HIP6013

FN4325 Rev 1.00

Page 8 of 11

November 3, 2005

Component Selection Guidelines

Output Capacitor Selection

An output capacitor is required to filter the output and supply

the load transient current. The filtering requirements are a

function of the switching frequency and the ripple current. The

load transient requirements are a function of the slew rate

(di/dt) and the magnitude of the transient load current. These

requirements are generally met with a mix of capacitors and

careful layout.

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) 1.0

F 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 HIP6013 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 output voltage

setting. Be sure to check both of these equations at the

minimum and maximum output levels for the worst case

response time.

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 current needed each time Q1 turns on. Place the small

ceramic capacitors physically close to the MOSFETs and

between the drain of Q1 and the anode of Schottky diode D2.

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

100

80

60

40

20

0

-20

-40

-60

10M

1M

100K

10K

1K

100

10

OPEN LOOP

ERROR AMP GAIN

COMPENSATION

FREQUENCY (Hz)

GAIN

20LOG

MODULATOR

GAIN

20LOG

(R2/R1)

CLOSED LOOP

GAIN

FIGURE 8. ASYMPTOTIC BODE PLOT OF CONVERTER GAIN

I =



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