RT8088AWSC datasheet(10/16 Pages) RICHTEK

10 / 16 page

DS8088A-01 August 2017

www.richtek.com

RT8088A

is a registered trademark of Richtek Technology Corporation.

Application Information

The basic RT8088A application circuit is shown in Typical

Application Circuit. External component selection is

determined by the maximum load current and begins with

the selection of the inductor value and operating frequency

followed by CIN and COUT.

Inductor Selection

The inductor value and operating frequency determine the

ripple current according to a specific input and output

voltage. The ripple current,

ΔIL, increases with higher VIN

and decreases with higher inductance, as shown in

equation below :

where f is the operating frequency and L is the inductance.

Having a lower ripple current reduces not only the ESR

losses in the output capacitors, but also the output voltage

ripple. Higher operating frequency combined with smaller

ripple current is necessary to achieve high efficiency. Thus,

a large inductor is required to attain this goal. The largest

ripple current occurs at the highest VIN. To guarantee that

the ripple current stays below the specified

ΔIL(MAX), the

inductor value should be chosen according to the following

equation :

The inductor's current rating (defined by a temperature

rise from 25

°C ambient to 40°C) should be greater than

the maximum load current and its saturation current should

be greater than the short-circuit peak current limit. Refer

to Table 1 for the suggested inductor selection.

Table 1. Suggested Inductors for Typical

Application Circuit

Component

Supplier

Part Number

Dimensions

(mm)

CYNTEC

PIFE20161B-

R33MS-39

2.0 X 1.6 X 1.2

Input and Output Capacitor Selection

An input capacitor, CIN, is needed to filter out the

trapezoidal current at the source of the high-side MOSFET.

To prevent large ripple current, a low ESR input capacitor

sized for the maximum RMS current should be used. The

RMS current is given by :

This formula has a maximum at VIN = 2VOUT, where IRMS =

IOUT(MAX)/2. This simple worst-case condition is commonly

used for design. Choose a capacitor rated at a higher

temperature than required. Several capacitors may also

be paralleled to meet the size or height requirements of

the design. Ceramic capacitors have high ripple current,

high voltage rating and low ESR, which makes them ideal

for switching regulator applications. However, they can

also have a high voltage coefficient and audible

piezoelectric effects. The high Q of ceramic capacitors

with trace inductance can lead to significant ringing. When

a ceramic capacitor is used at the input and the power is

supplied by a wall adapter through long wires, a load step

at the output can induce ringing at the input, VIN. At best,

this ringing can couple to the output and be mistaken as

loop instability. At worst, a sudden inrush of current

through the long wires can potentially cause a voltage

spike at VIN large enough to damage the part. Thus, care

must be taken to select a suitable input capacitor.

The selection of COUT is determined by the required ESR

to minimize output voltage ripple. Moreover, the amount

of bulk capacitance is also a key for COUT selection to

ensure that the control loop is stable. Loop stability can

be checked by viewing the load transient response. The

output voltage ripple,

ΔVOUT, is determined by :

OUT

x 1

f x L











OUT

L(MAX)

IN(MAX)

x 1

f x I





























OUT

RMS

OUT(MAX)

OUT



OUT

OSC OUT

ESR



 







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