RT5797B

Copyright © 2022 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

DS5797B-06

April 2022

www.richtek.com

13

Application Information

The RT5797B is a single-phase step-down converter. It

provides single feedback loop constant on-time, current

mode control with fast transient response. An internal

0.6V reference allows the output voltage to be precisely

regulated for low output voltage applications. A fixed

switching frequency (1MHz) oscillator and internal

compensation are integrated to minimize external

component count. Protection features include over

current protection, under voltage protection and over

temperature protection.

Output Voltage Setting

Connect a resistive voltage divider at the FB between

VOUT and GND to adjust the output voltage. The output

voltage is set according to the following equation :

( )

OUT

REF

R1

V

= V

1

R2

+

where VREF is the feedback reference voltage 0.6V

(typ.).

FB

GND

VOUT

R1

R2

Figure 1. Setting VOUT with a Voltage Divider

Chip Enable and Disable

The EN pin allows for power sequencing between the

controller bias voltage and another voltage rail. The

RT5797B remains in shutdown if the EN pin is lower

than 400mV. When the EN pin rises above the VEN trip

point, the RT5797B begins a new initialization and soft-

start cycle.

Internal Soft-Start

The RT5797B provides an internal soft-start function to

prevent large inrush current and output voltage

overshoot when the converter starts up. The soft-start

(SS) automatically begins once the chip is enabled.

During soft-start,

the internal

soft-start

capacitor

becomes charged and generates a linear ramping up

voltage across the capacitor. This voltage clamps the

voltage at the FB pin, causing PWM pulse width to

increase slowly and in turn reduce the input surge

current. The internal 0.6V reference takes over the loop

control once the internal ramping-up voltage becomes

higher than 0.6V.

Over Voltage Protection (OVP)

The RT5797BL provide over voltage protection function

when output voltage over 120%. The IC will be into

Latch-off mode.

UVLO Protection

The RT5797B has input Under Voltage Lockout

protection (UVLO). If the input voltage exceeds the

UVLO rising threshold voltage (2.25V typ.), the

converter resets and prepares the PWM for operation.

If the input voltage falls below the UVLO falling

threshold voltage during normal operation, the device

will stop switching. The UVLO rising and falling

threshold voltage has a hysteresis to prevent noise-

caused reset.

Input Capacitor Selection

High quality ceramic input decoupling capacitor, such

as X5R or X7R, with values greater than 22

F are

recommended for the input capacitor. The X5R and

X7R ceramic capacitors are usually selected for power

regulator capacitors because the dielectric material has

less capacitance variation and more temperature

stability.

Voltage rating and current rating are the key parameters

when selecting an input capacitor. Generally, selecting

an input capacitor with voltage rating 1.5 times greater

than the maximum input voltage is a conservatively safe

design.

The input capacitor is used to supply the input RMS

current, which can be approximately calculated using

the following equation :

OUT

OUT

IN_RMS

LOAD

IN

IN

VV

I

= I

1

VV





 −



