PS0021

OPERATION

Thermal Overload Protection

The PS0021 is a compact, high-efficiency, synchronous

boost converter in a SOT-26 package designed for space-

restricted applications. The part is available with

adjustable output voltages ranging from 2.5V to 5.0V. It

is able to start up with input voltages as low as 0.8V and

operate with input voltages down to 0.5V.

With its

internal synchronous rectifier and low on-resistance of

the internal NMOS switch, the device maintains high

efficiency over a wide range of load current. As shown in

the Block Diagram, the PS0021 consists of accurate band

gap core, error amplifier, start-up oscillator and control

logic unit along with PMOS and NMOS switches. With

current mode PWM control, the PS0021 has ultra-fast line

and improved load regulation. Moreover, the PS0021

provides real shutdown circuitry which disconnects the

output from the input during shutdown and results in the

discharge of the output to ground.

Thermal overload protection limits the total power

dissipation in the PS0021. When the junction temperature

shutdown logic and turns off most of the internal

circuitry. The thermal sensor turns the internal circuitry

on again after the IC’s junction temperature drops by

15

°C.

Thermal overload protection is designed to protect the

PS0021 in the event of a fault condition. For continuous

operation, do not exceed the absolute maximum junction

Current Limit Protection

The PS0021 includes a current limiter that monitors the

peak inductor current through the NMOS switch and

turns the NMOS switch off when the inductor peak

current exceeds 150mA.

Start-Up Mode

The PS0021 starts up typically at 0.8V. When it is turned

on, the device gets its start up bias from VIN. A start-up

oscillator, which runs typically at 650 KHz, takes the

output voltage high enough so that V

Light Load Operation

exceeds V

July 2007

www.picsemi.com

© 2007 Power IC. All Rights Reserved.

9

OUT

Once V

The PS0021 provides improved light load efficiency. The

internal zero current comparator monitors the inductor

current to the load and shuts off the PMOS switch once

this current reduces to some low value (typically 50mA).

This prevents the inductor current from reversing its

polarity, avoiding back-charging and thus improving the

efficiency at light loads. At that time, both the NMOS and

PMOS switches remain off. However, normal operation

starts again when the output voltage falls below the

regulation point. The IC automatically skips pulses at light

load, thus providing better efficiency.

exceeds V

OUT

V to V

IN

over.

Internal PMOS and NMOS Switches

The PS0021 features a 0.45

Ω NMOS switch and a 0.6Ω

PMOS switch. In normal operation, these switches are

alternatively turned on and thus initiate the charging of

the inductor from VIN and then discharging of it to the

output capacitor and the load. However, between the

event of one switch turning off and the other turning on,

a dead time is provided to avoid a huge in-rush current

from the output to ground via switches. During the dead

time, both switches remain off and the inductor

discharges via the body diode of the PMOS switch to the

output.