Datasheet

ManualsBrandsTEXAS INSTRUMENTS ManualsPMICsPMIC - DC/DC voltage regulator Holder WQFN 16

2 Iout

Co >

sw Vout

´ D

¦ ´ D

1 1

Co >

Voripple

8 sw

Iripple

´ ¦

Voripple

Resr <

Iripple

´ -

´ ´ ´ ¦

Vout (Vinmax Vout)

Icorms =

12 Vinm ax L1 sw

TPS54478

SLVSAS2 – JUNE 2011

www.ti.com

For this example, the transient load response is specified as a 3% change in Vout for a load step from 1 A (25%

load) to 3 A (75% load). For this example, ΔIout = 3 - 1 = 2.0A and ΔVout= 0.03 × 1.8 = 0.054 V. Using these

numbers gives a minimum capacitance of 74.1 μF. This value does not take the ESR of the output capacitor into

account in the output voltage change. For ceramic capacitors, the ESR is usually small enough to ignore in this

calculation.

Equation 24 calculates the minimum output capacitance needed to meet the output voltage ripple specification.

Where fsw is the switching frequency, Vripple is the maximum allowable output voltage ripple, and Iripple is the

inductor ripple current. In this case, the maximum output voltage ripple is 30 mV. Under this requirement,

Equation 24 yields 4.4 uF.

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(23)

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Where ΔIout is the change in output current, fsw is the regulators switching frequency and ΔVout is the

allowable change in the output voltage. (24)

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Equation 25 calculates the maximum ESR an output capacitor can have to meet the output voltage ripple

specification. Equation 25 indicates the ESR should be less than 28.6 mΩ. In this case, the ESR of the ceramic

capacitor is much less than 28.6 mΩ.

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

minimum value. For this example, two 47 μF 10 V X5R ceramic capacitors with 3 mΩ of ESR are used. The

estimated capacitance after derating is 2 x 45 µF = 90 µF.

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 RMS (Root Mean Square) value of the maximum ripple current. Equation 26 can be used

to calculate the RMS ripple current the output capacitor needs to support. For this application, Equation 26 yields

303 mA.

(25)

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(26)

INPUT CAPACITOR

The TPS54478 requires a high quality ceramic, type X5R or X7R, input decoupling capacitor of at least 10 μF of

effective capacitance and in some applications a bulk capacitance. 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 TPS54478.

The input ripple current can be calculated using Equation 27.

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.