ISL6445

FN9230 Rev 1.00

Page 13 of 15

Jun 3, 2008

Output Capacitor Selection

The output capacitors for each output have unique

requirements. In general, the output capacitors should be

selected to meet the dynamic regulation requirements

including ripple voltage and load transients. Selection of

output capacitors is also dependent on the output inductor,

so some inductor analysis is required to select the output

capacitors.

One of the parameters limiting the converter’s response to a

load transient is the time required for the inductor current to

slew to it’s new level. The ISL6445 will provide either 0% or

71% duty cycle in response to a load transient.

The response time is the time interval required to slew the

inductor current from an initial current value to the load

current level. During this interval the difference between the

inductor current and the transient current level must be

supplied by the output capacitor(s). Minimizing the response

time can minimize the output capacitance required. Also, if

the load transient rise time is slower than the inductor

response time, as in a hard drive or CD drive, it reduces the

requirement on the output capacitor.

The maximum capacitor value required to provide the full,

rising step, transient load current during the response time of

the inductor is:

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 13:

page 13.

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 at 1.4MHz 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

30kHz. This range is set by an internal, single compensation

zero at 6kHz. 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. Therefore,

In conclusion, the output capacitors must meet three 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,

and

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

to provide additional phase margin.

The recommended output capacitor value for the ISL6445 is

between 150µF to 680µF, to meet stability criteria with

external compensation. Use of aluminum electrolytic,

POSCAP, or tantalum type capacitors is recommended. Use

of low ESR ceramic capacitors is possible but would take

more rigorous loop analysis to ensure stability.

Output Inductor Selection

The PWM converters require output inductors. The output

inductor is selected to meet the output voltage ripple

requirements. The inductor value determines the converter’s

ripple current and the ripple voltage is a function of the ripple

current and output capacitor(s) ESR. The ripple voltage

10

20

30

40

50

6

INPUT VOLTAGE (V)

9

12

15

18

21

24

FIGURE 17. FET GATE CHARGE

 I

TRAN



2

–

 DV

OUT



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

=

(EQ. 12)

I



=

(EQ. 13)

1

2

 ESR

 f

Z



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

=

(EQ. 14)