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MIC3287
Micrel, Inc.
MIC3287
1.2MHz PWM White LED Driver with OVP in Thin SOT-23 and 2mm x 2mm MFLTM
General Description
The MIC3287 is a 1.2MHz pulse width modulated (PWM) step-up switching regulator that is optimized for constant current white LED driving applications. With a maximum output voltage of 24V and a switch current of over 350mA, the MIC3287 drives a string of up to 4 white LEDs in series, ensuring uniform brightness and eliminating several ballast resistors. The MIC3287 implements a constant frequency 1.2MHz PWM current-mode control scheme. The high frequency PWM operation saves board space by reducing external component sizes. The added benefit of the constant frequency PWM control scheme as opposed to variable frequency control schemes is much lower noise and input ripple injected back to the battery source. To optimize efficiency, the feedback voltage is set to only 250mV. The MIC3287 is also available with a maximum 24V over-voltage protection. The MIC3287 is available in low profile Thin SOT-23 5-pin and Thin SOT-23 6-pin packages. The MIC3287 has a junction temperature range of -40C to +125C. All support documentation can be found on Micrel's web site at: www.micrel.com.
Features
* * * * * * * * * * * * * 2.8V to 5.5V Input Voltage 350mA Switch Current 1.2 MHz PWM Operation 250mV Feedback Voltage 24V Output Over-Voltage Protection (OVP) Option <1% Line and Load Regulation <1A Shutdown Current Over-Temperature Protection Under-Voltage Lock Out (UVLO) Low Profile Thin SOT-23-5 Package Option Low Profile Thin SOT-23-6 Package Option 8-pin 2mm x 2mm MLFTM Package Option -40C to +125C Junction Temperature Range
Applications
* White LED Driver for Backlighting: - Cell phones - PDAs - GPS systems - Digital cameras - MP3 players - IP phones * LED Flashlights * Constant Current Power Supplies
Typical Application
10H
10H
5
MIC3287YD5
1-Cell Li Ion
1F
VIN EN
SW FB GND
2
1
MIC3287-24YD6
6
4
3
0.22F 250mV
1-Cell Li Ion
1F
VIN EN
SW OVP FB GND
4
1 5 3
4
0.22F
250mV
3-Series White LED Driver in Thin SOT-23-5
MLF and MicroLeadFrame are trademarks of Amkor Technology, Inc.
4-Series White LED Driver with Output OVP in Thin SOT-23-6
Micrel, Inc. * 2180 Fortune Drive * San Jose, CA 95131 * USA * tel + 1 (408) 944-0800 * fax + 1 (408) 474-1000 * http://www.micrel.com
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MIC3287
Micrel, Inc. Marking Code WGAA WHAA WLA Overvoltage Protection N/A 24V 24V Junction Temp. Range -40C to +125C -40C to +125C -40C to +125C
Ordering Information
Part Number MIC3287YD5 MIC3287-24YD6 MIC3287-24YML Package Thin SOT-23-5 Thin SOT-23-6 8-Pin 2mm x 2mm MLF Lead Finish Lead Free Lead Free Lead Free
Pin Configuration
FB GND SW 1 2 3 FB GND SW 3 2 1 VIN 1 OVP 2 4 EN 5 VIN 4 5 6 EN OVP VIN EN 3 AGND 4 EP 8 7 6 5 PGND SW NC FB
TSOT-23-5 (YD5)
TSOT-23-6 (YD6)
8-Pin 2mm x 2mm MLFTM (YML)
Ordering Information
Pin Number Pin Number TSOT-23-5 TSOT-23-6 1 2 2 3 4 5 -- -- -- 1 2 2 3 4 6 5 -- -- Pin Number 8-Pin MLF 7 4 8 5 3 1 2 6 EP Pin Name SW AGND PGND FB EN VIN OVP NC EP Pin Function Switch Node (Input): Internal power bipolar collector. Analog Ground (Return): Ground. Power Ground (Return): Ground. Feedback (Input): Output voltage sense node. Connect the cathode of the LED to this pin. A resistor from this pin to ground sets the LED current. Enable (Input): Logic high enables regulator. Logic low shuts down regulator. Supply (Input): 2.8V to 5.5V for internal circuitry. Overvoltage Protection (Input): Connect to the output. Not Connected Exposed Backside Pad: Connect to ground.
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MIC3287
Micrel, Inc.
Absolute Maximum Ratings(1)
Supply Voltage (VIN) ........................................................ 6V Switch Voltage (VSW) ......................................-0.3V to 34V Enable Pin Voltage (VEN) ................................... -0.3 to VIN FB Voltage (VFB) ............................................................. 6V Switch Current (ISW) ..................................................... 1.2A Ambient Storage Temperature (TS) .......... -65C to +150C ESD Rating(3) ................................................................ 2kV
Operating Ratings(2)
Supply Voltage (VIN) .........................................2.8V to 5.5V Output Voltage (VOUT)(4) ...................................VIN to VOUT Junction Temperature Range (TJ) ............ -40C to +125C Package Thermal Impedance Thin SOT-23-5 (JA)(5) ....................................... 207C/W Thin SOT-23-6 (JA)(5) ....................................... 134C/W 8-Pin MLF (JA) ................................................... 90C/W
Electrical Characteristics(6)
TA = 25C, VIN = VEN = 3.6V, VOUT = 10V, IOUT = 10mA, unless otherwise noted. Bold values indicate -40C TJ 125C. Symbol Parameter Condition Min 2.8 1.8 VEN = 0V(7) (7.5%) VFB > 500mV, (not switching) 231 2.1 2.1 0.04 250 -450 0.4 0.5 85 350 250 0.01 1.5 21 1 17 1.2 19 150 10 1 0.4 40 1.4 24 90 1 Typ VIN Supply Voltage Range Under Voltage Lockout Quiescent Current Shutdown Current Feedback Voltage Feedback Input Current Line Regulation Load Regulation DMAX ISW ISW VSW VEN IEN Maximum Duty Cycle Switch Current Limit Switch Saturation Voltage Switch Leakage Current Enable Threshold Enable Pin Current Oscillator Frequency Overvoltage Protection Overtemperature Threshold Shutdown Max 5.5 2.4 5 1 269 Units V V mA A mV nA % % % mA mV A V V A MHz V C C
VUVLO IVIN ISD IFB
VFB
VFB = 250mV 3V VIN
5V(8)
5mA IOUT 20mA(8) VIN = 3.6V
VIN = 3.6V, ISW = 300mA TURN ON TURN OFF VEN = 10V MIC3287 - 24YD6 MIC3287-24YML Hysteresis VEN = 0V, VSW = 10V
fSW
VOVP TJ
Notes:
1. 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. 2. This device is not guaranteed to operate beyond its specified operating ratings. 3. Devices are inherently ESD sensitive. Handling precautions required. Human body model. 4. For options with OVP. 5. Using 4 layer PCB. 6. Specification for packaged product only. 7. ISD = IVIN. 8. Guaranteed by design.
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Micrel, Inc.
Typical Characteristics
85 80
EFFICIENCY (%)
3 Series White LED Efficiency
VIN=3.6V
85 80
EFFICIENCY (%)
4 Series White LED Efficiency
VIN=3.6V
VIN=3.2V
12
ENABLE PIN CURRENT (A)
Enable Pin Current vs. Temperature
VEN=5V
75 VIN=3.2V 70 65 60 55 50 45 2
75 70 65 60 55 50 45 2
10 8 6 4 2
VIN=4.2V
VIN=4.2V
VEN=2.5V
VEN=1.8V
VEN=3.3V
L = 10H COUT = 0.22F
4 6 8 10 12 14 16 18 20 WHITE LED CURRENT (mA)
L = 10H COUT = 0.22F
4 6 8 10 12 14 16 18 20 WHITE LED CURRENT (mA)
0 -40 -25 -10 5 20 35 50 65 80 TEMPERATURE (C)
1.5
FREQUENCY (MHz)
Frequency vs. Temperature
260
FEEDBACK VOLTAGE (mV)
Feedback Voltage vs. Temperature
254
FEEDBACK VOLTAGE (mV)
Feedback Voltage vs. Input Voltage
1.4 1.3 1.2 1.1
VIN=4.2V
VIN=3.6V
VIN=3.2V
256 252 248 244 240 -40 -25 -10 5 20 35 50 65 80 TEMPERATURE (C)
253 252 251 250 249 248 2.8 3.2 3.6 4.0 4.4 4.8 5.2 5.6 INPUT VOLTAGE (V)
VIN=3.6V
1.0 -40 -25 -10 5 20 35 50 65 80 TEMPERATURE (C)
QUIESCENT CURRENT (mA)
0.003
SHUTDOWN CURRENT (uA)
Shutdown Current vs. Input Voltage
2.5 2.4 2.3 2.2 2.1
Quiescent Current vs. Input Voltage
900
CURRENT LIMIT (mA)
Curremt Limit vs. Temperature
850 800 750 700 650 600 -40 -25 -10 5 20 35 50 65 80 TEMPERATURE (C)
VIN=3.6V
0.002
0.001 2.8 3.2 3.6 4.0 4.4 4.8 5.2 5.6 INPUT VOLTAGE (V)
2.0 2.8 3.2 3.6 4.0 4.4 4.8 5.2 5.6 INPUT VOLTAGE (V)
840
CURRENT LIMIT (mA)
Current Limit vs. Input Voltage
820 800 780 760 740 720 2.8 3.2 3.6 4.0 4.4 4.8 5.2 5.6 INPUT VOLTAGE (V)
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MIC3287
Micrel, Inc.
Functional Diagram
VIN
FB
OVP*
EN
OVP VREF
Error Amplifier
SW
VREF
gm 250mV
PWM Generator
Current Amplifier
1.2MHz Oscillator
Ramp Generator
GND
* OVP available on TSOP-23-6 and MLFTM package options only.
MIC3287 Block Diagram
Functional Description
The MIC3287 is a constant frequency pulse width modulated (PWM) current-mode step-up regulator. The block diagram is shown above. The MIC3287 is composed of an oscillator, slope compensation ramp generator, current amplifier, gm error amplifier, PWM generator, and a 350mA minimum bipolar output transistor. The oscillator generates a 1.2MHz clock. The clock's two functions are to trigger the PWM generator that turns on the output transistor and to reset 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.
The gm error amplifier measures the LED current through the external sense resistor and amplifies the error between the detected signal and the internal 250mV reference voltage. 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, thus maintaining the constant frequency current-mode PWM control. The LED current is set by the feedback resistor: 250mV ILED = R FB The enable pin shuts down the internal power bipolar transistor and disables the control circuitry to reduce input current-to-leakage levels. Enable pin input current is zero at zero volts.
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MIC3287
Micrel, Inc.
External Component Selection
The table below shows recommended inductor and output capacitor combinations for optimal performance. Series LEDs 2 L 22H 15H 10H 6.8H 4.7H 3 22H 15H 10H 6.8H 4.7H 4 22H 15H 10H 6.8H 4.7H Min COUT 2.2F 1F 0.22F 0.22F 0.22F 2.2F 1F 0.22F 0.22F 0.27F 1F 1F 0.22F 0.27F 0.27F
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MIC3287 Dimming Control There are two techniques for dimming control. One is PWM dimming, and the other is continuous dimming. 1. PWM dimming control is implemented by applying a PWM signal to the EN pin, as shown in Figure 1. The MIC3287 is turned on and off by the PWM signal. With this method, the LEDs operate with either zero or full current. The average LED current is increased proportionally to the duty-cycle of the PWM signal. This technique has high-efficiency because the IC and the LEDs consume no current during the off time of the PWM signal. Typical PWM frequency should be between 100Hz and 10kHz. 2. Continuous dimming control is implemented by applying a DC control voltage to the FB pin of the MIC3287 through a series resistor, as shown in Figure 2. The LED intensity (current) can be dynamically varied with the applied DC control voltage. The DC voltage can come from a DAC signal, or a filtered PWM signal. The advantage of the filtered PWM signal approach is a high frequency PWM signal (>10kHz) can be used to control LED intensity (current).
VIN
Micrel, Inc. Open-Circuit Protection If the LEDs are disconnected from the circuit, or in the case that an LED fails, the sense resistor will then pull the FB pin to ground. This will cause the MIC3287 to switch with a high dutycycle, resulting in output over-voltage. This may cause the SW pin voltage to exceed its maximum voltage rating, damaging the IC and the external components. To ensure the highest level of protection, the MIC3287 has buit-in 24V over-voltage protection functionality in the TSOT-23-6 pin and 8-pin MLFTM package options. The OVP will clamp the output voltage to within the specified limits. Start-Up and Inrush Current If the inrush current needs to be limited, a soft-start circuit, similar to Figure 3, could be implemented. The soft-start capacitor, CSS, provides overdrive to the FB pin at start-up, resulting in gradual increase of switch duty cycle and limited in rush current.
VIN
CSS 2200pF VIN EN GND SW FB R 10k
VIN
SW FB GND
Figure 3. Soft-Start Circuit
PWM
EN
Figure 1. PWM Dimming Method
VIN
VIN EN
SW FB GND 5.11k 49.9k
DC Equivalent
Figure 2. Continuous Dimming
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MIC3287
Micrel, Inc.
Package Information
5-Pin TSOT (YD5)
6-Pin TSOT (YD6)
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MIC3287
Micrel, Inc.
8-Pin 2mm x 2mm MLFTM (ML)
MICREL INC. 2180 FORTUNE DRIVE
TEL
+ 1 (408) 944-0800 FAX + 1 (408) 474-1000 WEB http://www.micrel.com
SAN JOSE, CA 95131
USA
This 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) 2006 Micrel, Inc.
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M9999-010306

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