Datasheet

8
LT1932
1932f
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
®
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
•
.
()
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
APPLICATIO S I FOR ATIO
WUUU
Figure 6. Five Methods of LED Dimming
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.
Soft-Start/Controlling Inrush Current
For many applications, it is necessary to minimize the
inrush current at start-up. When first turned on and the
LED current is zero, the LT1932 will initially command the
maximum switch current of 500mA to 600mA, which may
give an inrush current too high for some applications. A
soft-start circuit (Figure 7) can be added to significantly
reduce the start-up current spike. Figure 8 shows that
without soft-start the input current reaches almost 600mA.
Figure 9 shows that when the soft-start circuit is added,
the input current has only a brief 300mA spike, and on
average does not exceed 100mA.
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 registered trademark of Linear Technology Corporation.