ISL6440

FN9040 Rev 2.00

Page 13 of 15

Oct 4, 2005

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:

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 300kHz 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 ISL6440 is

between 150

F to 680F, to ensure stability when using the

components in the typical application schematic. 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

expression is given in the capacitor selection section and the

ripple current is approximated by the following equation:

For the ISL6440, use Inductor values between 4.7

H to

10

H when using the component values in the Typical

Application Schematic for optimal compensation. Other

inductor values can be used but with a more rigorous design

for compensation.

Input Capacitor Selection

The important parameters for the bulk input capacitor(s) are

the voltage rating and the RMS current rating. For reliable

operation, select bulk 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.

The total RMS current supplied by the input capacitance is:

where,

DC is duty cycle of the respective PWM.

Depending on the specifics of the input power and its

impedance, most (or all) of this current is supplied by the

input capacitor(s). Figure 17 shows the advantage of having

the PWM converters operating out of phase. If the

converters were operating in phase, the combined RMS

current would be the algebraic sum, which is a much larger

 I

TRAN



2

–

 DV

OUT



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

=

I



=

1

2

 ESR

 f

Z



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

=

I

L

–

 V

OUT



 L

 V

IN



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

=

2

2

+

=

DC DC

2

–

=