ISL81601 datasheet(49/54 Pages) RENESAS

49 / 54 page

FN9299 Rev.3.1

Page 49 of 53

May 27, 2021

ISL81601

8. Component Selection Guideline

In Boost mode, the current to the output capacitor is not continuous. The output voltage ripple is much higher as

defined by Equation 33:

where

ILBoost is calculated in Equation 29 on page 48.

Place high frequency decoupling capacitors as close to the power pins of the load as physically possible. Be careful

not to add inductance in the circuit board wiring that could cancel the usefulness of these low inductance

components. Consult with the manufacturer of the load circuitry for specific decoupling requirements.

Use only specialized low-ESR capacitors intended for switching regulator applications for the bulk capacitors. In most

cases, multiple small case electrolytic capacitors perform better than a single large case capacitor.

The stability requirement on the selection of the output capacitor is that the ESR zero (fZ) is between 2kHz and

60kHz. The ESR zero can help increase phase margin of the control loop.

This requirement is shown in Equation 34:

In conclusion, the output capacitors must meet the following criteria:

• They must have sufficient bulk capacitance to sustain the output voltage during a load transient while the output

inductor current is slewing to the value of the load transient.

• The ESR must be sufficiently low to meet the desired output voltage ripple due to the supplied ripple current.

• The ESR zero should be placed in a large range to provide additional phase margin.

8.4

Input Capacitor Selection

The important parameters for the input capacitor(s) are the voltage rating and the RMS current rating. For reliable

operation, select input capacitors 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 1.5 times is a conservative guideline. In Buck mode the AC RMS input current varies

with the load giving in Equation 35:

where DC is duty cycle.

The maximum RMS current supplied by the input capacitance occurs at VIN = 2 X VOUT, DC = 50% as shown in

Equation 36:

In Boost mode, the input current is continuous. The RMS current supplied by the input capacitance is much

smaller.

Use a mix of input bypass capacitors to control the voltage ripple across the MOSFETs. Use ceramic capacitors for

the high frequency decoupling and bulk capacitors to supply the RMS current. Small ceramic capacitors can be

placed very close to the MOSFETs to suppress the voltage induced in the parasitic circuit impedances.

Solid tantalum capacitors can be used, but use caution with regard to the capacitor surge current rating. These

capacitors must be capable of handling the surge current at power-up.

VRIPPLE

IOUT

 V

OUT



VIN

----------------------------------------

I

LBoost

------------------------







ESR



(EQ. 33)

COUT

 ESR

 f



------------------------------------

(EQ. 34)

IRMS

DC DC

–

IOUT



(EQ. 35)

IRMS

--- IOUT



(EQ. 36)

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