HIP6004B

FN4567 Rev 3.00

Page 10 of 15

November 9, 2004

Modern microprocessors produce transient load rates above

1A/ns. High frequency capacitors initially supply the transient

and slow the current load rate 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. For example, Intel

recommends that the high frequency decoupling for the

Pentium Pro be composed of at least forty (40) 1µF ceramic

capacitors in the 1206 surface-mount package.

Use only specialized low-ESR capacitors intended for switching-

regulator applications for the bulk capacitors. The bulk

capacitor’s ESR will determine the output ripple voltage and the

initial voltage drop after a high slew-rate transient. 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.

Output Inductor Selection

The output inductor is selected to meet the output voltage

ripple requirements and minimize the converter’s response

time to the 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 reduce 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

HIP6004B will provide either 0% or 100% duty cycle in

response to a load transient. The response time is the time

required to slew the inductor current from an initial current

value to the transient current level. During this interval the

difference between the inductor current and the transient

current level must be supplied by the output capacitor.

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. With a +5V input source,

the worst case response time can be either at the application

or removal of load and dependent upon the DACOUT setting.

Be sure to check both of these equations at the minimum and

maximum output levels for the worst case response time. With

a +12V input, and output voltage level equal to DACOUT,

Input Capacitor Selection

Use a mix of input bypass capacitors to control the voltage

overshoot across the MOSFETs. Use small ceramic capacitors

for high frequency decoupling and bulk capacitors to supply the

capacitors physically close to the MOSFETs and between the

The important parameters for the bulk input capacitor are the

voltage rating and the RMS current rating. For reliable

operation, select the bulk capacitor 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 the DC load

current.

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 HIP6004B requires 2 N-Channel power MOSFETs. These

requirements, and thermal management requirements.

In high-current applications, the MOSFET power dissipation,

package selection and heatsink are the dominant design factors.

The power dissipation includes two loss components; conduction

loss and switching loss. The conduction losses are the largest

component of power dissipation for both the upper and the lower

MOSFETs. These losses are distributed between the two

MOSFETs according to duty factor (see the equations below).

DI =

Fs x L

x

HIP6004B