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LX1744

Dual Output Boost – LED Driver / LCD Bias

Microsemi Corporation 

INTEGRATED PRODUCTS

LX1744

Dual Output Boost – LED Driver / LCD Bias

PRODUCTION DATASHEET

APPLICATION NOTE

less than 100kHz, an external filter capacitor will be

needed (Figure 5). The value of CPWM is easily calculated

based on the PWM frequency and RPWM_1 using the

following equation.

CPWM

=

50

π ⋅ fPWM ⋅ RPWM _1

eq. 3

where

RPWM _1 << 2.5MΩ

eq. 4

LED DRIVER – OUTPUT CURRENT PROGRAMMING

Maximum LED current is easily programmed by

choosing the appropriate value for RLED (Figure 6). It is

recommended that a minimum value of 15Ω be used for

this resistor in order to prevent noise coupling issues on the

feedback line. Although, alternate values can be calculated

using the following equation:

300mV (VBRT) be used in order to minimize dissipative

losses in the LED current sense resistor (RLED).

Like the LCD bias adjustment (ADJ) pin, the BRT pin is

connected to an internal 50pF capacitor to ground that

works with an external resistor to create a low-pass filter,

allowing the BRT pin to driven directly by a PWM signal

whose frequency is greater than 100kHz. When this pin is

driven by a PWM signal whose frequency is less than

100kHz, an external filter capacitor is needed. This

capacitor is selected such that the ripple component of the

resultant voltage on the BRT pin is less than 10% of the

nominal input voltage.

For PWM frequencies greater than 100kHz, the external

BRT input resistor is calculated using the following

equation.

RBRT _1

=

2.5MΩ

⋅

⎜⎜⎝⎛

VPWM (DCMAX ) − VBRT(MAX)

VBRT(MAX)

⎟⎟⎠⎞

eq. 6

VBAT = 1.6V to 6.0V

VBRT(MAX)

R = LED

ILED(MAX)

L1

DRV

SRC

OVP

D1

ROVP_1

ROVP_2

C1

4.7µF

where VBRT is the selected maximum LED current sense

eq. 5 feedback threshold.

For PWM frequencies less than 100kHz, the external

BRT input resistors and filter capacitor (Figure 4) are

calculated using the following equations.

RBRT _1

=

RBRT _ 2

⋅

⎜⎜⎝⎛

VPWM (DCMAX ) − VBRT(MAX)

VBRT(MAX)

⎟⎟⎠⎞

eq. 7

LFB

LX1744

CS

BRT

RCS

RBRT_1

CBRT

RBRT_2

where RBRT_2 is selected and VBRT(MAX) is the selected

maximum LED current sense feedback threshold.

RLED

15Ω

CBRT

=

5

π ⋅ fPWM

⋅

⎜⎜⎝⎛

RBRT _1 + RBRT _ 2

RBRT _1 ⋅ RBRT _ 2

⎟⎟⎠⎞

eq. 8

Figure 6 – LED Current Programming

LED DRIVER – LED BRIGHTNESS ADJUSTMENT

The LX1744 features a full range dimming LED driver.

LED current regulation is accomplished by using the

applied BRT pin voltage as the LED current reference.

This reference voltage, in conjunction with the LED current

setting resistor (RLED), sets the LED output current.

Dimming can be accomplished in one of two ways: by

applying a variable DC voltage, or by varying the duty

cycle (DC) of a PWM control signal, directly to the BRT

pin.

It is recommended that a maximum signal voltage of

where VRIPPLE is selected to be 10% of VBRT, and fPWM is

the PWM signal frequency.

DIODE SELECTION

A Schottky diode is recommended for most applications

(e.g. Microsemi UPS5817). The low forward voltage drop

and fast recovery time associated with this device supports

the switching demands associated with this circuit

topology. The designer is encouraged to consider the

diode’s average and peak current ratings with respect to the

application’s output and peak inductor current

requirements. Further, the diode’s reverse breakdown

voltage characteristic must be capable of withstanding a

Copyright © 2000

Rev. 1.1b, 2005-03-01

Microsemi

Integrated Products Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 8

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