Datasheet

8
LT1932
Dimming Using a Filtered PWM Signal
While the direct PWM method provides the widest dim-
ming range and the purest white light output, it causes the
LT1932 to enter into Burst Mode
TM
operation. This opera-
tion may be undesirable for some systems, as it may
reflect some noise to the input source at the PWM fre-
quency. The solution is to filter the control signal by adding
a 10k resistor and a 0.1µF capacitor as shown in Figure 6,
converting the PWM to a DC level before it reaches the
R
SET
pin. The 10k resistor minimizes the capacitance seen
by the R
SET
pin.
Dimming Using a Logic Signal
For applications that need to adjust the LED brightness in
discrete steps, a logic signal can be used as shown in
Figure 6. R
MIN
sets the minimum LED current value (when
the NMOS is off):
R
V
I
MIN
LED MIN
=






225
01
•
.
()
R
INCR
sets how much the LED current is increased when
the NMOS is turned on:
R
V
I
INCR
LED INCREASE
=






225
01
•
.
()
APPLICATIO S I FOR ATIO
WUUU
Figure 6. Five Methods of LED Dimming
Dimming Using a DC Voltage
For some applications, the preferred method of brightness
control uses a variable DC voltage to adjust the LED
current. As the DC voltage is increased, current flows
through R
ADJ
into R
SET
, reducing the current flowing out
of the R
SET
pin, thus reducing the LED current. Choose the
R
ADJ
value as shown below where V
MAX
is the maximum
DC control voltage, I
LED(MAX)
is the current programmed
by R
SET
, and I
LED(MIN)
is the minimum value of I
LED
(when
the DC control voltage is at V
MAX
).
R
VV
II
ADJ
MAX
LED MAX LED MIN
=






225
01
•
–.
–
() ()
Regulating LED Current when V
IN
> V
OUT
The LT1932 contains special circuitry that enables it to
regulate the LED current even when the input voltage is
higher than the output voltage. When V
IN
is less than V
OUT
,
the internal NPN LED switch (transistor Q2 in Figure 1) is
saturated to provide a lower power loss. When V
IN
is
greater than V
OUT
, the NPN LED switch comes out of
saturation to keep the LED current in regulation.
LT1932
R
SET
V
DC
R
SET
4
1932 F06
R
ADJ
LT1932
R
SET
PWM
LT1932
PWM FILTERED PWM DC VOLTAGE LOGIC
R
SET
PWM
R
SET
4
R
PWM
LT1932
PWM
SHDN
PWM
5
R
SET
0.1µF
4
10k
R
PWM
LT1932
R
SET
R
MIN
LOGIC
SIGNAL
4
R
INCR
Burst Mode is a trademark of Linear Technology Corporation.