Aug 2002 White LED Driver in Tiny SC70 Package Achieves 84% Efficiency

DESIGN IDEAS
White LED Driver in Tiny SC70 Package
Achieves 84% Efficiency
by Pit-Leong Wong
Introduction
VIN
3V–5V
L
22µH
D
90
CIN
1µF
VDC
DIMMING
VIN
85
SW
91k
5.1k
LT1937
SHDN
FB
R1
4.02Ω
GND
CIN: TAIYO YUDEN JMK107BJ105
COUT: AVX 0603YD224
D: CENTRAL CMDSH-3
L: MURATA LQH3C220
(408) 573-4150
(843) 946-0362
(516) 435-1110
(814) 237-1431
LT1937
FB
D2
Figure 1. Li-Ion driver for
three or four white LEDs
5k
The most common application for the
LT1937 is to drive three or four white
LEDs from a single Li-Ion cell. The
circuit and the efficiency curve for
three and four white LEDs are shown
in Figure 1 and Figure 2, respectively.
The efficiencies remain above 80% for
most of the LED current range. In
these particular circuits, dimming is
controlled by a DC voltage. Dimming
can also be controlled by logic signals
and PWM signals. Consult the LT1937
SHDN
2V/DIV
VIN = 3.6V
THREE LEDs
15mA
Figure 3. External soft-start circuit
50µs/DIV
65
4-LED CIRCUIT
VIN = 3.6V
0
5
10
15
LED CURRENT (mA)
20
data sheet for more about LED dimming control. Figure 1 shows just
how simple it is to create a complete
LED driver solution with the LT1937
and a few external components.
Soft-Start Circuit
To minimize startup delay, no internal soft-startup circuit is included in
the LT1937, but it can be implemented with just a few additional
external components. Figure 3 shows
the components needed for a softstart LED driver that minimizes the
startup inrush current. The switching waveforms in Figure 4 show the
limited inrush current during startup.
The LT1937 provides constant current for LEDs, at efficiencies up to
84%, in an extremely compact and
easy to use package.
Figure 4. Soft-start waveforms
For more information on parts featured in this issue, see
http://www.linear.com/go/ltmag
Linear Technology Magazine • August 2002
70
3-LED
CIRCUIT
VIN = 3.0V
Conclusion
COUT
D2: MMBT
4-LED
CIRCUIT
VIN = 3.0V
Li-Ion Driver for Three or
Four White LEDs
FB
100mV/DIV
R1
6.34Ω
75
Figure 2. Efficiency of the circuit
in Figure 1
IIN
100mA/DIV
R2
1k
80
60
D1
2.2nF
3-LED CIRCUIT
VIN = 3.6V
COUT
0.22µF
EFFICIENCY (%)
The LT1937 step-up DC/DC converter
is designed to efficiently drive white
LED backlights while fitting into tight
spaces. The device, available in SC70
and ThinSOT™ packages, provides a
conversion efficiency of about 84%
when driving 3 LEDs from a Li-Ion
cell. The 1.2MHz switching frequency
of the LT1937 allows the use of tiny
external components. For instance, a
3-LED circuit requires only 0.22µF of
output capacitance and only 1µF of
input capacitance. An entire LED
driver solution can fit into a space
under 28mm2.
Series drive of the LEDs also produces matched light output from each
LED independent of the variable LED
forward voltage drop, VF.
The LED current is set at the FB
pin of the LT1937. A built-in 95mV
reference voltage minimizes power loss
in the current-setting resistor, increasing efficiency. Dimming is also
easily added with two additional resistors. The input range of 2.5V to
10V, and the internal switch voltage
rating of 36V, allow the device to drive
two to four LEDs from a Li-Ion cell
with plenty of voltage margin.
for
the latest information
on LTC products,
visit
www.linear.com
29
Similar pages