LX1745

Microsemi

Integrated Products Division

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

Page 8

Triple Output Boost – LED Driver / LCD Bias

Copyright

© 2000

Rev. 1.2, 2006-02-27

TM

®

APPLICATION NOTE

less than 100kHz, an external filter capacitor will be

following equation.

1

_

PWM

PWM

PWM

R

f

50

C

⋅

⋅

π

=

eq. 3

where

Ω

<<

M

5

.

2

R

1

_

PWM

eq. 4

Maximum LED current is easily programmed by

recommended that a minimum value of 15

Ω 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:

)

MAX

(

LED

)

MAX

(

BRTx

LED

I

V

R

=

eq. 5

C

1

4.7μF

L

1

LX1745

LFB

CS

DRV

SRC

R

LED

15Ω

OVP

BRT

D

1

R

OVP_1

R

OVP_2

R

CS

V

C

BRT

R

BRT_2

R

BRT_1

Figure 6 –

LED Current Programming

The LX1745 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

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

Like the LCD bias adjustment (ADJx) pins, 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.

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

−

⋅

Ω

=

)

MAX

(

BRT

)

MAX

(

BRT

MAX

PWM

1

_

BRT

V

V

)

DC

(

V

M

5

.

2

R

eq. 6

feedback threshold.

For PWM frequencies less than 100kHz, the external

BRT input resistors and filter capacitor (Figure 4) are

calculated using the following equations.

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

−

⋅

=

)

MAX

(

BRT

)

MAX

(

BRT

MAX

PWM

2

_

BRT

1

_

BRT

V

V

)

DC

(

V

R

R

eq. 7

maximum LED current sense feedback threshold.

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⋅

+

⋅

⋅

π

=

2

_

BRT

1

_

BRT

2

_

BRT

1

_

BRT

PWM

BRT

R

R

R

R

f

5

C

eq. 8

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