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LX1732

High Current PFM Boost Converter

Microsemi Corporation 

INTEGRATED PRODUCTS

LX1732

High Current PFM Boost Converter

PRODUCTION DATA SHEET

APPLICATION NOTE

FUNCTIONAL DESCRIPTION

The LX1732 implements a constant on-time and off-

time PFM architecture that can deliver a 5.0V output and

over 150mA of continuous output current. The free-

running oscillator and logic-circuit initiate the internal

MOSFET switching cycle during power-up via the VC pin

(see block diagram in Figure 2)). The current passing

through the LX pin is detected by the I-sense circuit and

compared with the feedback voltage status via the logic

circuit. The internal limit for the peak current is set at

1000mA (max). The MOSFET switch remains on for the

on-time duration or until the I-sense detects the maximum

peak current, or when the feedback threshold voltage is

detected at the FB pin. The feedback threshold voltage is

set by an external resistor divider network and compared

with the internal reference voltage. The LX1732

implements a non-load isolated shutdown mode using an

input-voltage referenced signal level via the SHDN pin.

Connect this pin to the input supply rail if the shutdown

feature is not required. The LX1732 features a low-battery

detection circuit that allows for sensing of the input voltage

supply. If the supply voltage at LBI drops below the

internal reference voltage, the internal MOSFET (open-

drain) sinks current to GND.

OUTPUT VOLTAGE PROGRAMMING

Resistors R1 and R2 of Figure 1 program the output

voltage. An optional 1000pF capacitor is recommended

across R1 to improve the transient response and reduce

output voltage ripple (see Figure 7). The value of R2 should

be less than 250KΩ. The value of R1 can be determined

using the following equation where VREF is found in the

ELECTRICAL CHARACTERISTICS TABLE:

R1 =

R2⎢⎢⎣⎡⎜⎜⎝⎛

VOUT

VREF

⎟⎟⎠⎞

−

⎤

1⎥

⎥⎦

DESIGN EXAMPLE:

Let R2 equal 100K and the required VOUT equal to

5.0V.

R1

=

100K

⎢⎢⎣⎡⎜⎜⎝⎛

5V

1.20

⎟⎟⎠⎞

−1⎥⎤

⎥⎦

= 316.6KΩ

DIODE SELECTION

A Schottky diode is recommended for use with the

LX1732 because it provides fast switching and superior

reverse recovery performance. The Microsemi UPS5817

(20V @ 1A) makes an effective choice for most

applications.

LOW BATTERY DETECTION PROGRAMMING

Program the Low Battery Detect voltage threshold by

selecting values for resistors R3 and R4 (see Figure 1) using

the formula below. Use a value of less than 250KΩ for R4 to

minimize threshold error due to the internal comparator’s

offset current. The value of R3 can be determined using the

following equation.

R3

=

R4⎢⎢⎣⎡⎜⎜⎝⎛

VLOW BATT

VREF

⎟⎟⎠⎞

⎤

− 1⎥

⎥⎦

The LBO pin’s open drain output requires a pull up resistor

(i.e., 100K typ.) to drive external CMOS logic circuits.

Connect the LBI pin to ground and omit resistors R3 and R4

when the Low Battery Detect function is not implemented.

DESIGN EXAMPLE:

Let R3 equal 100K and the required LB threshold equal

to 2.5V.

R3

=

R100⎢⎢⎣⎡⎜⎜⎝⎛

2.5V

0.6V

⎟⎟⎠⎞

−1⎥⎤

⎥⎦

=

316KΩ

CAPACITOR SELECTION

To minimize ripple voltage, output capacitors in the range

of 10uF to 100uF with a low series resistance (ESR) are

recommended. Multi-layer ceramic capacitors with X5R or

X7R dielectric make an effective choice because they feature

small size, very low ESR, a temperature stable dielectric, and

can be connected in parallel to increase capacitance. Other

low ESR capacitors such as solid tantalum, specialty

polymer, or organic semiconductor, make effective choices

provided that the capacitor is properly rated for the output

voltage and ripple current. Finally, choose an input capacitor

of sufficient size to effectively decouple the input voltage

source impedance (e.g., CIN > 47µF).

LAYOUT CONSIDERATIONS

The high peak currents and switching frequencies present

in DC/DC converter applications require careful attention to

device layout for optimal performance. Basic design rules

include: (1) maintaining wide traces for power components

(e.g., width > 50mils); (2) place CIN, COUT, the Schottky

diode, and the inductor close to the LX1732; (3) minimizing

trace capacitance by reducing the etch area connecting the

SW pin to the inductor; and (4) minimizing the etch length to

the LBI and FB pins to reduce noise coupling into these high

impedance sense inputs. Other considerations include

placing a 0.1uF capacitor between the LX1732 VOUT pin

and GND pin to reduce high frequency noise and decoupling

the VIN pin using a 0.1uF capacitor.

Copyright © 2000

Rev. 1.1c, 2005-03-03

Microsemi

Integrated Products

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

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