LT1932ES6 Ver la hoja de datos (PDF) - Linear Technology

Número de pieza

componentes Descripción

Lista de partido

LT1932ES6

Constant-Current DC/DC LED Driver in ThinSOT

Linear Technology 

LT1932

APPLICATIO S I FOR ATIO

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

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):

RMIN

=

225

•





0.1V

ILED(MIN)





RINCR sets how much the LED current is increased when

the NMOS is turned on:

RINCR

=

225

•





0.1V

ILED(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 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).

RADJ

=

225

•





VMAX

ILED(MAX)

– 0.1V

– ILED(MIN)





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.

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

5

PWM

PWM

8

LT1932

RSET

4 RPWM

RSET

PWM

LT1932

RSET

4 10k

RSET

RPWM

PWM

0.1µF

LT1932

RSET

4 RADJ

VDC

RSET

PWM

FILTERED PWM

DC VOLTAGE

Figure 6. Five Methods of LED Dimming

LT1932

RSET

4 RINCR

RMIN

LOGIC

SIGNAL

LOGIC

1932 F06

1932f

Share Link: