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LT1932
APPLICATIO S I FOR ATIO
R ADJ = 225 ? ? ?
R MIN = 225 ? ?
? I LED ( MIN ) ?
R INCR = 225 ? ? ?
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 0.1 μ F 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):
? 0 . 1 V ?
?
R INCR sets how much the LED current is increased when
the NMOS is turned on:
? 0 . 1 V ?
? I LED ( INCREASE ) ?
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 ).
? V MAX – 0 . 1 V ?
? I LED ( MAX ) – I LED ( MIN ) ?
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.
Burst Mode is a registered trademark of Linear Technology Corporation.
LT1932
SHDN
LT1932
R SET
LT1932
R SET
LT1932
R SET
LT1932
R SET
5
4
R PWM
4
10k
R PWM
4
R ADJ
4
R INCR
PWM
PWM
PWM
V DC
R SET
R SET
0.1 μ F
R SET
R MIN
LOGIC
SIGNAL
1932 F06
PWM
PWM
FILTERED PWM
DC VOLTAGE
LOGIC
Figure 6. Five Methods of LED Dimming
1932f
8
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LT1932ES6#TRMPBF (CUT REEL)
制造商:Linear Technology 功能描述:
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