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RT2660

www.richtek.com

DS2660-00

September 2015

©

Copyright 2015 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

Operation

The RT2660 is a synchronous low voltage step-down

converter that can support the V5IN range from 4.5V to

5.5V and the output current can be up to 6A. The RT2660

uses a constant on-time, current mode architecture. In

steady-state operation, the high-side N-MOSFET is turned

on when the current feedback reaches COMP level which

is the amplified difference between the reference voltage

and the feedback voltage.

The switching frequency of 600kHz or 1000kHz allows for

efficiency and size optimization when selecting the output

filter components. The switching frequency is set using

the mode pin.

The RT2660 reduces the external component count by

integrating the boot recharge MOSFET. The bias voltage

for the integrated high-side MOSFET is supplied by a

capacitor between the BOOT and SW pins. The boot

capacitor voltage is monitored by an BOOT detection circuit

and turns off the high-side MOSFET when the voltage falls

below a threshold voltage.

The SS pin is used to minimize inrush currents or provide

power supply sequencing during power up. A small value

capacitor should be coupled to the pin for slow start. The

SS pin is discharged before the output power up to ensure

a repeatable restart after an over-temperature fault, UVLO

fault or disable condition.

The error amplifier EA adjusts COMP voltage by comparing

the output voltage to the REFIN voltage. When the load

increases, it causes a drop in the feedback voltage relative

to the reference, then the COMP voltage rises to allow

higher inductor current to match the load current.

Bootstrap Voltage (Boot) and Low Dropout Operation

The RT2660 has an integrated boot regulator and requires

a small ceramic capacitor between the BOOT and SW

pins to provide the gate drive voltage for the high-side

MOSFET. The value of the ceramic capacitor should be

0.1

μF. A ceramic capacitor with an X7R or X5R grade

dielectric with a voltage rating of 10V or higher is

recommended because of the stable characteristics over

temperature and voltage. When the BOOT voltage is lower

than the threshold voltage, low-side MOSFET turns on

and built-in bootstrap MOSFET will recharge the BOOT

capacitor. The high-side MOSFET is always turned off

when BOOT voltage is lower than threshold voltage.

PWM Frequency and Adaptive On-Time Control

The on-time can be roughly estimated by the equation :

OUT

ON

IN

SW

V

1

T

=

Vf



Error Amplifier

The RT2660 has a transconductance amplifier. The error

The REFIN voltage can comes from external power source

or taps off the voltage divider from the 2V VREF. The

transconductance of the error amplifier is 1000

μA/V during

normal operation. The frequency compensation

components are placed between the COMP pin and

ground.

Auto-Zero Current Detector

The auto-zero current detector circuit senses the SW

waveform to adjust the zero current threshold voltage.

When the current of low-side MOSFET decreases to the

zero current threshold, the low-side MOSFET turns off to

prevent negative inductor current. In this way, the zero

current threshold can adjust for different conditions to get

better efficiency.

Protection Features

The RT2660 has many features to protect the device.

Under-Voltage Protection (UVLO)

The RT2660 continuously monitors the voltage on the V5IN

pin to ensure that the voltage level is high enough to bias

the device properly and to provide sufficient gate drive

potential to maintain high efficiency. The converter starts

with approximately 4.3V and has a nominal hysteresis of

440mV. If the 5V UVLO limit is reached, the converter

transitions the phase node into an off function. And the

converter remains in the off state until the device is reset

by cycling 5V until the 5V POR is reached (2.3V nominal).

The power input does not have an UVLO function.