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MIC3291
1.2MHz PWM White LED Driver with Internal Schottky Diode and Single-Wire Linear Brightness Control
General Description
Features
The MIC3291 is a PWM switching boost regulator that is * Single wire combines 16 level linear brightness and optimized for constant current white LED (WLED) driver shutdown control applications. The MIC3291 features an internal Schottky * 18V / 25V OVP options supports up to four and six diode, allowing an efficient DC/DC solution that requires WLEDs only four external components. * Startup in any one of 16 brightness levels The MIC3291 allows for a single-wire simple digital * Internal Schottky diode interface to control the dimming over 16 steps with a linear * 2.5V to 6.5V input voltage scale. The preprogramming feature allows the user to * 1.2MHz PWM operation select any one of the 16 WLED current levels as the start up brightness level. * Over 500mA switch current The feedback voltage of the MIC3291 is only 250mV, * 250mV reference voltage allowing high efficiency while retaining excellent accuracy * 5% LED current accuracy for the WLED current. * <1A shutdown current The MIC3291 implements a constant frequency 1.2MHz * UVLO and over-temperature protection PWM control scheme. The high frequency PWM operation * Thin SOT23-6L package option saves board space by reducing external component sizes. The 1.2MHz PWM scheme also reduces switching noise * 2mm x 2mm leadless MLF(R) package option and ripple to the input power source. * -40oC to +125oC junction temperature range The 2.5V to 6.5V input voltage range of the MIC3291 allows direct operation from single cell Li-Ion as well as 3 Applications to 4 cell NiCad/NiMH/Alkaline batteries. Battery life is preserved with a low 1A shutdown current. * Mobile handsets The MIC3291 is available in a low profile Thin SOT23 6* LCD handset backlighting (R) pin and 2mm x 2mm MLF 8-pin packages with a junction * Digital cameras temperature range of -40C to +125C. * Portable media players/MP3 players Data sheets and support documentation can be found on * Portable applications Micrel's web site at: www.micrel.com. ___________________________________________________________________________________________________________
Typical Application
White LED Driver with OVP and Digital Control
MLF and MicroLead Frame are registered trademarks of Amkor Technologies, Inc. Micrel Inc. * 2180 Fortune Drive * San Jose, CA 95131 * USA * tel +1 (408) 944-0800 * fax + 1 (408) 474-1000 * http://www.micrel.com
September 2010
M9999-092810 (408) 944-0800
Micrel, Inc.
MIC3291
Ordering Information
Part Number1 MIC3291-18YD6 MIC3291-25YD6 MIC3291-18YML MIC3291-25YML
Notes: 1. 2. 3. 4. Other options available. Contact Micrel Marketing for details. MLF is a GREEN RoHS compliant package. Lead finish is NiPdAu. Mold compound is Halogen Free. MLF
(R) (R)
Marking Code4 W18F W25F
WAF
Output Voltage Adjustable Adjustable Adjustable Adjustable
Over-Voltage Protection 18V 25V 18V 25V
Junction Temperature Range -40C to +125C -40C to +125C -40C to +125C -40C to +125C
Package2,3 TSOT23-6 TSOT23-6 2mm x 2mm MLF(R) 2mm x 2mm MLF(R)
WBF
= Pin 1 identifier.
Overbar ( )and Underbar symbols ( _ ) may not be to scale.
Pin Configuration
2mm x 2mm 8-Pin MLF(R) (ML) (Top View)
TSOT23-6 (D6) (Top View)
Pin Description
Pin Number 8-pin MLF 1 2 3 4 5 6 7 8 Pad
(R)
Pin Number SOT23-6 6 5 4 - - 3 1 2 -
Pin Name OUT VIN DC GND NC FB SW GND GND
Pin Name Output and over-voltage protection (output) Supply (Input): 2.5V to 6.5V for internal circuitry. Single pin digital control. See diagrams. Ground (Return): Ground. No connect (no internal connection to die) Feedback (Input): Output voltage sense node. Connect the cathode of the LED to this pin. Switch Node (Input): Internal power BIPOLAR collector. Ground (Return): Ground. Ground (Return): Backside pad.
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Absolute Maximum Ratings(1)
Supply Voltage (VIN) .................................................... 7.5V Switch Voltage (VSW) ..................................... -0.3V to 27V Digital Control Voltage (VDC) ..............................-0.3 to VIN FB Voltage (VFB)............................................................. 6V Switch Current (ISW)........................................................ 2A Ambient Storage Temperature (TS)......... -65C to +150C ESD Rating(2) ................................................................2KV
Operating Ratings(3)
Supply Voltage (VIN) ............................. ......2.5V to 6.5V Output Voltage (VOUT) ................... ............... (VIN to VOVP) Junction Temperature Range (TJ) ...... .... -40C to +125C Package Thermal Impedance JA 2mm x 2mm MLF(R)-8L ................................. 93C/W JA TSOT23-6 .................................................. 235C/W
Electrical Characteristics(4)
TA = +25oC, VIN = 3.6V, VOUT = 10V, IOUT = 20mA, unless otherwise noted. Bold values indicate -40C TJ +125C. Symbol Parameter Condition Min. Typ. Max. Units
VIN VUVLO IVIN ISD VFB IFB DMAX ISW VDC IDC tshutdown tMODE_UP tMODE_DOWN tstart_up tprog_low tprog_high tdelay Tprog_setup fSW
Supply Voltage Range Under-Voltage Lockout Quiescent Current Shutdown Current (DC Pin Low ) Feedback Voltage Feedback Input Current Line Regulation Load Regulation Maximum Duty Cycle Switch Current Limit DC Pin Thresholds DC Pin Current Shutdown Pulse Width Count UP Mode Pulse Width Count Down Mode Pulse Width Turn-On Delay Time Programming Pulse Width Low Programming Pulse Width High Minimum Delay for Mode Change First Pulse Window for Preprogramming Oscillator Frequency
2.5 1.8 VFB >500mV VDC = 0V for > 2ms. (5%) VFB = 250mV 2.5V VIN 4.5V 5mA IOUT 20mA VIN = 3.6V High Low VDC = 3.6V VIN = 2.8V to 5.5V VDC = Low VIN = 2.8V to 5.5V VDC = Low VIN = 2.8V to 5.5V VDC = Low VIN = 2.8V to 5.5V VIN = 2.8V to 5.5V VIN = 2.8V to 5.5V VIN = 2.8V to 5.5V VDC = High VIN = 2.8V to 5.5V 1260 100 420 140 1 1 140 86 500 1.1 237
2.1 2 0.01 250 450 0.5 0.5 90 750
6.5 2.4 4 1 263
V V mA A mV nA % % mA V A s
1200 0.4
5
10
160 500
s s s s s s
32 32
35 1 1.2
50 1.35
s MHz
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Electrical Characteristics(4) (Continued)
TA = +25oC, VIN = 3.6V, VOUT = 10V, IOUT = 20mA, unless otherwise noted. Bold values indicate -40C TJ +125C. Symbol Parameter Condition Min. Typ. Max. Units
VD IRD VOVP Tj
Notes: 1.
Schottky Forward Drop Schottky Leakage Current Over-Voltage Protection Over-Temperature Threshold Shutdown
ID = 150mA VR = 30V 3291- 18 only (nominal voltage) 3291- 25 only (nominal voltage) 15 22.1
0.8 16.5 23.5 150
1 4 18 25
V A V C
Absolute maximum ratings indicate limits beyond which damage to the component may occur. Electrical specifications do not apply when operating the device outside of its operating ratings. The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(Max), the junction-to-ambient thermal resistance, JA, and the ambient temperature, TA. The maximum allowable power dissipation will result in excessive die temperature, and the regulator will go into thermal shutdown. IC devices are inherently ESD sensitive. Handling precautions required. Human body model, 1.5k in series with 100pF. This device is not guaranteed to operate beyond its specified operating rating. Specification for packaged product only.
2. 3. 4.
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Typical Characteristics
275 FEEDBACK VOLTAGE (mV) 250 225 175 150 125 100 75 50 25 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 BRIGHTNESS LEVEL (0-15) EFFICIENCY (%) 200
Feedback Voltage vs. Brightness Level
100 90 80 60 50 40 30 20 10 0 0 70
6 Series White LED Efficiency
VIN = 2.8V
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0
3 Series White LED Efficiency
VIN = 2.8V
VIN = 4.2V VIN = 3.6V L = 10H COUT = 0.27F 5 10 15 20 25
VIN = 3.6V
VIN = 4.2V
L = 10H COUT = 0.27F 0 5 10 15 20 25
WHITE LED CURRENT (mA)
WHITE LED CURRENT (mA)
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0
6 Series White LED Efficiency
VIN = 4.2V
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0
3 Series White LED Efficiency
FEEDBACK VOLTAGE (mV)
260 258 256 254 252 250 248 246 244 242 240 25 2.5
Feedback Voltage vs. Input Voltage
VIN = 3.6V VIN = 2.8V
VIN = 3.6V VIN = 4.2V VIN = 2.8V
L = 22H COUT = 0.27F 5 10 15 20 25
L = 22H COUT = 0.27F 0 5 10 15 20
3
3.5
4
4.5
5
5.5
6
WHITE LED CURRENT (mA)
WHITE LED CURRENT (mA)
INPUT VOLTAGE (V)
1000 FEEDBACK VOLTAGE (mV) 900
Peak Switch Current vs. Input Voltage
INPUT CURRENT (mA)
350 300 250 200 150 100 50 0
Input Current vs. Input Voltage
L=10H
CURRENT LIMIT (A)
1 0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6
Current Limit vs. Input Voltage
800 700 600 500 400 300 200 2.5 3 3.5 4 4.5 5 5.5 INPUT VOLTAGE (V)
6 LEDs
3 LEDs 2.5 3 3.5 4 4.5 5 5.5 6
2.6 3.1 3.6 4.1 4.6 5.1 5.6 6.1 6.6 INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
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MIC3291
Functional Characteristics
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MIC3291
Functional Characteristics (Continued)
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MIC3291
Functional Diagram
MIC3291 Block Diagram
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MIC3291
Brightness Level 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 FB Voltage (mV) 250 235 220 205 190 175 160 145 130 115 100 85 70 55 40 25
Functional Description
The MIC3291 is a constant frequency, PWM current mode boost LED driver. It is composed of an oscillator, slope compensation ramp generator, current amplifier, gm error amplifier, PWM generator, bipolar output transistor, digital interface with D/A converter and Schottky rectifier diode. It features true single-wire linear digital control that may be used to vary the brightness of the output LEDs. The oscillator generates a 1.2MHz clock which triggers the PWM generator that turns on the output transistor and resets the slope compensation ramp generator. The current amplifier is used to measure the switch current by amplifying the voltage signal from the internal sense resistor. The output of the current amplifier is summed with the output of the slope compensation ramp generator. This summed current loop signal is fed to one of the inputs of the PWM generator. MIC3291 Block Diagram The gm error amplifier measures the LED current through the external sense resistor and amplifies the error between the detected signal and the reference voltage indicated by the digital interface. The output of the gm error amplifier provides the voltage loop signal that is fed to the other input of the PWM generator. When the current loop signal exceeds the voltage loop signal, the PWM generator turns off the bipolar output transistor. The next clock period initiates the next switching cycle, maintaining the constant frequency current mode PWM control. The LED current level at maximum brightness is set by the feedback resistor:
ILED = 250mV RLED
Table 1. Brightness Level vs. Ideal FB Voltage
Startup Presuming no presetting brightness command is issued (discussed in a later section), the MIC3291 will startup in its default state approximately 140s (tSTART_UP) after a logic level high has been applied and maintained at the DC pin. In the default state the LED drive current is at the maximum brightness level of 15 and brightness counter is set to count down mode. Any falling edges during the tPROG_SETUP period will cause the initial brightness level of the LEDs to be below the maximum brightness level. This is discussed in more detail in the Presetting Brightness section.
MIC3291 Digital Interface The MIC3291 incorporates an easy to use single-wire, serial programming interface allowing users to set LED brightness to one of sixteen levels spaced in a linear manner. In contrast to other solutions requiring a PWM drive signal to maintain LED brightness, the MIC3291 is "set and forget", relieving the controlling processor of the constant burden of supplying a drive signal. Additionally, brightness levels can be preset so that LEDs can be turned on at a particular brightness level.
Figure 2. Typical Startup Timing
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MIC3291
Shutdown Whenever a logic level LOW is applied to the DC input pin for a period greater than or equal to tSHUTDOWN(1260s), the MIC3291 will be in power saving shutdown mode.
Figure 5. Mode Change to Count Down
Figure 3. Shutdown Timing
Once the device is shutdown, the boost supply is disabled and the LEDs are turned off. Brightness level information stored in the MIC3291 prior to shutdown will be lost. Programming Pulse Counter Modes At power up the MIC3291 defaults to count down mode. In count down mode, the brightness level is decreased one level by applying two programming pulses, as shown in Figure 6. The counting mode can be changed to count up mode by pulling the DC pin LOW for a period equal to tMODE_UP (100s to 160s). The device will remain in count up mode until its state is changed to count down mode or by disabling the MIC3291.
Programming the Brightness Level MIC3291 is designed to start driving the output LEDs (tSTART_UP) in 140s at the maximum brightness level of 15. After startup, the internal control logic is ready to decrease the LED brightness upon receiving programming pulses (negative edges applied to DC pin). Since MIC3291 starts in count down mode, the brightness level is decreased one level by applying two programming pulses, as shown in Figure 6. Each programming pulse has a high (tPROG_HIGH) and a low (tPROG_LOW) pulse width that must be between 1s to 32s. Note that n+1 number of pulses are needed to decrease brightness by n level(s) since the first clock pulse is ignored. Ignoring the first clock pulse is necessary so mode change (tMODE_UP, tMODE_DOWN) pulses do not result in adjustments to the brightness level. The MIC3291 internal circuit can be changed from count down mode to count up mode and vice versa. The user may elect to send a mode change as shown in Figure 4 to set the MIC3291 to step up the brightness level with subsequent programming pulses. For proper operation, ensure the DC pin has remained HIGH for at least tDELAY(140s) before issuing a mode change command.
Figure 4. Mode Change to Count up
To change the state back to count down mode, pull the DC pin LOW for a period equal to tMODE_DOWN (420s to 500s). Now the internal circuitry will remain in count down mode until changed to count up mode as described previously.
Figure 6. Brightness Programming Pulses
Multiple brightness levels can be changed together with multiple programming pulses, as shown in Figure 7. When issuing multiple brightness level adjustment commands to the DC pin, ensure both tPROG_LOW and tPROG_HIGH are within 1s and 32s.
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MIC3291
Figure 9. Up Counter Rollover Figure 7. Decreasing Brightness Several Levels
As mentioned, MIC3291 can be programmed to set LED drive current to produce one of 16 distinct brightness levels. The internal logic keeps track of the brightness level with an up/down counter circuit. The Counter Rollover section explains how the brightness counter functions with continued programming edges. Counter Rollover The MIC3291 internal up/down counter contains registers from 0 to 15. When the brightness level is at 0 and a programming pulse forces the brightness to step down, then the counter will rollover to Level 15. This is illustrated in Figure 8.
One Step Brightness Changes For applications where a keypad button press is to be translated into a brightness level change, the following method of decreasing the brightness level may be useful. This "one step" brightness change procedure relieves the user from keeping track of the MIC3291's up/down counter mode. It combines a counter mode change with a programming pulse, therefore a one step decrease in brightness is assured no matter what the previous up/down counter mode was.
Figure 10. One step Brightness Decrease
Figure 8. Down Counter Rollover
Similarly, when the counter mode is set to count up and a programming pulse forces the brightness level to step up from level 15, then the counter will rollover to level 0 as illustrated in Figure 9.
This method is quite simple and the only requirement is that the first DC LOW period be equal to the tMODE_DOWN (420s to 500s) and immediately followed by a falling edge within tPROG_HIGH (1s to 32s) as shown in Figure 10 for one step brightness decrease. Similarly a one step increase can be assured by first generating a DC down pulse whose period is equal to the tMODE_UP (100s to 160s) and immediately followed by a falling edge within tPROG_HIGH (1s to 32s). Figure 11 illustrates the proper timing for execution of a one step brightness increase.
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MIC3291 Figure 12 shows the correct presetting sequence to set the MIC3291 brightness to Level 6 prior to startup. The sequence is initiated by driving the DC pin LOW for a period exceeding tSHUTDOWN (1260s) to insure that the part has entered the power saving shutdown state erasing all brightness level state and mode setting. Then the DC pin is driven HIGH and the first presetting pulse edge is entered within the tPROG_SETUP window. Notice that when using the presetting feature the first programming pulse is not ignored. This is because the counter's default mode is count down and a mode change cannot be performed in the presetting mode. (Note that the same timing requirements of standard brightness programming also apply during presetting brightness.)
Figure 11. One step Brightness Increase
Presetting Brightness The brightness level can be preset before the MIC3291 begins to drive the LEDs by sending a series of programming edges via the DC pin during the tSTART_UP (140s) period and between 35s to 50s after the DC pin is first pulled HIGH. The 15s timeframe between 35s and 50s is the tPROG_SETUP period. The MIC3291 does not drive current into the load until DC pin is kept HIGH for tSTART_UP (140s) after presetting has concluded in order to grant the user sufficient time to preset LED brightness. The first presetting pulse edge must occur somewhere between the timeframe of 35s to 50s after DC pin is first pulled HIGH otherwise the MIC3291 may continue to start at the full (default) brightness level.
Figure 12. Preset Timing
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MIC3291
Typical Application Circuit (MIC3291-xxYML)
Bill of Materials
Item C1 C2 R1 L1 U1
Notes:
Part Number GRM185R61A105KE36D GRM21BR71E474KCO1L CRCW060312R40FRT1 VLS252010T-100M MIC3291-xxYML
Manufacturer Murata(1) Murata Vishay
(1) (2)
Description Capacitor, 1F Ceramic, 10V, X5R, Size 0603 Capacitor, 0.47F Ceramic, 25V, X7R, Size 0805 12.4, 1%, 0603 10H, 650mA, 712m, L2.5mm x W2.0mm x H1.0mm 1.2MHz PWM Boost LED Driver with Single-Wire Linear Digital Control
Qty. 1 1 1 1 1
TDK(3) Micrel, Inc.(4)
1. Murata: www.murata.com. 2. Vishay: www.vishay.com. 3. TDK: www.tdk.com. 4. Micrel, Inc.: www.micrel.com.
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MIC3291
Typical Application Circuit (MIC3291-xxYD6)
Bill of Materials
Item C1 C2 R1 L1 U1
Notes:
Part Number GRM185R61A105KE36D GRM21BR71E474KCO1L CRCW060312R40FRT1 VLS252010T-100M MIC3291-xxYD6
Manufacturer Murata(1) Murata Vishay
(1) (2)
Description Capacitor, 1F Ceramic, 10V, X5R, Size 0603 Capacitor, 0.47F Ceramic, 25V, X7R, Size 0805 12.4, 1%, 0603 10H, 650mA, 712m, L2.5mm x W2.0mm x H1.0mm 1.2MHz PWM Boost LED Driver with Single-Wire Linear Digital Control
Qty. 1 1 1 1 1
TDK(3) Micrel, Inc.(4)
1. Murata: www.murata.com. 2. Vishay: www.vishay.com. 3. TDK: www.tdk.com. 4. Micrel, Inc.: www.micrel.com.
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MIC3291
PCB Layout Recommendations
Top Layer
Bottom Layer
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MIC3291
Package Information
6-Pin TSOT23 (D6)
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Package Information (Continued)
8-Pin MLF(R) (ML)
MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http://www.micrel.com
The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its use. Micrel reserves the right to change circuitry and specifications at any time without notification to the customer. Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A Purchaser's use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser's own risk and Purchaser agrees to fully indemnify Micrel for any damages resulting from such use or sale. (c) 2010 Micrel, Incorporated.
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