RT6203ENGSP datasheet(14/21 Pages) RICHTEK

14 / 21 page

RT6203E

DS6203E-00 January 2019

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

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 10% 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

V(V

)

L =



 

Once an inductor value is chosen, the ripple current (

ΔIL)

is calculated to determine the required peak inductor

current.

OUT

L(PEAK)

OUT_MAX

V(V

)

I =

and I

= I







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 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.

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 1. The peak-to-peak

voltage ripple on input capacitor can be estimated as

equation below :

CIN

OUT

= D I

+ I

ESR







where

OUT

D =





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

D1 D







CIN_MAX

Where V

200mV



CIN Ripple Voltage

CIN Ripple Current

VCIN



(1-D) x IOUT

D x IOUT

(1-D) x tSW

D x tSW

VESR = IOUT x ESR

Figure 1. CIN Ripple Voltage and Ripple Current

In addition, the input capacitor needs to have a very low

ESR and must be rated to handle the worst-case RMS

input current of :

OUT

RMS

OUT_MAX

OUT







It is commonly to use the worse IRMS

≅ IOUT/2 at VIN =

2VOUT for design. Note that ripple current ratings from

capacitor manufacturers are often based on only 2000

hours of life which makes it advisable to further de-rate

the capacitor, or choose a capacitor rated at a higher

temperature than required.

Several capacitors may also be paralleled to meet size,

height and thermal requirements in the design. For low

input voltage applications, sufficient bulk input capacitance

is needed to minimize transient effects during output load

changes.

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