Datasheet
7
LT1932
Open-Circuit Protection
For applications where the string of LEDs can be discon-
nected or could potentially become an open circuit, a zener
diode can be added across the LEDs to protect the LT1932
(see Figure 4). If the device is turned on without the LEDs
present, no current feedback signal is provided to the LED
pin. The LT1932 will then switch at its maximum duty
cycle, generating an output voltage 10 to 15 times greater
than the input voltage. Without the zener, the SW pin could
see more than 36V and exceed its maximum rating. The
zener voltage should be larger than the maximum forward
voltage of the LED string.
If the R
SET
pin is used, increasing the duty cycle will
decrease the brightness. Using this method, the LEDs are
dimmed using R
SET
and turned off completely using
SHDN. If the R
SET
pin is used to provide PWM dimming,
the approximate value of R
PWM
should be (where V
MAX
is
the “high” value of the PWM signal):
RR
V
V
PWM SET
MAX
=
•
.
–
015
1
In addition to providing the widest dimming range, PWM
brightness control also ensures the “purest” white LED
color over the entire dimming range. The true color of a
white LED changes with operating current, and is the
“purest” white at a specific forward current, usually 15mA
or 20mA. If the LED current is less than or more than this
value, the emitted light becomes more blue. For color
LCDs, this often results in a noticeable and undesirable
blue tint to the display.
When a PWM control signal is used to drive the SHDN
pin
of the LT1932 (see Figure 6), the LEDs are turned off and
on at the PWM frequency. The current through them
alternates between full current and zero current, so the
average current changes with duty cycle. This ensures
that when the LEDs are on, they can be driven at the
appropriate current to give the purest white light. Figure
5 shows the LED current when a 5kHz PWM dimming
control signal is used with the LT1932. The LED current
waveform cleanly tracks the PWM control signal with no
delays, so the LED brightness varies linearly with the
PWM duty cycle.
V
IN
SW
D1
15mA
1932 F04
L1
6.8µH
V
IN
LT1932
SHDN
61
4
R
SET
1.50k
C1
4.7µF
C2
1µF
24V
2
35
LED
R
SET
GND
Figure 4. LED Driver with Open-Circuit Protection
Dimming Using a PWM Signal
PWM brightness control provides the widest dimming
range (greater than 20:1) by pulsing the LEDs on and off
using the control signal. The LEDs operate at either zero or
full current, but their average current changes with the
PWM signal duty cycle. Typically, a 5kHz to 40kHz PWM
signal is used. PWM dimming with the LT1932 can be
accomplished two different ways (see Figure 6). The
SHDN pin can be driven directly or a resistor can be added
to drive the R
SET
pin.
If the SHDN pin is used, increasing the duty cycle will
increase the LED brightness. Using this method, the LEDs
can be dimmed and turned off completely using the same
control signal. A 0% duty cycle signal will turn off the
LT1932, reducing the total quiescent current to zero.
V
PWM
2V/DIV
I
LED
10mA/DIV
50µs/DIV 1932 F05
Figure 5. PWM Dimming Using the SHDN Pin
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