AP3595

Document number: DS36749 Rev. 1 - 2

17 of 23

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January 2014

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AP3595

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Although the inductor value and frequency are increased and the ripple current and voltage are reduced, a tradeoff

exists between the inductor’s

ripple current and the regulator load transient response time. A smaller inductor will give the regulator a faster load transient response at the

switching loss of the MOSFETs and the power dissipation of the converter. The maximum ripple current occurs at the maximum input voltage. A

good starting point is to choose the ripple current to be approximately 30% of the maximum output current. Once the inductance value has been

chosen, select an inductor that is capable of carrying the required peak current without going into saturation. In some types of inductors, especially

core that is made of ferrite, the ripple current will increase abruptly when it saturates. This results in a larger output ripple voltage.

15. Output Capacitor Selection

Output voltage ripple and the transient voltage deviation are factors that have to be taken into consideration when selecting output capacitors.

Higher capacitor value and lower ESR reduce the output ripple and the load transient drop. Therefore, selecting high performance low ESR

capacitors is recommended for switching regulator applications. In addition to high frequency noise related to MOSFET turn-on and turn-off, the

output voltage ripple includes the capacitance

inductor’s current. The ripple voltage of output capacitors can be represented by:

SW

OUT

P

P

COUT

f

C

I

V













8

ESR

P

P

ESR

R

I

V











These two components constitute a large portion of the total output voltage ripple. In some applications, multiple capacitors have to be paralleled

to achieve the desired ESR value. If the output of the converter has to support another load with high pulsating current, more capacitors are

needed in order to reduce the equivalent ESR and suppress the voltage ripple to a tolerable level. As mall decoupling capacitor in parallel for by

passing the noise is also recommended, and the voltage rating of the output capacitors must be considered too.

To support a load transient that is faster than the switching frequency, more capacitors are needed for reducing the voltage excursion during load

step change.

For getting same load transient response, the output capacitance of two-phase converter only needs to be around half of output capacitance of

single-phase converter.

Another aspect of the capacitor selection is that the total AC current going through the capacitors has to be less than the rated RMS current

specified on the capacitors in order to prevent the capacitor from overheating.

16. Input Capacitor Selection

Use small ceramic capacitors for high frequency decoupling and bulk capacitors to supply the surge current needed each time high-side MOSFET

turns on. Place the small ceramic capacitors physically close to the MOSFETs and between the drain of high-side MOSFET and the source of low-

side MOSFET.

The important parameters for the bulk input capacitor are the voltage rating and the RMS current rating. For reliable operation, select the bulk

capacitor with voltage and current ratings above the maximum input voltage and largest RMS current required by the circuit. The capacitor voltage

rating should be at least 1.25 times greater than the maximum input voltage and a voltage rating of 1.5 times is a conservative guideline. For two-

phase converter, the RMS current of the bulk input capacitor is roughly calculated as the following equation:

)

2

1

(

2

2

D

D

I

I

OUT

RMS









For a through hole design, several electrolytic capacitors may be needed. For surface mount design, solid tantalum capacitors can be used, but

caution must be exercised with regard to the capacitor surge current rating.

17. MOSFET Selection

and thermal management requirements.

In high current applications, the MOSFET power dissipation, package selection, and heat sink are the dominant design factors. The power

dissipation includes two loss components: conduction loss and switching loss.