Microsemi

Integrated Products Division

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

Page 7

Copyright

© 2000

Rev. 1.1b, 2005-03-03

LX1741

High Efficiency High Voltage Boost Controller

I N T E GRA T ED

PR ODUC T S

APPLI CATI ON I N FORMATI O N

The LX1741 is a Pulse Frequency Modulated (PFM)

boost converter that is optimized for large step up voltage

applications like LCD biasing. It operates in a pseudo-

hysteretic mode with a fixed switch “off time” of 300ns.

Converter switching is enabled when the feedback voltage,

Diagram). When this occurs, comparator A activates the

off-time controller. The off-time controller and the current

limiter circuit activate comparator B which toggles the

NDRV output circuit. The NDRV output is switched “on”

(and remains “on”) until the inductor current ramps up to

the peak current level. This current level is set via the

SRC inputs.

The load is powered from energy stored in the output

capacitor during the inductor charging cycle. Once the peak

inductor current value is achieved, the NDRV output is

turned off (off-time is typically 300ns) allowing a portion of

the energy stored in the inductor to be delivered to the load.

This causes the output voltage to continue to rise at the

input to the feedback circuit (i.e., comparator A). If the

voltage at the FB input is still less than 1.29V at the end of

the off-time period, the NDRV output switches the external

is greater than the internal reference.

This switching

behavior is shown in Figure 9 and 11.

The application of an external voltage source at the ADJ

pin allows for output voltage adjustment over a typical

range of approximately ±15%. The designer can select one

of two possible methods.

One option is to vary the

reference voltage directly at the ADJ pin by applying a DC

voltage from 0.9 to 1.5V. The second option is to connect a

PWM logic signal to the ADJ pin (e.g., see Figure 2). The

LX1741 includes an internal 50pF capacitor to ground that

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

(i.e., filter out the AC component of a pulse width

The adjustment voltage level is selectable (with limited

accuracy) by implementing the voltage divider created

between the external series resistor and the internal 2.5MΩ

resistor. If the DC voltage at the ADJ pin drops below

0.6V, the device will revert to the internal reference voltage

level of 1.29V. A typical adjustment curve is shown in

Figure 8 (see section titled: Characteristic Curves).

Disabling the LX1741 is achieved by driving the SHDN pin

with a low-level logic signal thus reducing the device power

consumption to less than 1µA.

Selecting the appropriate values for R1 and R2 in the

voltage divider connected to the feedback pin programs the

output voltage. Using a value of 49.9K for R2 works well

in most applications.

R1 can be determined by the

()

V

V

-

V

R2

R1

REF

REF

OUT

×

=

Let R2 equals 49.9K and the required VOUT equal to

18V.

()

Ω

=

×

Ω

=

K

4

.

646

49.9K

R1

1.29V

1.29V

-

18V

Setting the level of peak inductor current to, at least, 1.5x

the expected maximum DC input current will minimize the

inductor size, the input ripple current, and the output ripple

voltage. The designer is encouraged to use inductors that

will not saturate at the peak inductor current level. An

inductor value of 47µH is recommended. Choosing a lower

value emphasizes peak current overshoot while choosing a

higher value emphasizes output ripple voltage. The peak

switch current is defined using a resistor placed between the

CS

SCALE

D

IN

MIN

PEAK

R

I

t

L

V

I

I

⎟⎟

⎟

⎠

⎞

⎜⎜

⎜

⎝

⎛

⎟

⎟

⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎜

⎜

⎝

⎛

+

+

=

to internal operation of comparator A. A typical value at

per KΩ. All of these parameters have an effect on the final

4.02KΩ using nominal values for all other parameters.

4.02KΩ

kΩ

31mA

620ns

H

47

3.0V

145mA

×

+

×

+

=

⎟

⎠

⎞

⎜

⎝

⎛

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

µ

145mA + 39.6mA + 124.6ma = 309.2mA.