RT8477A datasheet(11/13 Pages) RICHTEK

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RT8477A

DS8477A-02 April 2015

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

OUT

ESR +

8f C



   





Schottky Diode Selection

The Schottky diode, with their low forward voltage drop

and fast switching speed, is necessary for RT8477A

applications. In addition, power dissipation, reverse voltage

rating and pulsating peak current are important parameters

of the Schottky diode that must be considered. The diode's

average current rating must exceed the average output

current. The diode conducts current only when the power

switch is turned off (typically less than 50% duty cycle).

Thermal Considerations

For continuous operation, do not exceed absolute

maximum junction temperature. The maximum power

dissipation depends on the thermal resistance of the IC

package, PCB layout, rate of surrounding airflow, and

difference between junction and ambient temperature. The

maximum power dissipation can be calculated by the

following formula :

PD(MAX) = (TJ(MAX)

− TA) / θJA

where TJ(MAX) is the maximum junction temperature, TAis

the ambient temperature, and

θJAis the junction to ambient

thermal resistance.

For recommended operating condition specifications, the

maximum junction temperature is 125

°C. The junction to

ambient thermal resistance,

θJA, is layout dependent. For

SOP-8 (Exposed Pad) package, the thermal resistance,

θJA, is 30.6°C/W on a standard JEDEC 51-7 four-layer

thermal test board. The maximum power dissipation at TA

= 25

°C can be calculated by the following formula :

PD(MAX) = (125

°C − 25°C) / (30.6°C/W) = 3.26W for

SOP-8 (Exposed Pad) package

The maximum power dissipation depends on the operating

ambient temperature for fixed TJ(MAX) and thermal

resistance,

θJA. The derating curve in Figure 1 allows the

designer to see the effect of rising ambient temperature

on the maximum power dissipation.





 







OUT

L =

20.3 I

For Boost application :





 







OUT

L =

20.3 I

The inductor must also be selected with a saturation

current rating greater than the maximum inductor current

during normal operation. The maximum inductor current

can be calculated by the following equations.

For Buck application :





 





OUT

PEAK

OUT

I= I

2L f

For Boost application :







 

 



OUT

PEAK

OUT

V2 L f

where

η is the efficiency of the power converter.

Capacitor Selection

The input capacitor reduces current spikes from the input

supply and minimizes noise injection to the converter. For

most RT8477A applications, a 4.7

μF ceramic capacitor is

sufficient. A value higher or lower may be used depending

on the noise level from the input supply and the input

current to the converter. In Buck application, the output

capacitor is typically ceramic and selection is mainly

based on the output voltage ripple requirements. The

output ripple,

ΔVOUT, is determined by the following

equation :

As a design example, to design the peak to peak inductor

ripple to be

±30% of the output current, the following

equations can be used to estimate the size of the needed

inductance :

For Buck application :

For Buck-Boost application :





OUT

PEAK

OUT

+ V

V2 L f

+ V







 

 



For Buck-Boost application :

OUT

L =

20.3 I

+ V



 







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