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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 0.1

µF capacitor as shown in Figure 6,

converting the PWM to a DC level before it reaches the

RSET pin. The 10k resistor minimizes the capacitance seen

by the RSET 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. RMIN sets the minimum LED current value (when

the NMOS is off):

MIN

LED MIN









225

•

()

RINCR sets how much the LED current is increased when

the NMOS is turned on:

INCR

LED INCREASE









225

•

()

APPLICATIO S I FOR ATIO

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 RADJ into RSET, reducing the current flowing out

of the RSET pin, thus reducing the LED current. Choose the

RADJ value as shown below where VMAX is the maximum

DC control voltage, ILED(MAX) is the current programmed

by RSET, and ILED(MIN) is the minimum value of ILED (when

the DC control voltage is at VMAX).

ADJ

MAX

LED MAX

LED MIN









225

•

–.

–

()

(

)

Regulating LED Current when VIN > VOUT

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 VIN is less than VOUT,

the internal NPN LED switch (transistor Q2 in Figure 1) is

saturated to provide a lower power loss. When VIN is

greater than VOUT, the NPN LED switch comes out of

saturation to keep the LED current in regulation.

LT1932

RSET

VDC

RSET

1932 F06

RADJ

LT1932

RSET

PWM

LT1932

PWM

FILTERED PWM

DC VOLTAGE

LOGIC

RSET

PWM

RSET

RPWM

LT1932

PWM

SHDN

PWM

RSET

0.1

µF

10k

RPWM

LT1932

RSET

RMIN

LOGIC

SIGNAL

RINCR

Burst Mode is a trademark of Linear Technology Corporation.

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