LX1744

Microsemi

Integrated Products Division

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

Page 7

Dual Output Boost – LED Driver / LCD Bias

I N T E GR A T ED

PRODUCT S

Copyright

© 2000

Rev. 1.1b, 2005-03-01

APPLI CATI ON NOTE

The LX1744 is a dual output Pulse Frequency Modulated

(PFM) boost converter that is optimized for large step-up

voltage applications like LCD biasing and LED drive.

Operating in a pseudo-hysteretic mode with a fixed

switch “off time” of 300ns, converter switching is enabled

reference voltage or the ADJ pin voltage managed by the

reference logic block (see Block Diagram). When this

occurs, the feedback comparator activates the switching

logic, pulling the gate of the power MOSFET high. This in

turn connects the boost inductor to ground causing current

to flow building up the energy stored in the inductor. The

output remains “on”, until the inductor current ramps up to

the peak current level set either by the CS pin programming

reference threshold for the LCD bias output. During this

switch cycle, the load is powered from energy stored in the

output capacitor. Once the peak inductor current value is

achieved, the driver output is turned off, for the fixed off-

time period of 300ns, allowing a portion of the energy

stored in the inductor to be delivered to the load causing

output voltage to rise at the input to the feedback circuit. If

the voltage at the feedback pin is less than the internal

reference at the end of the off-time period, the output

switches the power MOSFET “on” and the inductor

charging cycle repeats until the feedback pin voltage is

greater than the internal reference.

Typical converter

switching behavior is shown in Figure 12.

Selecting the appropriate values for LCD Bias output

voltage divider (Figure 3), connected to the feedback pin,

programs the output voltage.

LX1744

V

SW

V

OUT

R

1

R

2

FB

Figure 3 –

LCD Bias Output Voltage

Using a value between 40k

in most applications.

REF

REF

OUT

2

1

V

V

-

V

R

R

=

eq. 1

The LX1744 allows for the dynamic adjustment of the

of the voltage output via an adjustment pin (ADJ). Any

voltage applied to the adjustment pin works in conjunction

with the internal reference logic.

The LX1744 will

automatically utilize the internal reference when no signal

is detected or when the adjustment signal voltage is below

approximately 0.6V.

This adjustment pin includes an internal 50pF capacitor

to ground (Figure 4) that works with an external resistor to

100KHz) signal input to be used for the voltage adjustment

signal. (Consequently a DC bias signal can also be used).

LX1744

ADJ

Reference

Logic

50pF

2.5M

Ω

R

ADJ_1

Figure 4 –

LCD Bias Adjustment Input

Different PWM signal levels can be accommodated by

is equal to the reference voltage (eq. 2)

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

Ω

Ω

⋅

⋅

=

1

_

PWM

PWM

ADJ

R

M

5

.

2

M

5

.

2

Cycle

Duty

V

V

eq. 2

LX1744

ADJ

C

ADJ

R

ADJ_1

R

ADJ_2

Figure 5 –

LCD Bias Adjustment Input Filter

Ideally the resultant ripple on the ADJ pin should be

approximately 1% or 40dB down from the nominal

reference. When using a PWM with a frequency that is