Information

17
Portable Power Conversion Design Guide
Backlighting Solutions
Switching Regulators
Switching Regulators
Efficient backlighting in portable applications presents a number of unusual power supply design challenges. Like all
portable circuits as in all portable designs, board area is at a premium and minimizing power consumption is essential for
maximizing battery run time. Commonly, LED forward voltages are unique potentials within the circuit; they may be higher
or lower than the battery voltage, and different from the voltages needed to power the microcontrollers and peripheral
components. In addition, since the current flow through LEDs is exponentially dependent on voltage, brightness control and
LED longevity both require accurate, measured and controllable current delivery.
LED Backlighting with MCP1643: Synchronous Boost Constant Current Regulator
MCP1643 is a compact, high-efficiency, fixed-frequency, synchronous step-up LED driver with constant current that can
operate from one- and two-cell alkaline, NiMH and NiCd batteries. With an output of up to 5V, the device can drive a single
white or blue LED; or pairs of red, green, and yellow LEDs connected in series. In addition, the 550 mA load current
capability is enough to drive high-brightness LEDs with high current requirements, or sets of matched LEDs connected in
parallel. Current is read from the voltage on a low-impedance sense resistor connected to the feedback pin, with minimal
power losses compared to other current measurement techniques. For visible blinking or dimming, the enable pin can be
toggled with a PWM signal to switch the power supply and LED load on and off. (At frequencies approximately 50 Hz or
faster the blinking light should appear dimmed, and not visibly flickering.) Finally, the over-voltage protection feature limits
the output to 5V, protecting the power circuit in the event of an open circuit or load failure.
MCP1643 LED Drive Application with Multiple Parallel LEDs
■ Constant current drive capability with a low component
count, area-efficient circuit
■ 1 MHz PWM synchronous boost operation with up to
550 mA output current
■ Low-voltage reference input to maximize LED efficiency
conversion (VFB = 120 mV)
■ Capable of start up with only 0.65V input, and
continuous operation with an input above 0.5V
■ Low 1.2 µA shutdown current
■ Over-voltage protection halts device operation (floating
output) if the LED fails or is disconnected
■ Up to 90% efficiency
■ 240 µs soft start time
■ Available in tiny footprint 8-lead 2 × 3 DFN or 8-pin
MSOP packages
MCP1643 LED Drive Application with a Single LED
GND
V
IN
EN
SW
V
FB
VOUT
C
OUT
4.7 μF
C
IN
4.7...10 μF
LED
R
SET
4.7 Ω
L
1
4.7 μH
ILED =
ILED = 25 mA
0.12V
R
SET
OFF
ON
MCP1643 LED Output Current as a Function of
Input Voltage and Sense Resistor
0
50
100
150
200
250
300
350
400
450
500
0.6 0.9 1.2 1.5 1.8 2.1 2.4
LED Current (mA)
Input Voltage (V)
R
SET
= 5Ω
R
SET
= 1.2Ω
R
SET
= 0.82Ω
R
SET
= 0.41Ω
R
SET
= 0.25Ω
LED V
F
= 3.5V @ I
F
= 700 mA
GND
V
IN
EN
SW
V
FB
VOUT
C
OUT
4.7 μF
C
IN
4.7...10 μF
WLED1
R
SET
2.4 Ω
L
1
4.7 μH
OFF
ON
Battery input
(One or
Two Cells)
I
LED =
ILED = 50 mA
0.12V
R
SET
WLED2
R
2
2.4 Ω
WLED3
R
3
2.4 Ω
DZ
D
V
Z = 2.4V ILED
2
= 50 mA ILED
3
= 50 mA