ISL8117

FN8666 Rev 4.00

Page 20 of 24

Apr 19, 2018

The output voltage ripple is due to the inductor ripple current and

the ESR of the output capacitors as defined by Equation 16:

where

High frequency decoupling capacitors should be placed 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 set by an internal, single compensation zero at 8.8kHz. The

ESR zero can be a factor of five on either side of the internal zero

and still contribute to increased phase margin of the control loop.

This requirement is shown in Equation 17:

In conclusion, the output capacitors must meet the following

criteria:

1. 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.

2. The ESR must be sufficiently low to meet the desired output

voltage ripple due to the output inductor current.

3. The ESR zero should be placed in a rather large range, to

provide additional phase margin.

The recommended output capacitor value for the ISL8117 is

between 100µF to 680µF, to meet the stability criteria with

external compensation. Use of aluminum electrolytic (POSCAP)

or tantalum type capacitors is recommended. Use of low ESR

ceramic capacitors is possible with loop analysis to ensure

stability.

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. The AC RMS input current varies with the

load given in Equation 18:

where DC is duty cycle of the PWM.

The maximum RMS current supplied by the input capacitance

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 caution must be

exercised with regard to the capacitor surge current rating. These

capacitors must be capable of handling the surge current at

power-up.

I



=

(EQ. 16)

1

2

 ESR

 f

Z



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

=

(EQ. 17)

DC DC

2

–



=

(EQ. 18)

1

2



=

(EQ. 19)