RT5752 datasheet(22/32 Pages) RICHTEK

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RT5752

DS5752-03 November 2023

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

OUT

MAX

OFF_MIN

and D



The actual on-time will be slightly longer as the IC

compensates for voltage drops in the circuit, but we can

neglect both of these since the on-time increase

compensates for the voltage losses. Calculate the output

voltage sag as :



OUT

SAG

OUT

IN(MIN)

MAX

OUT

L( I

)









The amplitude of the capacitive soar is a function of the

load step, the output capacitor value, the inductor value

and the output voltage :

OUT

SOAR

OUT

L( I

)





Because some modern digital loads can exhibit nearly

instantaneous load changes, the amplitude of the ESR

step up or down should be taken into consideration.

Output Voltage Setting

Set the desired output voltage using a resistive divider

from the output to ground with the midpoint connected to

FB, as shown in Figure 5. The output voltage is set

according to the following equation :

VOUT = 0.6V x (1 + RFB1 / RFB2)

Figure 5. Output Voltage Setting

Place the FB resistors within 5mm of the FB pin. For

output voltage accuracy, use divider resistors with 1% or

better tolerance.

EN Pin for Start-Up and Shutdown Operation

For automatic start-up, the EN pin can be connected to

the input supply VIN directly. The large built-in hysteresis

band makes the EN pin useful for simple delay and timing

circuits. The EN pin can be externally connected to VIN

by adding a resistor REN and a capacitor CEN, as shown in

Figure 6, to have an additional delay. The time delay can

be calculated with the EN's internal threshold, at which

switching operation begins.

An external MOSFET can be added for the EN pin to be

logic-controlled, as shown in Figure 7. In this case, a pull-

up resistor, REN, is connected between VIN and the EN

pin. The MOSFET Q1 will be under logic control to pull

down the EN pin. To prevent the device being enabled

when VIN is smaller than the VOUT target level or some

other desired voltage level, a resistive divider (REN1 and

REN2) can be used to externally set the input under-voltage

lockout threshold, as shown in Figure 8.

Figure 6. Enable Timing Control

RT5752

GND

RFB1

VOUT

RFB2

GND

VIN

REN

CEN

RT5752

capacitors) and the output capacitance is large enough to

prevent excessive sag and soar on very fast load step

edges, with the chosen inductor value.

The amplitude of the ESR step up or down is a function of

the load step and the ESR of the output capacitor :

VESR _STEP =

ΔIOUT x RESR

The amplitude of the capacitive sag is a function of the

load step, the output capacitor value, the inductor value,

the input-to-output voltage differential, and the maximum

duty cycle. The maximum duty cycle during a fast transient

is a function of the on-time and the minimum off-time since

the ACOT® control scheme will ramp the current using

on-times spaced apart with minimum off-times, which is

as fast as allowed. Calculate the approximate on-time

(neglecting parasites) and maximum duty cycle for a given

input and output voltage as :

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