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PWM Output Capacitors

Modern microprocessors produce transient load rates above

1A/ns. 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 (effective series

resistance) and voltage rating requirements rather than

actual capacitance requirements.

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 on

specific decoupling requirements.

Use only specialized low-ESR capacitors intended for

switching-regulator applications for the bulk capacitors. The

bulk capacitor’s ESR determines the output ripple voltage

and the initial voltage drop following a high slew-rate

transient’s edge. An aluminum electrolytic capacitor’s ESR

value is related to the case size with lower ESR available in

larger case sizes. However, the equivalent series inductance

(ESL) of these capacitors increases with case size and can

reduce the usefulness of the capacitor to high slew-rate

transient loading. Unfortunately, ESL is not a specified

parameter. Work with your capacitor supplier and measure

the capacitor’s impedance with frequency to select a suitable

component. In most cases, multiple electrolytic capacitors of

small case size perform better than a single large case

capacitor.

Linear Output Capacitors

The output capacitors for the linear regulators provide

dynamic load current. The linear controllers use dominant

pole compensation integrated into the error amplifier and are

insensitive to output capacitor selection. Output capacitors

should be selected for transient load regulation.

PWM Output Inductor Selection

The PWM converter requires an output inductor. The output

inductor is selected to meet the output voltage ripple

requirements and sets the converter’s response time to a

load transient. The inductor value determines the converter’s

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

current. The ripple voltage and current are approximated by

the following equations:

Increasing the value of inductance reduces the ripple current

and voltage. However, the large inductance values increase

the converter’s response time to a load transient.

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

load transient is the time required to change the inductor

current. Given a sufficiently fast control loop design, the

HIP6021 will provide either 0% or 100% 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 post-transient 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.

The response time to a transient is different for the

application of load and the removal of load. The following

equations give the approximate response time interval for

application and removal of a transient load:

response time to the removal of load. Be sure to check both

of these equations at the minimum and maximum output

levels for the worst case response time.

Input Capacitor Selection

The important parameters for the bulk input capacitors 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 a voltage rating of 1.5 times is a

conservative guideline. The RMS current rating requirement

for the input capacitor of a buck regulator is approximately

1/2 of the summation of the DC load current.

Use a mix of input bypass capacitors to control the voltage

overshoot across the MOSFETs. Use ceramic capacitance

for the high frequency decoupling and bulk capacitors to

supply the RMS current. Small ceramic capacitors can be

placed very close to the upper MOSFET to suppress the

voltage induced in the parasitic circuit impedances.

For a through-hole design, several electrolytic capacitors

(Panasonic HFQ series or Nichicon PL series or Sanyo

MV-GX or equivalent) may be needed. For surface mount

designs, 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. The TPS series available from

AVX, and the 593D series from Sprague are both surge

current tested.

MOSFET Selection/Considerations

The HIP6021 requires 5 external transistors. Two N-channel

MOSFETs are used in the synchronous-rectified buck

topology of PWM1 converter. It is recommended that the

AGP linear regulator pass element be a N-channel MOSFET

as well. The GTL and memory linear controllers can also

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HIP6021