LT3587 データシートの表示（PDF） - Linear Technology

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LT3587

High Voltage Monolithic Inverter and Dual Boost

Linear Technology 

LT3587

APPLICATIONS INFORMATION

Besides acting as a fault output indicator, the Fault pin

is also an input pin. If this pin is externally forced low

below 400mV, the LT3587 behaves as if a fault event has

been detected and all the channels turn off. In order to

turn the part back on, remove the external voltage that

forces the pin low and reset the part. Figure 11 shows the

waveforms when the Fault pin is externally forced low and

the subsequent resetting of the part.

VFLT

5V/DIV

ENSS1/ENSS3

5V/DIV

VVOUT1

10V/DIV

VNEG

10V/DIV

VVOUT3

20V/DIV

FLT FORCED LOW

PART RESET

100ms/DIV

3587 F11

Figure 11. Waveforms When the

Fault Pin is Externally Forced Low

Dimming Control For Boost3 Current Regulator as an

LED Driver

As shown on the front page application and the Block Dia-

gram, one of the most common applications for the Boost3

channel when configured as a boost current regulator is

a backlight LED driver. In an LED driver application, there

are two different ways to implement a dimming control of

the LED string. The LED current can be adjusted either by

using a digital to analog converter (DAC) with a resistor

RIFB3 or by using a PWM signal.

Using a DAC and a Resistor

For some applications, the preferred method of brightness

control is using a DAC and a resistor. The Boost3 configura-

tion for using this method is shown in Figure 12.

Since the programmed VOUT3 current is proportional to

the current through RIFB3, the LED current can be adjusted

according to the following formula:

IVOUT3 = (0.8V – VDAC-OUT) • 200/RIFB3

A higher DAC output voltage level results in lower LED

current and hence lower overall brightness. Conversely,

a lower DAC output voltage results in higher LED current

and higher brightness. Note that the DAC output impedance

should be low enough to be able to sink approximately

1/200 of the desired maximum LED current without any

appreciable error for accurate dimming control.

Note also that the maximum output current is limited by

the output disconnect current limit to 110mA (typ).

PWM Dimming

Changing the forward current flowing in the LEDs not

only changes the brightness intensity of the LEDs, it also

changes the color. The chromaticity of the LEDs changes

with the change in forward current. Many applications

cannot tolerate any shift in the color of the LEDs. Control-

ling the intensity of the LEDs with a direct PWM signal

allows dimming of the LEDs without changing the color.

In addition, direct PWM dimming offers a wider dimming

range to the user.

VVIN

2.5V TO 5V

10μH

VIN

SW3

CAP3

LT3587

VOUT3

IFB3

EN/SS3

DAC

LTC2630

RIFB3

8.06k

VDAC-OUT

1μF

LED DRIVER

3587 F12

Figure 12. Dimming Using a DAC and a Resistor

3587fc

16

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