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Ultra-Small Inductorless Regulator Drives White LED Backlights(1).pdf

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ideas for design
INNOVATIVE DESIGNS FROM READERS
Ultra-Small Inductorless Regulator
Drives White LED Backlights
David Kim
Linear Technology Corp., 1630 McCarthy Blvd., Milpitas, CA 95035; (408) 432-1900
CIRCLE 521
H
igh-intensity white LEDs are
ideal for backlighting of small,
color LCD displays used in
handheld devices such as cellular
phones and PDAs. When compared to
other backlight solutions, white LEDs
provide higher reliability and lower
power consumption.
But portable devices powered by a
single Li-ion cell require a step-up volt-
age conversion to drive white LEDs
over the entire voltage range of the Li-
ion cell (2.7 to 4.2 V). This is because
the forward voltage drop of a typical
white LED is 3.6 to 4 V. Shown here is a
constant-current backlight supply
using the LTC3200 inductorless regula-
tor to drive four white LEDs.
The LTC3200’s inductorless architec-
ture and 2-MHz switching frequency
result in a small circuit size—less than
1.1 mm in height and 28 mm
2
in area.
The constant-current source illustrated
in the figure accurately controls the
June 18, 2001 • ELECTRONIC DESIGN
101
IDEAS FOR DESIGN
R4
50k
Brightness control
PWM input (200 Hz to 2 KHz)
Q1
2N7002LT1
1
C1
0.22 F
2
LTC3200EMS8
8
VOUT
C+
D1
VIN
FB
7
D2
D3
D4
LED current regardless of variations in
the forward voltage drop of the LEDs.
This results in constant light output
from different white LEDs.
The LED current is programmed via
the R
PROGRAM
resistor. Using the feed-
back voltage of the LTC3200 (V
FEED-
BACK
= 1.4 V), the LED current is regu-
lated according to the formula:
I
LED
= V
FEEDBACK
/R
PROGRAM
In this circuit, an 86.6- resistor is
used to set the LED current at 16 mA.
To maintain the same current in each
LED, the paralleled LEDs also are bal-
lasted with 86.6- resistors.
The brightness of the LEDs can be
adjusted by changing the duty cycle of
the PWM signal at the external MOSFET
connected to the shutdown pin. A lower
duty cycle will result in lower bright-
ness, whereas a higher duty cycle will
result in higher brightness. For the best
results, a PWM signal in the range of
200 Hz to 1 kHz should be used.
2.7 to 4.2 V
Input
C3
1 F
3 C–
SHDN 6
C2
1 F
RPROGRAM
86.6
R1
86.6
R2
86.6
R3
86.6
4
PGND
SGND
5
D1 to D4 = Nichia NSCW100
C1 = Taiyo Yuden JMK107BJ224MA
C2, C3 = Taiyo Yuden JMK107BJ105MA
With this inductorless step-up circuit, constant-current output control drives white LEDs
regardless of the magnitude or variations in their forward-voltage characteristics.
102
ELECTRONIC DESIGN • June 18, 2001