April 2010 1 M9999-042910
Micrel, Inc MIC3287
MIC3287
1.2MHz PWM White LED Driver with OVP in
Thin SOT-23 and 2mm x 2mm MFL®
General Description
The MIC3287 is a 1.2MHz pulse width modulated (PWM)
step-up switching regulator that is optimized for constant cur-
rent white LED driving applications. With a switch current of
over 350mA, the MIC3287-24 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 op-
eration saves board space by reducing external component
sizes. The added benet 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 efciency, 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 prole Thin SOT-23 5-pin
and Thin SOT-23 6-pin packages. The MIC3287 has a junc-
tion temperature range of –40°C to +125°C.
All support documentation can be found on Micrel’s web site
at: www.micrel.com.
Typical Application
10µH
0.22µF
250mV
1-Cell
Li Ion 1µF
2
3
1
4
5
MIC3287YD5
VIN
EN
SW
FB
GND
3-Series White LED Driver in Thin SOT-23-5
Features
• 2.8V to 6.5V Input Voltage
• 350mA Switch Current
• 1.2 MHz PWM Operation
• 250mV Feedback Voltage
• Optional 24V Output Over-Voltage Protection (OVP)
• Up to 36V Output Voltage without OVP
• <1% Line and Load Regulation
• <1µA Shutdown Current
• Over-Temperature Protection
• Under-Voltage Lock Out (UVLO)
• Low Prole Thin SOT-23-5 Package Option
• Low Prole Thin SOT-23-6 Package Option
• 8-pin 2mm x 2mm MLF® Package Option
• –40°C to +125°C 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
Micrel, Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
MLF and MicroLeadFrame are registered trademarks of Amkor Technology, Inc.
10µH
0.22µF
1µF1-Cell
Li Ion
MIC3287-24YD6
VIN
EN
SW
OVP
FB
GND 250mV
5
1
3
2
4
6
4-Series White LED Driver
with Output OVP in Thin SOT-23-6
Micrel, Inc. MIC3287
April 2010 2 M9999-042910
Pin Conguration
FB GND
EN VIN
SW
31
5
2
4
FB GND
EN VIN
SW
31
6
2
4 5
OVP
TSOT-23-5 (YD5) TSOT-23-6 (YD6)
1VIN
OVP
EN
AGND
8 PGND
SW
FB
NC
7
6
5
2
3
4EP
8-Pin 2mm x 2mm MLF™ (YML)
Ordering Information
Part Number
Marking
Code
Overvoltage
Protection
Junction
Temp. Range
Package
Lead Finish
MIC3287YD5 WGAA N/A -40°C to +125°C Thin SOT-23-5 Lead Free
MIC3287-24YD6 WHAA 24V -40°C to +125°C Thin SOT-23-6 Lead Free
MIC3287-24YML WLA 24V -40°C to +125°C 8-Pin 2mm x 2mm MLF Lead Free
Pin Description
Pin Number
TSOT-23-5
Pin Number
TSOT-23-6
Pin Number
8-Pin MLF
Pin Name
Pin Function
1 1 7 SW Switch Node (Input): Internal power bipolar collector.
2 2 4 AGND Analog Ground (Return): Ground.
2 2 8 PGND Power Ground (Return): Ground.
3 3 6FB 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.
4 4 3 EN Enable (Input): Logic high enables regulator. Logic low shuts down regu-
lator.
561 VIN Supply (Input): 2.8V to 6.5V for internal circuitry.
— 5 2 OVP Overvoltage Protection (Input): Connect to the output.
— — 5 NC Not Connected
— — EP EP Exposed Backside Pad: Connect to ground.
April 2010 3 M9999-042910
Micrel, Inc MIC3287
Absolute Maximum Ratings(1)
Supply Voltage (VIN) ........................................................ 7V
Switch Voltage (VSW) ......................................–0.3V to 40V
Enable Pin Voltage (VEN) ................................... –0.3 to VIN
FB Voltage (VFB) ............................................................. 6V
Switch Current (ISW) ..................................................... 1.2A
Ambient Storage Temperature (TS) .......... –65°C to +150°C
ESD Rating(3) ................................................................ 2kV
Operating Ratings(2)
Supply Voltage (VIN) .........................................2.8V to 6.5V
Output Voltage (VOUT) .........................................VIN to 36V
Output Voltage (VOUT)(4) ...................................VIN to VOVP
Junction Temperature Range (TJ) ............ –40°C to +125°C
Package Thermal Impedance
Thin SOT-23-5 (θJA)(5) ....................................... 207°C/W
Thin SOT-23-6 (θJA)(5) ....................................... 134°C/W
8-Pin MLF (θJA) ................................................... 90°C/W
Electrical Characteristics(6)
TA = 25°C, VIN = VEN = 3.6V, VOUT = 10V, IOUT = 10mA, unless otherwise noted. Bold values indicate –40°C ≤ TJ ≤125°C.
Symbol Parameter Condition Min Typ Max Units
VIN Supply Voltage Range 2.8 6.5 V
VUVLO Under Voltage Lockout 1.8 2.1 2.4 V
IVIN Quiescent Current VFB > 500mV, (not switching) 2.1 5 mA
ISD Shutdown Current VEN = 0V(7) 0.04 1 µA
VFB Feedback Voltage (±7.5%) 231 250 269 mV
IFB Feedback Input Current VFB = 250mV –450 nA
Line Regulation 3V ≤ VIN ≤ 5V(8) 0.4 1 %
Load Regulation 5mA ≤ IOUT ≤ 20mA(8) 0.5 %
DMAX Maximum Duty Cycle 85 90 %
ISW Switch Current Limit VIN = 3.6V 350 mA
VSW Switch Saturation Voltage VIN = 3.6V, ISW = 300mA 250 mV
ISW Switch Leakage Current VEN = 0V, VSW = 10V 0.01 1 µA
VEN Enable Threshold TURN ON 1.5 V
TURN OFF 0.4 V
IEN Enable Pin Current VEN = 10V 21 40 µA
fSW Oscillator Frequency 1 1.2 1.4 MHz
VOVP Overvoltage Protection MIC3287-24YD6 17 19 24 V
MIC3287-24YML
TJ Overtemperature 150 °C
Threshold Shutdown Hysteresis 10 °C
Notes:
1. Absolute maximum ratings indicate limits beyond which damage to the component may occur. Electrical specications 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, θJA, 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 specied 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. Specication for packaged product only.
7. I
SD = IVIN.
8. Guaranteed by design.
Micrel, Inc. MIC3287
April 2010 4 M9999-042910
Typical Characteristics
45
50
55
60
65
70
75
80
85
2468101214161820
EFFICIENCY (%)
WHITE LED CURRENT (mA)
3 Series
White LED Efficiency
VIN=3.2V
VIN=3.6V
VIN=4.2V
L = 10µH
COUT = 0.22µF
45
50
55
60
65
70
75
80
85
2468101214161820
EFFICIENCY (%)
WHITE LED CURRENT (mA)
4 Series
White LED Efficiency
VIN=3.2V
VIN=3.6V
VIN=4.2V
L = 10µH
COUT = 0.22µF
0
2
4
6
8
10
12
-40
ENABLE PIN CURRENT (µA)
TEMPERATURE (°C)
Enable Pin Current
vs. Temperature
-25 -10 5 20 35 50 65 80
VEN=1.8V
VEN=2.5V
VEN=3.3V
VEN=5V
1.0
1.1
1.2
1.3
1.4
1.5
FREQUENCY (MHz)
Frequency
vs. Temperature
-40
TEMPERATURE (°C)
-25 -10 5 20 35 50 65 80
VIN=3.2V
VIN=3.6V
VIN=4.2V
240
244
248
252
256
260
FEEDBACK VOLTAGE (mV)
Feedback Voltage
vs. Temperature
-40
TEMPERATURE (°C)
-25 -10 5 20 35 50 65 80
VIN=3.6V
248
249
250
251
252
253
254
2.8
FEEDBACK VOLTAGE (mV)
INPUT VOLTAGE (V)
Feedback Voltage
vs. Input Voltage
3.2 3.6 4.0 4.4 4.8 5.2 5.6
2.0
2.1
2.2
2.3
2.4
2.5
2.8
QUIESCENT CURRENT (mA)
Quiescent Current
vs. Input Voltage
INPUT VOLTAGE (V)
3.2 3.6 4.0 4.4 4.8 5.2 5.6
600
650
700
750
800
850
900
CURRENT LIMIT (mA)
Curremt Limit
vs. Temperature
-40
TEMPERATURE (°C)
-25 -10 5 20 35 50 65 80
VIN=3.6V
720
740
760
780
800
820
840
2.8
CURRENT LIMIT (mA)
Current Limit
vs. Input Voltage
INPUT VOLTAGE (V)
3.2 3.6 4.0 4.4 4.8 5.2 5.6
0.001
0.002
0.003
2.8
SHUTDOWN CURRENT (uA)
Shutdown Current
vs. Input Voltage
INPUT VOLTAGE (V)
3.2 3.6 4.0 4.4 4.8 5.2 5.6
April 2010 5 M9999-042910
Micrel, Inc MIC3287
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 amplier, gm er-
ror amplier, 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 com-
pensation ramp generator. The current amplier is used to
measure the switch current by amplifying the voltage signal
from the internal sense resistor. The output of the current
amplier is summed with the output of the slope compensa-
tion ramp generator. This summed current-loop signal is fed
to one of the inputs of the PWM generator.
Functional Diagram
GND
VREF
PWM
Generator
Ramp
Generator
1.2MHz
Oscillator
SW
ENFB OVP*VIN
250mV
g
m
OVP
∑
VREF
Error
Amplifier
Current
Amplifier
* OVP available on TSOP-23-6 and MLF™ package options only.
MIC3287 Block Diagram
The gm error amplier measures the LED current through the
external sense resistor and amplies the error between the
detected signal and the internal 250mV reference voltage.
The output of the gm error amplier 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 main-
taining the constant frequency current-mode PWM control.
The LED current is set by the feedback resistor:
I
LED =
250mV
RFB
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.
Micrel, Inc. MIC3287
April 2010 6 M9999-042910
External Component Selection
The table below shows recommended inductor and output
capacitor combinations for optimal performance.
Series LEDs L Min COUT
2 22µH 2.2µF
15µH 1µF
10µH 0.22µF
6.8µH 0.22µF
4.7µH 0.22µF
3 22µH 2.2µF
15µH 1µF
10µH 0.22µF
6.8µH 0.22µF
4.7µH 0.27µF
4 22µH 1µF
15µH 1µF
10µH 0.22µF
6.8µH 0.27µF
4.7µH 0.27µF
April 2010 7 M9999-042910
Micrel, Inc 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-efciency
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 ltered PWM signal. The advantage
of the ltered PWM signal approach is a high
frequency PWM signal (>10kHz) can be used to
control LED intensity (current).
PWM
VIN
EN
SW
FB
GND
V
IN
Figure 1. PWM Dimming Method
VIN
EN
SW
FB 5.11k
49.9k
GND
DC�
Equivalent
V
IN
Figure 2. Continuous Dimming
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 duty-
cycle, 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 MLF™
package options. The OVP will clamp the output voltage to
within the specied limits.
Start-Up and Inrush Current
If the inrush current needs to be limited, a soft-start circuit, simi-
lar 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.
Figure 3. Soft-Start Circuit
Micrel, Inc. MIC3287
April 2010 8 M9999-042910
Package Information
5-Pin TSOT (YD5)
6-Pin TSOT (YD6)
April 2010 9 M9999-042910
Micrel, Inc MIC3287
MICREL INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA
t e l + 1 (408) 944-0800 f a x + 1 (408) 474-1000 w e b http://www.micrel.com
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 specications at any time without notication 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 signicant 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.
© 2006 Micrel, Inc.
8-Pin 2mm x 2mm MLF® (ML)