TPS65279V datasheet(28/46 Pages) TI

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out max

0.25

(

)

inmin

out

inrms

out

inmin

out

inmax

out

corms

inmax

)

12 V

L f

× ×

oripple

esr

oripple

8 f

TPS65279V

SLVSBW1B – FEBRUARY 2013 – REVISED JANUARY 2014

www.ti.com

Equation 11 calculates the minimum output capacitance needed to meet the output voltage ripple specification.

Where fSW is the switching frequency, Voripple is the maximum allowable output voltage ripple, and Ioripple is the

inductor ripple current.

(11)

Equation 12 calculates the maximum ESR an output capacitor can have to meet the output voltage ripple

specification.

(12)

Additional capacitance de-ratings for aging, temperature and DC bias should be factored in which increases this

minimum value.

Capacitors generally have limits to the amount of ripple current they can handle without failing or producing

excess heat. An output capacitor that can support the inductor ripple current must be specified. Some capacitor

data sheets specify the root mean square (RMS) value of the maximum ripple current. Equation 13 can be used

to calculate the RMS ripple current the output capacitor needs to support.

(13)

Input Capacitor Selection

The TPS65279V requires a high quality ceramic, type X5R or X7R, input decoupling capacitor of at least 10-µF

of effective capacitance on the PVIN input voltage pins. In some applications additional bulk capacitance may

also be required for the PVIN input. The effective capacitance includes any DC bias effects. The voltage rating of

the input capacitor must be greater than the maximum input voltage. The capacitor must also have a ripple

current rating greater than the maximum input current ripple of the TPS65279V. The input ripple current can be

calculated using Equation 14.

(14)

The value of a ceramic capacitor varies significantly over temperature and the amount of DC bias applied to the

capacitor. The capacitance variations due to temperature can be minimized by selecting a dielectric material that

is stable over temperature. X5R and X7R ceramic dielectrics are usually selected for power regulator capacitors

because they have a high capacitance to volume ratio and are fairly stable over temperature. The output

capacitor must also be selected with the DC bias taken into account. The capacitance value of a capacitor

decreases as the DC bias across a capacitor increases. For this example design, a ceramic capacitor with at

least a 25-V voltage rating is required to support the maximum input voltage. TPS65279V may operate from a

single supply. The input capacitance value determines the input ripple voltage of the regulator. The input voltage

ripple can be calculated using Equation 15.

(15)

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