DN315 - White LED Driver in Tiny SC70 Package Delivers High Efficiency and Uniform LED Brightness

White LED Driver in Tiny SC70 Package Delivers High Efficiency
and Uniform LED Brightness – Design Note 315
David Kim
Introduction
The LT®1937 step-up white LED driver is an ideal solution for small battery-powered portable devices such as
cellular phones, PDAs and digital cameras. The LT1937
features an internal 36V switch that is capable of driving up to eight LEDs in series, but it is optimized for
Li-Ion powered color display backlight applications that
use two to four white LEDs. The LT1937 guarantees a
constant light intensity and color in each LED, regardless of differences in their forward voltage drops due
to a constant current step-up architecture that directly
regulates the LED current. The constant 1.2MHz switching allows for the use of tiny external components and
minimizes input and output ripple voltage—meeting
the noise level requirements of products with sensitive
wireless circuitry. The superior internal compensation
of LT1937 lowers the output capacitor requirement to a
single 0.22μF ceramic, saving space and cost. The low
95mV feedback voltage and an efficient internal switch
minimize power losses in the LT1937. The result is a
typical efficiency of 84%. The LT1937 is available in the
tiny SC70 or 1mm tall ThinSOT™ package.
Li-Ion Powered Driver for Three White LEDs
Figure 1 shows a white LED driver circuit that is
intended for small wireless devices. The constant current step-up series LED architecture of this circuit has
much better efficiency than the alternative switched
capacitor based parallel LED architecture. The circuit
is designed to provide 15mA of constant current to
L1
22μH
SW
SHDN
GND
R1 (Ω)
5
19.1
10
9.53
12
7.87
15
6.34
20
4.75
Easy Dimming Control
The brightness of the LED can be adjusted using a PWM
signal, a filtered PWM signal, a logic signal or a DC
voltage. The brightness control using the PWM signal
to SHDN pin and PWM dimming waveforms are shown
in Figure 2. With the PWM signal applied to the SHDN
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and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
Conversion Efficiency
90
85
C2
0.22μF
LED 1
LT1937
OFF ON
ILED (mA)
D1
C1
1μF
VIN
Table 1. R1 Resistor Value Selection
EFFICIENCY (%)
VIN
3V TO 5V
drive three LEDs in series from a Li-Ion battery or 5V
adapter input. The 1.2MHz constant frequency and
superior internal compensation results in 0603 size
ceramic input and output capacitors and a tiny ferrite
core inductor (a chip inductor can be used to save even
more space). The constant LED current is set with the
R1 resistor at the feedback pin. By using a simple LED
current calculation, ILED = 95mV/R1 or R1 = 95mV/ILED,
a resistor value selection table is easily calculated (see
Table 1). For accurate LED current, high precision (1%)
resistors are needed.
15mA
LED 2
LED 3
FB
R1
6.34Ω
DN315 F01a
C1, C2: X5R OR X7R DIELECTRIC
D1: CENTRAL SEMICONDUCTOR CMDSH-3
L1: MURATA LQH3C-220 OR EQUIVALENT
80
VIN = 3V
70
65
60
0
Figure 1. Li-Ion Powered Driver for Three White LEDs
07/03/315_conv
VIN = 3.6V
75
5
10
15
LED CURRENT (mA)
20
DN315 F01b
DC Voltage Dimming
LT1937
SHDN
LT1937
DN315 F02a
FB
PWM
R3
91k
1kHz PWM Signal
R2
5.1k
VDC
R1
6.34Ω
FB
100mV/DIV
DN315 F03a
Filtered PWM Dimming
SHDN
2V/DIV
LT1937
FB
200μs/DIV
DN315 F02b
10k
10kHz PWM Signal
R2
5.1k
R3
91k
PWM
R1
6.34Ω
0.1μF
FB
100mV/DIV
DN315 F03b
Logic Signal Dimming
SHDN
2V/DIV
LT1937
FB
20μs/DIV
DN315 F02c
RINC
Figure 2. PWM Dimming Control Using the SHDN Pin
pin, the LT1937 is turned on or off by this signal. The
average LED current increases proportionally with the
duty cycle of the PWM signal, where 0% duty cycle sets
the LED current to zero and a 100% duty cycle sets it to
full current. The typical frequency range recommended
for PWM dimming is for a 1kHz to 10kHz signal with at
least a 1.5V amplitude.
Figure 3 shows alternative LED brightness control
methods using a DC voltage, filtered PWM signal and
logic signal. The DC voltage dimming control shown
in Figure 3 is designed to control LED current from
0mA to 15mA using the 0V to 2V DC voltage at the
VDC input. As the voltage at the VDC input increases,
the voltage drop on R2 increases and voltage drop on
R1 decreases, resulting in a decrease of LED current.
The filtered PWM dimming works the same way except
that the VDC input now comes from a filtered PWM
signal. The 10k, 0.1μF RC filters the PWM signal so
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LOGIC
SIGNAL
2N7002
R1
DN315 F03c
Figure 3. Dimming Control Methods
that it is close to DC and the duty cycle of the PWM
signal changes the DC voltage level. The LED current
can also be adjusted in discrete steps using a logic
signal dimming method shown in Figure 3. R1 sets the
minimum LED current when the NMOS is off and RINC
increases LED current by reducing the resistor value
when the NMOS is turned on.
Conclusion
The LT1937 is a white LED driver optimized for driving
two to four LEDs from a Li-Ion battery input. With its
36V, 1.2MHz internal switch and superior internal compensation, the LT1937 is well suited for small wireless
devices requiring very small circuit size, high efficiency
and uniform LED brightness.
For applications help,
call (408) 432-1900
dn315f_conv LT/TP 0703 351.5K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2003
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