ISL8117AEVAL2Z datasheet(21/24 Pages) RENESAS

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ISL8117A

FN8752 Rev 1.00

Page 21 of 23

Feb 26, 2024

The maximum capacitor value required to provide the full, rising

step, transient load current during the response time of the

inductor is shown in Equation 27:

Where COUT is the output capacitor(s) required, LO is the output

inductor, ITRAN is the transient load current step, VIN is the input

voltage, VO is output voltage and DVOUT is the drop in output

voltage allowed during the load transient.

High frequency capacitors initially supply the transient current

and slow the load rate of change seen by the bulk capacitors. The

bulk filter capacitor values are generally determined by the ESR

(Equivalent Series Resistance) and voltage rating requirements

as well as actual capacitance requirements.

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

the ESR of the output capacitors as defined by Equation 28:

Where IL is calculated in Equation 26.

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.

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.

The recommended output capacitor value for the ISL8117A 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 1.25x

greater than the maximum input voltage and 1.5x is a

conservative guideline. The ACRMS input current varies with the

load giving in Equation 29:

Where DC is duty cycle of the PWM.

The maximum RMS current supplied by the input capacitance

occurs at VIN = 2 x VOUT, DC = 50% as shown in Equation 30:

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.

COUT





ITRAN



2

2 VIN VO

–





DVOUT





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

(EQ. 27)

VRIPPLE IL ESR





(EQ. 28)

IRMS

DC DC2

–



(EQ. 29)

IRMS

--- IO



(EQ. 30)

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