2.90 2.80 2.70 2.60
245 244 243 242 241 240 239
4.33 4.32 4.31 4.30
238 4.29 -40 25 105 237 -40 25 105
VIN Quiescent Supply Current (mA)
Comp SINK SOURCE
246
SS SINK/SOURCE
VIN UVLO (V)
2.50 -40 25 105
Temperature (oC)
Temperature (oC)
Temperature (oC)
7.50 7.00
COMP SINK / SOURCE Current vs. Temperature VIN = 12V
6.0 5.0 4.0 3.0 2.0 1.0 0.0
SS SINK / SOURCE Current vs. Temperature
175
SW Saturation Voltage at A vs. Temperature
170
SW Saturation Voltage at 1A (mV)
VIN = 12V SOURCE
SINK 6.50 SINK
6.00 5.50
165 160 155 150 145 140 135 -40 25 105
o
VIN = 4.5V VIN = 12V VIN = 21V
SOURCE
5.00 SOURCE 4.50 4.00 -40 25 105
o
SINK
-40
25
105
o
Temperature ( C)
Temperature ( C)
Temperature ( C)
VOUT Pull Down Current Source vs. Temperature
0.93 0.91
LED Current Source SCP Threshold vs. Temperature
2.410 2.405
Average LED Current Source Setting vs. Temperature
150.7 150.6
VIN = 12V
LED Current Source Setting (mA)
LED Current Source SCP Threshold (V)
VOUT Pull down Current Source (mA)
0.89 0.87 0.85 0.83 0.81 0.79 0.77 0.75 -40 25
VIN = 4.5V
2.400 2.395 2.390 2.385 2.380 2.375 2.370 2.365 2.360
150.5 150.4 150.3 150.2 150.1 150.0 149.9 149.8 149.7
VIN = 21V VOUT = VIN + 3V
105
VIN = 12V RIOSET = 1.78kW
-40 25 105
-40
25
105
Temperature (oC)
Temperature (oC)
Temperature (oC)
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SC441A
Typical Characteristics (continued)
LED Current Source Saturation Voltage vs. LED Current
900
OVPRTN Saturation Voltage vs. Temperature
75 70
OVPRTN Saturation Voltage (V)
1.58 1.56 1.54 1.52
OVPIN Threshold Voltage vs. Temperature
LED Current Source Saturation Voltage (mV)
800 700 600 500 400 300 200 100 0 10
TA = 25 oC
IOVPRTN = 100uA
65 60 55 50 45 40
1.50 1.48 1.46
-40 25 105
VIN = 12V
-40 25 105
20
40
60
80
100
120
140
160
LED Current (mA)
Temperature (oC)
Temperature (oC)
FFLAG Saturation Voltage vs. Temperature
260 240
FFLAG Saturation Voltage (mV)
Efficiency ( PBOOST_OUTPUT / PINPUT )
92 90
Efficiency (%)
IFFLAG = 2mA
VIN = 21V
PWM Dimming Minimum Pulse Width(uS)
94
8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 0.2
PWM Dimming Minimum Pulse Width vs. PWM Dimming Frequency
220 200 180 160 140 120 -40 25 105
88 86 84 82 80 80 160 240 320 400
VIN = 12V
VIN = 5V
480
560
600
0.5
1
2
5
10
20
30
40
50
Temperature (oC)
Boost Section Output Current (mA)
Condition: VOUT = 36V
PWM Dimming Frequency (kHz)
Condition: VIN = 2V, VOUT = 30V / 20mA x 4 strings
PWM Dimming Minimum Pulse Width vs. PWM Dimming Frequency
PWM Dimming Minimum Pulse Width(uS)
PWM Dimming Minimum Duty Cycle vs. PWM Dimming Frequency
22
PWM Dimming Minimum Duty Cycle(%)
PWM Dimming Minimum Duty Cycle vs. PWM Dimming Frequency
14
PWM Dimming Minimum Duty Cycle(%)
5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 0.5 1 2 5 10 20 30 40 50
20 18 16 14 12 10 8 6 4 2 0 0.2 0.5 1 2 5 10 20 30 40 50
12 10 8 6 4 2 0 0.5 1 2 5 10 20 30 40 50
PWM Dimming Frequency (kHz)
Condition: VIN = 2V, VOUT = 30V / 50mA x 4 strings
PWM Dimming Frequency (kHz)
Condition: VIN = 2V, VOUT = 30V / 20mA x 4 strings
PWM Dimming Frequency (kHz)
Condition: VIN = 2V, VOUT = 30V / 50mA x 4 strings
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150mA
10mA
SC441A
Typical Characteristics (continued) 10mA
PWM Dimming Minimum Pulse Width vs. PWM Dimming Frequency
PWM Dimming Minimum Pulse Width(uS) PWM Dimming Minimum Pulse Width(uS)
PWM Dimming Minimum Pulse Width vs. PWM Dimming Frequency
PWM Dimming Frequency (kHz)
Condition: VIN = 2V, VOUT = 34.5V / 0mA x 4 strings
PWM Dimming Frequency (kHz)
Condition: VIN = 2V, VOUT = 34.5V / 50mA x 4 strings
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SC441A
Typical Characteristics (continued)
Start up Shut Down
VIN (5V/DIV) VIN (10V/DIV) VSS (2V/DIV) VSW (20V/DIV) VOUT (20V/DIV) VSS (2V/DIV)
VSW (20V/DIV) VOUT (20V/DIV)
Time (20mS/DIV) Conditions: VIN = 19V, Output = 30V / 150mA x 4 LED strings
Time (200mS/DIV) Conditions: VIN = 12V, Output = 27V / 20mA x 4 LED strings
Main Power Switching Waveform
Main Power Switching Waveform
VSW (20V/DIV)
VSW (10V/DIV)
VOUT Accoupling (200mV/DIV)
VOUT Accoupling (100mV/DIV)
Time (1uS/DIV) Conditions: VIN = 12V, Output = 36V / 150mA x 4 LED strings
Time (400nS/DIV) Conditions: VIN = 12V, Output = 27V / 20mA x 4 LED strings
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SC441A
Typical Characteristics (continued)
OTP and OTP Recovery
VIN = 12V VSS (2V/DIV) VSW (20V/DIV) VOUT (20V/DIV) VIN (10V/DIV) VOUT (20V/DIV) IO1 (5V/DIV) IO2 (1V/DIV)
LED Short Circuit Protection
Time (40mS/DIV) Conditions: VIN = 12V, Output = 36V / 150mA x 4 LED strings
Time (10mS/DIV) Conditions: VIN = 12V, IO1 has one LED short circuit, Output = 36V / 150mA x 4 LED strings
LED Open Circuit Protection
PWM Dimming
VIN = 12V
VIN (5V/DIV) VOUT (20V/DIV) IO1 (10V/DIV) IO2 (1V/DIV)
PWM (2V/DIV) SW (20V/DIV) IO1 (5V/DIV)
Time (10mS/DIV) Conditions: VIN = 12V, IO2 LED String is open circuit, Output = 36V / 150mA x 3 LED strings
Time (10mS/DIV) Conditions: VIN = 12V, Output = 36V / 50mA x 4 LED strings
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SC441A
Pin Descriptions
Pin # 2 3 4 5 6 7 8 9 0 2 3 4 5,8 6,7 9 20 EDP Pin Name IO2 IO IOGND AGND SS COMP EN IOSET PWM VOUT FFLAG VIN OVPRTN OVPIN PGND SW IO4 IO3 Pin Function Provides constant current source to LED string 2. Provides constant current source to LED string . Constant current source ground. Analog ground Soft-start pin The output of the internal trans-conductance error amplifier. Enable the device including regulator and LED drivers. Current source IO value set pin. By selecting the resistor connected from this pin to GND, the corresponding maximum current on all 4 strings are set. PWM dimming control pin for LED strings. Internal pull down current source in over voltage fault. Connect this pin directly to Boost output. Power failure signal output with open collector. Held low under normal operation. Power input voltage pin. Bypassed with capacitors close to the pin. Over-Voltage Protection sense signal return path pin. Over-Voltage Protection sense signal input. Power ground Collector of the internal power switch. Provides constant current source to LED string 4. Connect to VIN for 3 strings operation. Provides constant current source to LED string 3. Solder to the ground plane of the PCB.
Note: Any unused IO pin should be pulled up to VIN.
EN
0
STATUS
backlight disable backlight enable
Note: When EN = 0; the boost is turned OFF and disabled.
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SC441A
5 4 3 2
Block Diagram
FFLAG HICCUP SW DISABLE2 OSC
S Q
SW
R
ONE IO CHANNEL SHOWN LED OPEN/SHORT/OVERSHOOT CIRCUIT PROTECTION
FAULT-1
+
IO1
DISABLE1
IOSET
IOGND PWM VOUT OVPIN OVPRTN 0.9mA 4.5uA SS HICCUP AGND 1uA TSD OVP Detect Fault HICCUP 3V3 CONTROL LOGIC HICCUP UVLO UVLO & TSD Bandgap EN OVP Fault VIN
Figure 1. SC441A Block Diagram
5 4 3 2
(c) 2009 Semtech Corporation
-
LED CURRENT SETTING
CURRENT SOURCE
SS
-
ISENSE
PGND PGND COMP
ILIM
+
+
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SC441A
Applications Information
SC441A Detailed Description
The SC44A contains a high frequency, current-mode boost regulator and four programmable current sources. The LED current source value is set using an external resistor while the PWM controller maintains the output voltage at a level keeping the current regulated through the LEDs. Since the SC44A receives feedback from all of the LED current sources, all LED strings can be turned on at any given time. A typical application would use 3-0 backlight LEDs for each string, driven up to 50mA. voltage increases. Proper decoupling is required on the VIN pin, especially for a lower input voltage condition. A 22F, 6.3V rated X5R ceramic capacitor is recommended for a 5V input system. The internal LED current source (IO ~ IO4) helps to regulate the LED current to its set point while the output voltage increases; a suitable amount of error information will be generated on the internal error amplifier. The COMP pin voltage keeps rising and once the LED current reaches its set point, the error information will not be generated by the LED current source. The COMP pin voltage stays level while keeping the LED current in its set point. If the EN pin voltage is pulled below 0.4V, the SC44A will stay in shutdown mode drawing less than A from its input power supply. During the normal operation, when PWM pin is pulled below 0.4V, the device operates in standby mode, drawing 3mA (typical) current from the input. Under this condition, since the EN pin is pulled high, soft-start is initiated and the SS pin voltage is raised to its end value. Following this, when the PWM signal goes high to enable the SC44A, the COMP pin voltage will rise quickly since it is not limited by the SS pin. A proper capacitance (0nF ~ 00nF) is required to prevent output voltage overshoot on the COMP pin and its external RC network.
Operation
The SC44A controls the boost converter set point based on instantaneous requirements of four current sources. Therefore, only a single inductor and power switch is needed to provide power to the entire lighting subsystem, increasing efficiency and reducing part count. A digital interface to output control is high-bandwidth, supporting digital PWM dimming at 50Hz to 50kHz dimming frequency, while aggressively shutting the entire supply current down to 3mA (typical), when all LED strings are turned off. High frequency switching provides high output power using a tiny .0mm high inductor, maximizing efficiency for space-constrained and cost-sensitive applications. Additionally, both converter and output capacitor are protected from open-LED conditions by over-voltage protection.
Shut Down
LED Current Programming
The SC44A features programmable LED current regulators. The LED current set points are chosen using external resistors tied to the IOSET pin. The relationship between the programming resistor value and the output current set point can be described as follows: RIOSET = (0.26V) / ILED Where RIOSET is in k. ILED is the LED current in Amperes. The four output channels have the same output current.
If the VIN pin voltage falls below its UVLO, or the voltage on the EN pin goes low, the device will run in shutdown mode as the internal switch and the LED current sources will immediately turn off. The SS capacitor is discharged by the internal current source of the SS pin. The SS pin voltage decreases to 0V while the output voltage falls to the same level as the input voltage. If the PWM pin voltage goes low while SC44A is in normal operation, then the SC44A will run in standby mode. The Internal switcher and the LED current source will immediately turn off.
NOTE- The PWM signal does not affect the SS pin nor its final value.
Start-Up
During start-up, when the VIN pin voltage reaches its UVLO threshold and both the EN and PWM signals are set to high, the SS pin begins to source 4.5A as its voltage begins to rise from 0V to its end value of 2.5V. The output voltage of the internal trans-conductance error amplifier (COMP), increases and clamps to the SS pin voltage. When the SS pin voltage reaches its switching threshold, output
(c) 2009 Semtech Corporation
Main Power Operation
SC44A is a 700KHz fixed-frequency, peak current-mode step-up switching regulator with an integrated 2.5A (minimum) power transistor.
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SC441A
Applications Information (continued)
Referring to the Block Diagram, Figure , the clock from the oscillation section resets the latch and turns on the power transistor. Switch current is sensed with an integrated sense resistor. The sensed current is summed with the slope-compensating ramp and fed into the modulating ramp input of the PWM comparator. The latch is set and the power transistor conduction is terminated when the modulating ramp intersects with the error amplifier output (COMP). The current-mode switching regulator is a dual-loop feedback control system. In the inner current loop, the EA output (COMP) controls the peak inductor current. In the outer loop, the error amplifier regulates the output voltage to keep the LED current at setting point. The double reactive poles of the output LC filter are reduced to a single real pole by the inner current loop, allowing the simple loop compensation network to accommodate a wide range of input and output voltages. It is well known that, in Boost converter, Vo is greater than or equal to Vin. In normal continuous conduction mode (CCM) operation, there are some choices. ) Leave some room between Vin range and Vo. 2) Operate the Boost converter at normal load (less Ro) 3) Increases the Boost inductance (L).
Over-Current Protection
SC44A provides cycle-by-cycle current limiting for its internal switch. If the switch current exceeds 3.32A (the typical current-limit trip point), then the current-limit comparator ILIM, will set the latch immediately turning off internal power. All LED current sources keep operating in an over-current condition. Due to separate pulse-width modulating and current limiting paths, the OCP trip point is not affected by slope compensation (i.e. trip point is not affected by switching duty cycle).
Over-Voltage Protection (OVP)
Vo Vin
1 1D
Where, D is the duty ratio of the PWM power switch. As Vin increases, in order to regulate Vo to a given constant value, D decreases. When Vin approaches Vo, D surely leads to 0. In practice, due to the minimum on-time of the PWM power switch, D usually could not approach 0 with infinitely small granularity. At some point, it either produces one pulse with minimum on-time or generates 0 by skipping the pulse. Such point could be theoretically calculated for SC44A as follows. For CCM: Vin 0.92 Vo. For DCM (Discontinuous conduction mode):
Vin 2 1 1 1.6 *10 2 * Ro L Vo
SC44A includes an external programming over-voltage protection circuit to prevent damage to the IC and output capacitor in the event of an open-circuit condition. The boost converter's output voltage is detected at the OVPIN pin. If the voltage at the OVPIN pin exceeds .52V (typical), the boost converter will shut down and a 0.9mA pull-down current will be applied to the VOUT pin to quickly discharge the output capacitor. This added protection prevents a condition where the output capacitor and Schottky diode must endure high voltage for an extended time, which can pose a reliability risk for the user's system. Refer to evaluation application circuit in page5. The output over voltage trip point can be programmed by R5 and R7 resistor divider. The relationship can be described as follows:
OVP _ trip OVPIN _ TH R5 R7 R7
Where OVPIN_TH is .52V typical. An OVP event causing a fault could disable the boost converter enabling the device to a strong pull-down. This event would cause the FFLAG pin to go high and the softstart capacitor to discharge. When the soft-start capacitor voltage falls below 0.5V, and the output voltage falls to VIN, SC44A enters a soft-start process. The OVP detection circuitry provides a disconnect function during the shutdown state to prevent any leakage from the output. The external OVP resistor divider should be connected between VOUT and OVPRTN with the central
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Where, Ro is the Boost equivalent output resistance (=Vo/ Io), L is the Boost inductor (in uH). In many Boost converter designs and operations, pulse skipping is normally allowed at light load conditions. Some designers even purposely let the Boost power converter enter the pulse skipping in order to save power at light load conditions. If some designers do not want pulse skipping mode, based on the conditions provided above,
(c) 2009 Semtech Corporation
SC441A
Applications Information (continued)
tap connected to OVPIN. Note: If this disconnect function is not desired, bypass the OVPRTN pin and connect the end of the OVP resistor divider directly to GND. The OVPIN pin is sensitive to noise, and a proper decoupling capacitor (nF ~ 0nF) is required. The combined impedance of the resistor divider for OVPIN should be greater than 200k. If the thermal shutdown temperature of 50C is reached, a hiccup sequence is initiated where the boost converter and all IO current sources are turned off. SS is discharged by a A current source, and a 0.9mA discharge path is turned on to pull down VOUT. As temperature falls below the TSD trip point, SC44A will retry when SS falls below 0.5V and VOUT falls to VIN.
LED Short-Circuit Protection
PWM Dimming
If one or more LEDs are detected as short-circuit, that string will be latched off. Voltage is monitored if it exceeds 2.35V on the internal LED current source (IO pins). (The IO voltage on an abnormal LED string will rise earlier than other normal LED strings). If the voltage exceeds 2.35V on any IO pin, the IO current source will latch off and the FFLAG will go high. The latch is reset if VIN falls below UVLO or it will recycle the EN signal. Other LED strings operate normally. If all IO pin voltages reach .07V then the internal main switch will be off until any of the IO voltages is lower than 970mV.
The PWM input needs to be held high for normal operation. PWM dimming can be done by cycling the PWM input at a given frequency where a "low" on the PWM input turns off all IO current sources and a "high" turns on all IO current sources. The short and open detection latches are blanked for approximately 2s as the PWM input transitions from low-to-high to prevent a false fault detection during PWM dimming. The PWM pin can be toggled by external circuitry to allow PWM dimming. In a typical application, a microcontroller sets a register - or counter, that varies the pulse-width on a GPIO pin. The SC44A allows dimming over two decades in frequency (50Hz-50kHz), in order to allow compatibility with a wide range of devices, including the newest dimming strategies that avoid the audio band by using high frequency PWM dimming. In this manner, a wide range of illumination can be generated while keeping the instantaneous LED current at its peak value for luminescent efficiency and color purity. Furthermore, advanced lighting effects such as backlight dim-on can be implemented as the SC44A can resolve 0s (minimum), PWM dimming pulse-width. As far as the maximum PWM dimming pulse-width, it depends on the PWM dimming frequency. Clearly, it is trivial to get 00% LED brightness by pulling PWM pin "High" constantly. When the user tries to dim the LED brightness using PWM signal from 00% down, he or she needs to observe the following. When the PWM dimming signal is actively switching from "High" to "Low" and to "High", there is a minimal OFF time (T_off_min, 200ns, guaranteed by design) requirement of the PWM dimming signal with this IC. Such minimal OFF time sets the maximum PWM duty ratio before hitting to 00% in the following way. Dmax 1 Toff _ min f PWM For example, if the PWM dimming frequency f_ PWM=200Hz, the D_max=99.996%. If f_PWM=25kHz, the D_max=99.5%. With most practical dimming interfaces,
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LED Open-Circuit Protection
If any LED string is detected as an open-circuit, that string will latch off. If any given string is open, the IO current source will go to deep saturation; the COMP and FFLAG pins will be driven high and the boost converter duty cycle will increase causing VOUT to rise. At some point VOUT will rise high enough to cause all the IO pin voltages of the intact strings to reach the shorted LED detection level and latch off those strings. Because of the LED open string VOUT will continue to rise until it reaches the programmed OVP level. When OVP is reached, the voltage on the IO pins are monitored and if any IO voltage is less than 0.2V that string will be identified as open and will latch off. Only when VIN falls below UVLO, or an EN signal is recycled, and if thermal shutdown occurs, can this latch be reset. A hiccup cycle is then initiated and the SS pin is discharged slowly with a A current source and a 0.9mA discharge path (turned on to pull down VOUT). When SS falls below 0.5V and VOUT falls below to VIN, the shorted LED detection latches are reset and a new soft-start sequence is initiated to resume normal operation.
Thermal Shutdown (TSD)
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SC441A
Applications Information (continued)
the needed dimming steps and resolutions, it is uncommon to run into the above D_max before reaching 00%. While most applications will not run into D_max, the designer should be aware of possible parasitic elements from PWM dimming interface to the PWM pin of SC44A. Usually, simply checking signal D_max at PWM pin of SC44A is sufficient. 5 4 3 2 1 The linear dimming control is available for SC44A D by applying an external control voltage on IOSET pin through an external resistor-like circuit (shown below). External environment brightness compensation can also be achieved when the control voltage is generated by a light sensor circuit.
C
One may use 3.9nF or less in the circuit. In some applications, circuit designers tend to select the decoupling capacitors in the range of (00pF ~ nF). For some low LED current (e.g. 0mA) applications, it is recommended to add M-0Mohm resistor from IO pin to GND in order to reduce IO pin voltage during PWM dimming.
Linear Dimming
R_EXT V_EXT R_IOSET
IOSET
When two or more SC44As are operating in parallel for a large-sized panel application, audible noise may be D observed due to non-synchronous switching frequency. The ripple voltage on the input voltage rail will be modulated by the beat frequency resulting in audible noise. This situation can be resolved by adding an input inductor between input voltage rail and the SC44A VIN pin. This situation can also be improved by adding more input C decoupling capacitors.
Parallel Operation
B
The IOSET voltage is 0.5V when linear dimming is used and the minimum IOSET current must be higher than 27A (i.e. 5mA per LED string). The external control voltage slew rate must slow at V/0ms.
A
LED Strings Connection
Generally, LED strings are connected to IO ~ IO4 pins through a mechanical connector which, generally, cannot support an electrical connection thereby resulting in 5 4 3 2 1 significant noise. Consequently, the SC44A LED shortcircuit protection may false trip when the noise level is large. Certain ceramic decoupling capacitor on pins IO ~ IO4 to GND are useful to prevent the SC44A from noise influence. As a general guideline, the decoupling capacitance should be limited as follows.
Cdcple 0.6uS I LED * Vo
The inductance value of the inductor affects the converter's steady state operation, transient response, and its loop stability. Special attention needs to be paid to three B specifications of the inductor, its value, its DC resistance and saturation current. The inductor's inductance value also determines the inductor ripple current. The converter can operate in either CCM or DCM depending on its working conditions. The inductor DC current or input current A can be calculated as,
Inductor Selection
IIN - Input current; IOUT - Output current; VOUT - Boost output voltage; VIN - Input voltage; - Efficiency of the boost converter. Then the duty ratio is,
Where, I_LED is the LED current per string, Vo is the Boost output voltage and C_dcple is the suggested decoupling capacitor value. For example, if I_LED=0mA, Vo=3.5V, the calculated upper bound of C_dcple is about 444pF. One could use 390pF or less in the circuit. If I_LED=00mA, Vo=3.5V, the calculated upper bound of C_dcple is about 4.44nF.
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VD - Forward conduction drop of the output rectifying diode When the boost converter runs in DCM ( L < Lboundary), it takes the advantages of small inductance and quick transient response while avoiding the bandwidth limiting instability of the RHP zero found in CCM boost converters.
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SC441A
Applications Information (continued)
The inductor peak current is,
I L - peak = VIN D FS L
The converter will work in CCM if L > Lboundary. Generally the converter has higher efficiency under CCM and the inductor peak current is,
the mechanics of regulation and frequency dependence may be complex, actual selection of output capacitor can be simplified because this capacitor is mainly selected for the output ripple of the converter. Assume a ceramic capacitor is used. The minimum capacitance needed for a given ripple can be estimated by,
C OUT (VOUT VIN ) IOUT VOUT FS VRIPPLE
For many applications, an inductor with value of 4.7H to 22H should be fine, such as for the typical case shown on page . The inductor peak current must be less than its saturation rating. When the inductor current is close to the saturation level, its inductance can decrease 20% to 35% from the 0A value depending on the vendor specifications. Using a small value inductor forces the converter under DCM in which case the inductor current ramps down to zero before the end of each switching cycle. It reduces the boost converter's maximum output current, and produces large input voltage ripple. An inductor with larger inductance will reduce the bandwidth of the feedback loop, possibly higher DC resistance (DCR). Inductor's DCR plays a significant role for the total efficiency since the power transistor is integrated inside the SC44A. Of course, there is a trade-off between the DCR and inductor size. Table 2 lists recommended inductors and their vendors. Table 2. Recommended Inductors
Inductor
DR74, 4.7H ~ 5H IHLP-2525CZ-0, 4.7 ~ 0H DS85LC, 6.8H ~ 0H
VRIPPLE - Peak to peak output ripple; IOUT - Output current; VOUT - Boost output voltage; VIN - Input voltage; FS - Switching frequency. During load transient, the output capacitor supplies or absorbs additional current before the inductor current reaches its steady state value. Larger capacitance helps with the overshoot and undershoots during load transient, and loop stability. Recommended ceramic capacitor manufacturers are listed in Table 3. Table 3. Recommended Ceramic Capacitor Manufacturers
Vendor
Kemet Murata
Phone
408-986-0424 84-237-43 408-573-450
Website
www.kemet.com www.murata.com www.t-yuden.com
Website
www.cooperet.com www.vishay.com www.tokoam.com
Taiyo Yuden
Output Rectifying Diode Selection
Output Capacitor Selection
The next task in SC44A design is targeting the proper amount of ripple voltage due to the constant-current LED loads. The two error amplifiers that control the PWM converter sense the delta between requested current and actual current in each output current regulator. On a cycle-by-cycle basis, a small amount of output ripple ensures good sensing and tight regulation, while the output current regulators keep each LED current at a fixed value. Overall, this allows usage of small output caps while ensuring precision LED current regulation. Although
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Schottky diodes are the ideal choice for SC44A due to their low forward voltage drop and fast switching speed. Table 4 shows several different Schottky diodes that work well with the SC44A. Make sure that the diode has a voltage rating greater that the possible maximum ouput voltage. The diode conducts current only when the power switch is turned off. A diode of 2A will be sufficient for most designs. Table 4. Recommended Rectifying Diodes
Part
SS23 SS24
Vendor
Vishay www.vishay.com www.semtech.com 6
SC441A
Applications Information (continued)
Layout Guidelines
The SC44A contains a boost converter and the placements of the power components outside the SC44A should follow the layout guidelines of a general boost converter. The evaluation application circuit on page 7 will be used as an example. C2 and C3 are input decoupling capacitor for SC44A VIN pin and main power input. C2,C3 should be placed as close as possible to the VIN pin to achieve the best decoupling performance. To minimize the switching noise, The switching loop formed by input decoupling capacitors, internal switch, output Schottky diode and output capacitors must be minimized. The LED current programming resistor(R6), compensation network (R9,C5,C7) and soft start capacitor (C6) should be placed as close as possible to SC44A. The C4~C7 are decoupling capacitors for LED current source which prevent IO pins from noise influence. C4~C7 should be placed close to each corresponding IO pin. Use an isolated local AGND plane underneath the controller and tie it to the negative side of output capacitor through R4 for better noise immunity.
(c) 2009 Semtech Corporation
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SC441A
5 4 3 2 1
Evaluation Application Circuit
VOUT R1 0R R3 VIN R8 1R C2 10uF/25V C3 N.P. L1 6.8uH C4 2.2uF/25V R5 220K VOUT C1 R7 1nF 10K 10K 11 12 13 14 15 16 17 18 IO3 IO4 R10 1R 19 20 R12 1R C14 1nF C15 1nF FFLAG VIN OVPRTN OVPIN PGND SW SW PGND IO4 VOUT PWM IOSET EN 10 9 8 7 6 5 4 3 2 1 R11 1R C6 100nF C7 N.P. C5 47n IO1 IO2
B
VIN
D
R2 10K
R4 10K PWM
R6 RIOSET EN
SC441A
COMP SS AGND IOGND IO1
C
VOUT
D1
R9 1.5K
C8 4.7uF/50V
C9 4.7uF/50V
IO3
EDP
IO2 U1
21
R13 1R C17 1nF R14 0R
C16 1nF
Evaluation Board Bill of Materials
Item 2 35 4 5 6 7 8 9 0 2 3 4 5 Reference C, C4, C5, C6, C7 C2 C8, C9 C4 C5 C6 D L R, R4 R2, R3, R4, R7 R5 R6 R9 R8, R0, R, R2, R3 U Quantity 5 2 2 4 5
4
Description 50V ceramic capacitor, X7R 25V ceramic capacitor, X5R
3 50V ceramic capacitor, X5R
Package SM_0603 SM_206
2 SM_206
Part nF 0uF 4.7uF 2.2uF 47nF 00nF B340 6.8H 0R 0K 220K RIOSET .5K R SC44A
Vendor Panasonic Panasonic
1 Panasonic
A
25V ceramic capacitor, X5R 6.3V ceramic capacitor, X7R 6.3V ceramic capacitor, X7R 40V, 3A Schottky diode 6.8H, 6.6A % SMD resistor 5% SMD resistor % SMD resistor % SMD resistor 5% SMD resistor 5% SMD resistor Controller
SM_0805 SM_0603 SM_0603 SMA DR25 SM_0603 SM_0603 SM_0603 SM_0603 SM_0603 SM_0603 EDP TSSOP-20
Panasonic Panasonic Panasonic Diodes or Any Copper or Any Any Any Any Any Any Any SEMTECH
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(c) 2009 Semtech Corporation
SC441A
Outline Drawing - TSSOP-20 EDP
A N 2X E/2 E1 PIN 1 INDICATOR ccc C 2X N/2 TIPS 123 E e D
DIM
A A1 A2 b c D E1 E e F H L L1 N 01 aaa bbb ccc
DIMENSIONS INCHES MILLIMETERS MIN NOM MAX MIN NOM MAX
.047 1.20 .001 .006 0.025 0.15 .031 .042 0.80 1.05 .007 .012 0.19 0.30 .003 .007 0.09 0.20 .251 .255 .259 6.40 6.50 6.60 .169 .173 .177 4.30 4.40 4.50 .252 BSC 6.40 BSC .026 BSC 0.65 BSC .144 .150 .154 3.66 3.81 3.91 .112 .118 .122 2.85 3.00 3.10 .018 .024 .030 0.45 0.60 0.75 (.039) (1.0) 20 20 0 8 0 8 .004 0.10 .004 0.10 .008 0.20
e/2 B
aaa C SEATING PLANE
D A2 A
C
bxN
A1 bbb C A-B D
F EXPOSED PAD H GAGE PLANE BOTTOM VIEW 0.25
SIDE VIEW
SEE DETAIL
A
H c L (L1) DETAIL
01
NOTES: 1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. DATUMS -A- AND -B- TO BE DETERMINED AT DATUM PLANE -H3. DIMENSIONS "E1" AND "D" DO NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
A
(c) 2009 Semtech Corporation
(c) 2009 Semtech Corporation
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SC441A
Land Pattern -TSSOP-20 EDP
F
DIM
(C) H G Z C F G H P X Y Z
DIMENSIONS INCHES MILLIMETERS
(.222) .157 .161 .126 .026 .016 .061 .283 (5.65) 4.00 4.10 3.20 0.65 0.40 1.55 7.20
Y P X
NOTES:
1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES ARE MET. 3. THERMAL VIAS IN THE LAND PATTERN OF THE EXPOSED PAD SHALL BE CONNECTED TO A SYSTEM GROUND PLANE. FAILURE TO DO SO MAY COMPROMISE THE THERMAL AND/OR FUNCTIONAL PERFORMANCE OF THE DEVICE.
Contact Information
Semtech Corporation Power Management Products Division 200 Flynn Road, Camarillo, CA 9302 Phone: (805) 498-2 Fax: (805) 498-3804
www.semtech.com
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