RT5753

21

DS5753-05 November 2023

www.richtek.com

©

Copyright 2023 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

Application Information

The output stage of a synchronous buck converter is

composed of an inductor and capacitor, which stores and

delivers energy to the load, and forms a second-order low-

pass filter to smooth out the switch node voltage to

maintain a regulated output voltage.

Inductor Selection

The inductor selection trade-offs among size, cost,

efficiency, and transient response requirements. Generally,

three key inductor parameters are specified for operation

with the device: inductance value (L), inductor saturation

current (ISAT), and DC resistance (DCR).

A good compromise between size and loss is to choose

the peak-to-peak ripple current equals to 20% to 50% of

the IC rated current. The switching frequency, input

voltage, output voltage, and selected inductor ripple current

determines the inductor value as follows :

OUT

IN

OUT

IN

SW

L

V(V

V

)

L =

Vf

I



 

Once an inductor value is chosen, the ripple current (

ΔIL)

is calculated to determine the required peak inductor

current.

OUT

IN

OUT

L

IN

SW

V(V

V

)

I=

Vf

L







L_PEAK

OUT_MAX

L

1

I

= I

+

I

2



IL(PEAK) should not exceed the minimum value of IC's upper

current limit level. Besides, the current flowing through

the inductor is the inductor ripple current plus the output

current. During power up, faults or transient load

conditions, the inductor current can increase above the

calculated peak inductor current level calculated above.

In transient conditions, the inductor current can increase

up to the switch current limit of the device. For this reason,

the most conservative approach is to specify an inductor

with a saturation current rating equal to or greater than

the switch current limit rather than the peak inductor

current.

For the selected inductor, the inductor’s saturation and

thermal rating should meet or greater than the ripple

current (

ΔIL). For more conservative, the rating for inductor

saturation current must be equal to or greater than switch

current limit of the device rather than the inductor peak

current.

For EMI sensitive application, choosing shielding type

inductor is preferred.

Input Capacitor Selection

Input capacitance, CIN, is needed to filter the pulsating

current at the drain of the high-side power MOSFET. CIN

should be sized to do this without causing a large variation

in input voltage. The waveform of CIN ripple voltage and

ripple current are shown in Figure 4. The peak-to-peak

voltage ripple on input capacitor can be estimated as

equation below :

where

OUT

IN

V

D =

V





CIN

OUT

OUT

IN

SW

1D

V

= D I

+ I

ESR

Cf











For ceramic capacitors, the equivalent series resistance

(ESR) is very low, the ripple which is caused by ESR can

be ignored, and the minimum input capacitance can be

estimated as equation below :





IN_MIN

OUT_MAX

CIN_MAX

SW

D1 D

C

I

Vf

=







where

ΔVCIN_MAX is maximum input ripple voltage.

Figure 4. CIN Ripple Voltage and Ripple Current

CIN Ripple Current

CIN Ripple Voltage

VCIN



(1-D) x IOUT

D x IOUT

(1-D) x tSW

D x tSW

VESR = IOUT x ESR

Richtek’s component specification does not include the following information in the Application Information section.

Thereby no warranty is given regarding its validity and accuracy. Customers should take responsibility to verify their

own designs and reserve suitable design margin to ensure the functional suitability of their components and systems.