RT8101GS datasheet(12/15 Pages) RICHTEK

12 / 15 page

RT8101/A

Preliminary

www.richtek.com

DS8101/A-01 March 2007

3) Input Capacitor

The selection of input capacitor is mainly based on its

maximum ripple current capability. The buck converter

draws pulsewise current from the input capacitor during

the on time of S1 as shown in Figure 8. The RMS value of

ripple current flowing through the input capacitor is

described as :

(6)

The input capacitor must be capable of handling this ripple

current. Sometimes, for higher efficiency the low ESR

capacitor is necessarily.

(A)

D(1

rms

OUT

Figure 9. The related waveforms of output capacitor

2) Output Capacitor

The selection of output capacitor depends on the output

ripple voltage requirement. Practically, the output ripple

voltage is a function of both capacitance value and the

equivalent series resistance (ESR) rC. Figure 9 shows

the related waveforms of output capacitor.

The AC impedance of output capacitor at operating

frequency is quite smaller than the load impedance, so

the ripple current (

ΔIL) of the inductor current flows mainly

through output capacitor. The output ripple voltage is

described as :

where

ΔVOR is caused by ESR and ΔVOC by capacitance.

For electrolytic capacitor application, typically 90 to 95%

of the output voltage ripple is contributed by the ESR of

output capacitor. So Equation (4) could be simplified as :

ΔVOUT = ΔIL x rC

Users could connect capacitors in parallel to get calculated

ESR.

(2)

(3)

(4)

(5)

OUT

I dt

(1 D)T

Δ= Δ

+ Δ

Δ= Δ

× Δ

−

∫

VOUT

VOR

dIL

ΔIL

1/2

ΔIL x rC

VOC

ΔVOC

ΔIL

VIN-VOUT

IOUT

dIL

Thermal Considerations

For continuous operation, do not exceed absolute

maximum operation junction temperature 125

°C. The

maximum power dissipation depends on the thermal

resistance of IC package, PCB layout, the rate of

surroundings airflow and temperature difference between

junction to ambient. The maximum power dissipation can

be calculated by following formula :

PD(MAX) = ( TJ(MAX)

− TA ) / θJA

Where TJ(MAX) is the maximum operation junction

temperature 125

°C, TAis the ambient temperature and the

θJA is the junction to ambient thermal resistance.

For recommended operating conditions specification of

RT8101/A, where TJ(MAX) is the maximum junction

temperature of the die (125

°C) and TA is the maximum

ambient temperature. The junction to ambient thermal

resistance

θJA is layout dependent.

The maximum power dissipation at TA = 25

°C can be

calculated by following formula :

PD(MAX) = ( 125

°C − 25°C) / (120°C/W) = 0.83W for

SOP-8 packages

PD(MAX) = ( 125

°C − 25°C) / (75°C/W) = 1.33W for

PSOP-8 packages

The maximum power dissipation depends on operating

ambient temperature for fixed TJ (MAX) and thermal

resistance

θJA. For RT8101/A packages, Figure 10 allows

the designer to see the effect of rising ambient temperature

on the maximum power allowed.

RT8101GS Datasheet(PDF) 12 Page - Richtek Technology Corporation

RT8101GS Datasheet(HTML) 12 Page - Richtek Technology Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Datasheet Download

Link URL