RT9201 datasheet(10/13 Pages) RICHTEK

10 / 13 page

RT9201

DS9201-12 August 2007

www.richtek.com

Application Information

RT9201 is a high-efficiency pulse-width-modulated (PWM)

step-down DC-DC converter. Capable of delivering 600mA

output current over a wide input voltage range from 2.5V

to 5.5V, the RT9201 is ideally suited for portable electronic

devices that are powered from 1-cell Li-ion battery or from

other power sources within the range such as cellular

phones, PDAs and handy-terminals.

Chip Enable/Disable and Soft Start

Four operational modes are available: PWM, PSM, Low-

Drop-Out and shut-down modes. Pulling EN pin lower

than 0.4V shuts down the RT9201 and reduces its

quiescent current to 0.1

µA.

Pulling EN pin higher than 1.4V enables the RT9201 and

initiates the softstart cycle. A softstart capacitor connected

to SS pin along with the internal 300k

Ω resistor

determines VREF ramp-up speed and the softstart

behavior. A softstart capacitor between 330pF to 1000pF

is recommended for smooth start-up of RT9201.

PWM Operation

During normal operation, the RT9201 regulates output

voltage by switching at a constant frequency and then

transferring the power to the load in each cycle by PWM.

The RT9201 uses a slope-compensated, current-mode

PWM controller capable of achieving 100% duty cycle.

At each rising edge of the internal oscillator, the Control

Logic cell sends a PWM ON signal to the Driver cell to

turn on internal PMOSFET. This allows current to ramp

up through the inductor to the load, and stores energy in

a magnetic field. The switch remains on until either the

current-limit is tripped or the PWM comparator signals

for the output in regulation. After the switch is turned off,

the inductor releases the magnetic energy and forces

current through the NMOSFET synchronous rectifier to

the output-filter capacitor and load. The output-filter

capacitor stores charge when the inductor current is

above the average output current and releases charge

when the inductor current is below the average current

to smooth the output voltage across the load.

A Zero Detector monitors inductor current by sensing

voltage drop across the NMOSFET synchronous rectifier

when it turns on. The NMOSFET turns off and allows the

converter entering discontinuous conduction mode when

the inductor current decreases to zero. The zero current

defection on threshold is about 50mA.This reduces

conduction loss and increase power conversion efficiency

at light load condition.

PSM Operation

The minimum on-time of RT9201 is approximately 200ns.

Consequently, the converter will enter pulse-skipping-

mode (PSM) during extreme light load condition or when

modulation index (VOUT/VIN) is extreme low. This could

reduce switching loss and further increase power

conversion efficiency.

For example : VIN = 3.8V, VOUT = 2.5V & ILOAD = 4mA

Low Dropout Mode Operation

The maximum on-time can exceed one oscillator cycle,

which permits operation up to 100% duty cycle. As the

input voltage drops, the duty cycle increases until the

PMOSFET is held on continuously. Dropout voltage in

100% duty cycle is the output current multiplied by the

on-resistance of the internal switch and inductor, around

300mV (IOUT = 600mA).

When the converter operates around the boundary of

Low-Dropout-Mode and PWM, sub-harmonic oscillation

and large ripple may occur at output voltage as shown in

Figure 1. Figure 2 illustrates the boundary of stable PWM

and Low-Dropout-Mode operations for typical output

voltages. Above each boudary is the stable PWM mode

region with respect to that output voltage. To prevent sub-

harmonic oscillation, please make sure that VIN and VOUT

locate in the stable PWM operation region. If the input

voltage range must cross the boundary region, increasing

output capacitors with low ESR can alleviate the output

ripple voltage.

RT9201 Datasheet(PDF) 10 Page - Richtek Technology Corporation

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