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1.5A SW Current, 40V Precision WLED Driver
Features
n n n n n n n n n n n
PAM2841
Description
The PAM2841 is a white LED driver, capable of driving 10 or more WLEDs in series (depending on forward voltage of the LEDs) with a range of input voltages from 2.7V to 5.5V. The PAM2841 features over current protection , over voltage protection , under voltage lockout and over temperature protection, which prevent the device from damage. LED dimming can be done by four methods as described in the Application Information hereinafter.
Capable of Driving 10 or more WLEDs Chip Enable with Soft-start Analog and PWM Dimming Peak Efficiency up to 90% Low Quiescent Current Fixed Frequency of 1MHz Over Current Protection Over Voltage Protection Thermal Protection UVLO Tiny Pb-Free Packages (RoHS Compliant): MSOP-8 and DFN 2X2
Applications
n WLED Driver System
Typical Application
V IN
C1 10 F L1 22 H D1(SS14)
PGND VIN
C2 1F
SW OVP FB GND
R1 910k
C3 1F
PAM2841
ENA Comp
R2 27k
10 LEDs
C4 10nF R3 10
V IN
C1 10 F
D1(SS14) L1 22 H
PGND VIN
C2 1F
SW OVP FB GND
R1 910k
C3 1F
PAM2841
3x9 LEDs
R2 27k
ENA Comp
R3 C4 10nF 1.1
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1.5A SW Current, 40V Precision WLED Driver
Block Diagram
Comp OVP VIN SW
PAM2841
200mV Reference
FB
+ GM -
Comparator
PWM +
PWM Logic And Driver
Ramp Generator
+
CS
-
ENA
Shutdown And Soft-start
1.0MHz Oscillator
GND
PGND
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1.5A SW Current, 40V Precision WLED Driver
Pin Configuration & Marking Information
TOP View MSOP-8
PAM2841
1
8
2 3 4
7 6 5
XXX: Internal Code Y: Year W: Weekly
DFN2X2
1 8
P2841 XXXYW
EMX YW
2 3 4 7 6 5
EM: Product Code X: Internal Code Y: Year W: Weekly
Pin number 1 2 3 4 5 6 7 8
Name PGND VIN ENA Comp GND FB OVP SW Power Ground Input Voltage Chip Enable, Active High
Compensation Node
Description
Chip Ground Feedback Over Voltage Drain of Main Switch.
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1.5A SW Current, 40V Precision WLED Driver
Absolute Maximum Ratings
These are stress ratings only and functional operation is not implied . Exposure to absolute maximum ratings for prolonged time periods may affect device reliability . All voltages are with respect to ground . Supply Voltage.............................................6.0V Output Current............................................50mA I/O Pin Voltage Range.........GND-0.3V to V DD+0.3V Maximum SW Pin Voltage...............................40V Storage Temperature.....................-40 C to 150 C O Maximum Junction Temperature..................150 C O Soldering Temperature.......................300 C,5sec
O O
PAM2841
Recommended Operating Conditions
Supply Voltage Range........................2.8V to 5.5V O O Operation Temperature Range..........-40 C to 85 C Junction Temperature Range.............0 OC to 125 OC
Thermal Information
Parameter Thermal Resistance (Junction to Ambient) Thermal Resistance (Junction to Case) Symbol JA JC Package MSOP DFN MSOP DFN Maximum 180 102 40 20 Unit C/W C/W C/W C/W
Electrical Characteristic
V EN=V DD=5.0V, 10 LEDs, T A=25 C , unless otherwise noted .
O
Parameters Supply Voltage Range Quiescent Current Shutdown Current
Symbol VDD IQ ISD
Test Conditions No Switching VENA=low R3=5.1
MIN. 2.7
TYP. 200 40 30 20 10
MAX. 5.5 300 1
UNITS V A A
Output Current
I
R3=6.8 R3=10 R3=20 VINx1.1 VENA=high ISW=100mA Switch On 0.7 Open Load Chip Shutdown Chip On VIN falling 1.2 2.0 194
mA
Output Voltage Range Feedback Voltage SW On Resistance SW Current Limit SW Frequency Maximum Duty Cycle Over Voltage Protection Threshold Enable Threshold Voltage Under Voltage Lockout UVLO Hysterisis Over Temperature Shutdown Over Temperature Hysterisis
VO VFB RDS(ON) ILIM fSW DC OVP VEL VEH UVLO VTH OTS OTH
40 200 0.35 1.5 1.0 95 1.2 0.4 2.2 0.2 150 30 2.4 1.3 206 0.5
V mV A MHz % V V V V
C C
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1.5A SW Current, 40V Precision WLED Driver
Typical Operating Characteristics
T A=25 C,V DD=5V, unless otherwise noted . 1. Efficiency VS Input Voltage
I LED=20mA, L=22 H
90% 85% 80%
PAM2841
2. Efficiency VS Load Current
6 LED, L=22 H
95 % 90 % 85 %
Efficiency
6LED 8LED
Efficiency
75% 70% 65% 60% 55% 50% 2 3 4 Input Voltage(V) 5
80 % 75% 70% 65% 60%
10LED 12LED
V DD =5V VDD=3.6V
6
0
10
20
30
40
50
60
Output Current(mA)
3. Efficiency VS Input Voltage
I LED=20mA, 10 LED
0.9 0.85 0.8
Efficiency
4. LED Current V S Output Voltage
L= 47 H
0.5
VDD=5V VDD=3V
0.4
LED Current (A)
L=4.7uH L=10uH L=22uH L=33uH L=47uH 2 3 4 Input Voltage(V) 5 6
0.3
0.75 0.7 0.65 0.6
0.2
0.1
0 8 12 16 20 24 28 32 36 Output Voltage (V)
Power Analog Microelectronics , Inc
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1.5A SW Current, 40V Precision WLED Driver
Typical Operating Characteristics
T A=25 C,V DD=5V, unless otherwise noted . 5. Feedback Voltage VS Input voltage
10 LED, ILED=20mA
0.3 0.25 0.2
0.3 0.25 0.2
PAM2841
6. Feeback Voltage VS Temperature
10 LED, ILED=20mA
Vfb (V)
0.15 0.1 0.05 0 2 3 4 Input Voltage (V) 5 6
Vfb(V)
0.15 0.1 0.05 0 0 20 40 60 80 100 120 140
Temperature
7. LED current VS Input voltage
L=22 H, 10 LED
25
1.2 1
8. Frequency VS Input Voltage
10 LED
20
Frequency(Mhz)
0.8 0.6 0.4 0.2 0
ILED(mA)
15
10 R=10 R=15 R=20 0 2 3 4 Vin(V) 5 6
5
2
3
4 Input Voltage(V)
5
6
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08/2008 Rev 1.3
1.5A SW Current, 40V Precision WLED Driver
Typical Operating Characteristics
T A=25 C,V DD=5V, unless otherwise noted . 9. Quiescent Current VS Input Voltage
200 180
Quiescent Current (uA)
PAM2841
10. Frequency VS Temperature (10 LED, I LED=20mA)
1.13 1.12
160 140 120 100 80 60 40 20 0 2 3 4 Input voltage (V) 5 6
Frequency(Mhz)
1.11 1.1 1.09 1.08 1.07 1.06 1.05 0 20 40 60 80 100 120 140
Temperature
11. LED Current VS Duty Cycle of PWM (PWM@EN Pin)
20 18 16
LED Current (mA)
Theoretic
12. LED Current VS Duty Cycle of PWM (PWM@COMP Pin)
25
20
LED Current(mA)
14 12 10 8 6 4 2 0 0% 20% 40%
f=200Hz f=100Hz
15
10
f=100HZ f=200Hz theoretic 60% 80% 100%
5
0 0%
20%
40%
60%
80%
100%
Duty Cycle of PWM
Duty Cycle of PWM
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1.5A SW Current, 40V Precision WLED Driver
Typical Operating Characteristics
T A=25 C,V DD=5V, unless otherwise noted . 13. Start-up Waveform
Vin=5V,10LED,Iled=20mA
PAM2841
14. Steady State Waveform
Vin=5V,10LED,Iled=20mA
ENA DC coupling SW DC coupling IL DC coutpling IL DC coupling VOUT DC coupling Vout AC coupling
15.Open load
16. Open load
Vout AC coupling
Vout DC coutpling
OVP AC coupling
SW DC coupling
SW DC coutpling
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1.5A SW Current, 40V Precision WLED Driver
Application Information
Inductor Selection The selection of the inductor affects steady state operation as well as transient behavior and loop stability. These factors make it the most important component in power regulator design. There are three important inductor specifications, inductor value, DC resistance and saturation current. Considering inductor value alone is not enough. The inductor value determines the inductor ripple current. Choose an inductor that can handle the necessary peak current without saturation, the inductor DC current given by: Iin_dc=Vout*Iout/(Vin* ) =efficiency. V RIPPLE=V RIPPLE(C)+V RIPPLE(ESR) Inductor values can have 20% tolerance with no current bias. When the inductor current approaches saturation level, its inductance can decrease 20% to 35% from the 0A value depending on how the inductor vendor defines saturation current. Using an inductor with a smaller inductance value causes discontinuous PWM when the inductor current ramps down to zero before the end of each switching cycle. This reduces the boost converter's maximum output current, causes large input voltage ripple and reduces efficiency. Large inductance value provides much more output current and higher conversion efficiency. For these reasons, an inductor within 4.7H to 22H value range is recommended. Schottky Diode Selection The high switching frequency of the PAM2841 demands a high-speed rectification for optimum efficiency. Ensure that the diode average and peak current rating exceeds the average output current and peak inductor current. In addition, the diode's reverse breakdown voltage must exceed the open protection voltage. Input and Output Capacitor Selection Input Capacitor At least a 1 F input capacitor is recommended to reduce the input ripple and switching noise for normal operating conditions. Larger value and lower ESR (Equivalent Series Resistance) may be needed if the application require very low input ripple. It follows that ceramic capacitors are a good choice for applications. Note that the input capacitor should be located as close as possible V RIPPLE(C) 2 I OUT) 1/2*(L/(C OUT *((V OUT(MAX) -V IN(MIN) ))))*(I
2 PEAK
PAM2841
to the device. Output Capacitor The output capacitor is mainly selected to meet the requirement for the output ripple and loop stability. This ripple voltage is related to the capacitor's capacitance and its equivalent series resistance (ESR). A output capacitor of 1F minimum is recommended and maybe need a larger capacitor. The total output voltage ripple has two components: the capacitive ripple caused by the charging and discharging on the output capacitor, and the ohmic ripple due to the capacitor's equivalent series resistance (ESR):
-
V RIPPLE(ESR)=I PEAK*R ESR(COUT) Where I PEAK is the peak inductor current. Multilayer ceramic capacitors are an excellent choice as they have extremely low ESR and are available in small footprints. Capacitance and ESR variation with temperature should be considered for best performance in applications with wide operating temperature ranges. Dimming Control There are 4 different types of dimming control methods: 1). Using an External PWM Signal to EN Pin With the PWM signal applied to the EN pin, the PAM2841 is alternately turned on or off by the PWM signal. The LEDs operate at either zero or full current. The average LED current changes proportionally with the duty cycle of the PWM signal. A 0% duty cycle turns off the PAM2841 and leads to zero LED current. A 100% duty cycle generates full current.Also the recommend dimming frequency is between 100Hz and 200Hz. I AVE=I STATE* (T ON -T STARTUP)/(T ON+T OFF) where T ON: on time of a period T STARTUP: 0.85ms T OFF: off time of a period I STATE: on state current (full current)
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1.5A SW Current, 40V Precision WLED Driver
L1 VIN C1 10uF U12 1 22uH C3 1uF 1 2 3 1uF C2 4 8 7 6 5 R1 910k 2 1 D1 2
PAM2841
0
PGND VIN ENA COMP
SW OV OVP FB GND
0
R2 27k
PAM2841 PWM
C4 10n
Figure. 1 2). Using an External PWM Signal to NMOS Gate. When PWM signal is at high level, N MOSFET turned on, then pull comp pin down, then the LED current should be zero. When PWM signal is at low level, N MOSFET turned off. The circuit uses resistor R1 to set the on state current. The average LED current changes proportionally with the duty cycle of the PWM signal. A 100% duty cycle turns off the PAM2841 and leads to zero LED current. A 0% duty cycle generates full current. I AVE=I STATE* (T ON -T STARTUP)/(T ON+T OFF) where T ON: on time of a period T STARTUP: 0.08ms T OFF: off time of a period I STATE: on state current (full current) Also the recommend frequency is between 100 and 500Hz. Frequency<100Hz can naturally causes LEDs to blink visibly.
L1 VIN C1 10uF U12 1 22uH C3 1uF 1 2 3 1uF C2 4 8 7 6 5 R1 910k 2 1 D1 2
3). Using a DC Voltage For some applications, a simple and direct way to control brightness is use an external variable DC voltage to vary the voltage drop on feedback resistor. This will make the PAM2841 adjust the output current to follow the change of feedback voltage. The circuit is shown in Figure 3. As the DC voltage increases, the voltage drop on R4 increases and the voltage drop on R3 decreases. Thus, the LED current decreases. The selection of R4 and R5 will make the current from the variable DC source much smaller than the LED current and much larger than the FB pin current. For VCC range from 0V to 2V, the selection of resistors in Figure 3 gives dimming control of LED current from 0mA to 20mA.
L1 VIN C1 10uF U33 1 22uH C3 1uF 1 2 3 1uF C2 4 8 7 6 5 R1 910k 2 1 D1 2
0
PGND VIN ENA COMP
SW OV OVP FB GND
0
R2 27k
PAM2841 ENA C4 10n R5 90k
R4 5k R3 10
Figure. 3
DC
4). Using a Filtered PWM Signal The filtered PWM signal can be considered as an adjustable DC voltage. Such regulated signal is often with some grade of ripple because of some simple configuration of circuit. With appropriated arrangement of PWM frequency and level, and filter parameters, it can be used to replace the variable DC voltage source in dimming control. The circuit is shown in Figure 4.
L1 VIN C1 10uF U33 1 22uH C3 1uF 1 2 3 1uF C2 4 8 7 6 5 R1 910k 2 1 D1 2
0
PGND VIN ENA COMP
SW OV OVP FB GND
0
R2 27k
PAM2841 ENA Q1 PWM C4 10n R3 10
ENA C4 10n
0
PGND VIN ENA COMP
SW
OVP OV
FB GND
0
R2 27k
PAM2841
R4 R5 90k R6 PWM 10k C5 100nF 5k R3 10
Figure. 2
5V 0V
0
Figure. 4
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1.5A SW Current, 40V Precision WLED Driver
Layout Considerations As for all switching power supplies, especially those in high frequency and high current ones, layout is an important design step. If layout is not carefully done, the regulator could suffer from instability as well as noise problems. (1) Use separate traces for power ground and signal ground. Power ground and signal ground are connected together to a quite ground (input connector) (2) To prevent radiation of high frequency resonance, proper layout of the high frequency switching path is essential. Minimize the length and area of all traces connecting to the SW pin. The loop including the PWM switch, schottky diode and output capacitor, contains high current rising and falling in nanosecond and thus it should be kept as short as possible. (3) The input capacitor should be close to both the VIN pin and the GND pin in order to reduce the IC supply ripple. (4) Keep the signal ground traces short and as close to the IC as possible. Small signal components should be placed as close as possible to the IC, thus minimizing control signal noise interference. Layout Example Top Layer
PAM2841
Bottom Layer
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1.5A SW Current, 40V Precision WLED Driver
Ordering Information
PAM2841
PAM2841 X X
Shipping Package Type
Part Number PAM2841SR PAM2841GR Marking P2841 EMX YW Package MSOP-8 DFN 2x2-8 Shipping 3,000 units/Tape & Reel 3,000 units/Tape & Reel
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1.5A SW Current, 40V Precision WLED Driver
Outline Dimensions
MSOP-8
e
PAM2841
HE
DETAIL A
b D
E
A
L L1
DETAIL A(S=3:1)
REF. A A1 A2 D E HE
Millimeter Min -0.05 0.78 2.90 2.90 4.75 Max 1.10 0.15 0.94 3.10 3.10 5.05
A1
REF. L L1 B C e
Millimeter Min 0.40 0.95 BSC 0.22 0.08 0.65 BSC 0.38 0.23 Max 0.70
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C
A2
1.5A SW Current, 40V Precision WLED Driver
Outline Dimensions
DFN 2x2
D B
PAM2841
A
4
INDEX AREA (D/2 xE/2)
9
TOP VIEW
E A
C
SEATING PLANE
8
0.08 C
SIDE VIEW
e
Nxb
Pin#1 ID
4 INDEX AREA (D/2 xE/2)
E2
A1
D2
BOTTOM VIEW
SYMBOL A A1
MIN 0.70 0.00
COMMON DIMENSION NOM MAX 0.75 0.80 0.02 0.05 Summary Table Lead Body Count Size 8 2X2
Lead Pitch (e) 0.50
Pin #1 ID R0.20
D BSC E BSC MIN b NOM MAX MIN D2 NOM MAX MIN E2 NOM MAX MIN L NOM MAX N
2.00 2.00 0.18 0.25 0.30 1.05 1.20 1.30 0.45 0.60 0.70 0.20 0.30 0.40 8
NxK
Unit: Millimeters
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NxL

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