LT1932ES6 查看數據表（PDF） - Linear Technology

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LT1932ES6

Constant-Current DC/DC LED Driver in ThinSOT

Linear Technology 

LT1932

APPLICATIO S I FOR ATIO

Open-Circuit Protection

For applications where the string of LEDs can be discon-

nected or could potentially become an open circuit, a zener

diode can be added across the LEDs to protect the LT1932

(see Figure 4). If the device is turned on without the LEDs

present, no current feedback signal is provided to the LED

pin. The LT1932 will then switch at its maximum duty

cycle, generating an output voltage 10 to 15 times greater

than the input voltage. Without the zener, the SW pin could

see more than 36V and exceed its maximum rating. The

zener voltage should be larger than the maximum forward

voltage of the LED string.

L1

6.8µH

D1

VIN

C1

4.7µF

6

1

VIN

SW

LT1932

5

SHDN

3

LED

RSET

GND

15mA

4

2

RSET

1.50k

24V

C2

1µF

1932 F04

Figure 4. LED Driver with Open-Circuit Protection

Dimming Using a PWM Signal

PWM brightness control provides the widest dimming

range (greater than 20:1) by pulsing the LEDs on and off

using the control signal. The LEDs operate at either zero or

full current, but their average current changes with the

PWM signal duty cycle. Typically, a 5kHz to 40kHz PWM

signal is used. PWM dimming with the LT1932 can be

accomplished two different ways (see Figure 6). The

SHDN pin can be driven directly or a resistor can be added

to drive the RSET pin.

If the SHDN pin is used, increasing the duty cycle will

increase the LED brightness. Using this method, the LEDs

can be dimmed and turned off completely using the same

control signal. A 0% duty cycle signal will turn off the

LT1932, reducing the total quiescent current to zero.

If the RSET pin is used, increasing the duty cycle will

decrease the brightness. Using this method, the LEDs are

dimmed using RSET and turned off completely using

SHDN. If the RSET pin is used to provide PWM dimming,

the approximate value of RPWM should be (where VMAX is

the “high” value of the PWM signal):

RPWM

=

RSET

•



VMAX

0.15V

–

1

In addition to providing the widest dimming range, PWM

brightness control also ensures the “purest” white LED

color over the entire dimming range. The true color of a

white LED changes with operating current, and is the

“purest” white at a specific forward current, usually 15mA

or 20mA. If the LED current is less than or more than this

value, the emitted light becomes more blue. For color

LCDs, this often results in a noticeable and undesirable

blue tint to the display.

When a PWM control signal is used to drive the SHDN pin

of the LT1932 (see Figure 6), the LEDs are turned off and

on at the PWM frequency. The current through them

alternates between full current and zero current, so the

average current changes with duty cycle. This ensures

that when the LEDs are on, they can be driven at the

appropriate current to give the purest white light. Figure

5 shows the LED current when a 5kHz PWM dimming

control signal is used with the LT1932. The LED current

waveform cleanly tracks the PWM control signal with no

delays, so the LED brightness varies linearly with the

PWM duty cycle.

VPWM

2V/DIV

ILED

10mA/DIV

50µs/DIV

1932 F05

Figure 5. PWM Dimming Using the SHDN Pin

1932f

7

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