HIP6021

FN4684 Rev 1.00

Page 13 of 15

March 8, 2005

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 each drive a

MOSFET or a NPN bipolar as a pass transistor. All these

gain, saturation voltages, gate supply requirements, and

thermal management considerations.

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