LT1932 Linear Technology, LT1932 Datasheet - Page 8

• Manufacturer Part Number: LT1932
• Description: Constant-Current/ Constant-Voltage 1.4MHz Step-Up DC/DC Converter
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
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
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 0.1 F capacitor as shown in Figure 6,
converting the PWM to a DC level before it reaches the
R
by the R
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
the NMOS is off):
R
the NMOS is turned on:
8
SET
INCR
PWM
LT1932
R
R
SHDN
MIN
INCR
pin. The 10k resistor minimizes the capacitance seen
5
sets how much the LED current is increased when
SET
MIN
225
225
pin.
PWM
sets the minimum LED current value (when
•
•
I
LED MIN
I
U
LED INCREASE
0 1
.
(
(
V
0 1
PWM
LT1932
R
.
U
SET
)
4
V
R
SET
R
PWM
TM
)
operation. This opera-
W
PWM
Figure 6. Five Methods of LED Dimming
FILTERED PWM
U
LT1932
R
SET
4
R
SET
10k
R
0.1 F
PWM
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
of the R
R
DC control voltage, I
by R
the DC control voltage is at V
Regulating LED Current when V
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
the internal NPN LED switch (transistor Q2 in Figure 1) is
saturated to provide a lower power loss. When V
greater than V
saturation to keep the LED current in regulation.
Burst Mode is a trademark of Linear Technology Corporation.
ADJ
R
SET
PWM
ADJ
value as shown below where V
SET
, and I
ADJ
DC VOLTAGE
225
pin, thus reducing the LED current. Choose the
LED(MIN)
LT1932
into R
R
OUT
•
SET
4
R
I
SET
, the NPN LED switch comes out of
LED MAX
SET
LED(MAX)
R
ADJ
(
is the minimum value of I
V
, reducing the current flowing out
MAX
)
V
DC
–
– .
MAX
is the current programmed
I
0 1
LED MIN
IN
).
V
(
LOGIC
> V
LT1932
MAX
R
SET
IN
)
4
OUT
R
MIN
is less than V
is the maximum
R
INCR
LED
1932 F06
(when
LOGIC
SIGNAL
IN
OUT
is
,