2-292

where

current

BW - desired converter bandwidth (not to exceed 0.25 to

0.30 of the switching frequency)

The design procedure for this output should follow the

following steps:

1. Choose number and type of output capacitors to meet the

output transient requirements based on the dynamic

loading characteristics of the output.

2. Determine the equivalent ESR of the output capacitor

bank and calculate minimum output inductor value.

3. Verify that chosen inductor meets this minimum value

criteria (at full output load). It is recommended the cho-

sen output inductor be no more than 30% saturated at

full output load.

Oscillator Synchronization

The PWM controllers use a triangle wave for comparison

with the error amplifier output to provide a pulse-width

modulated signal. Should the output voltage of the two

converters be programmed close to each other, then cross-

talk between the converters could cause non-uniform

PHASE pulse-widths and increased output voltage ripple.

The HIP6020 avoids this problem by synchronizing the two

converters 180 degrees out of phase for output voltage

settings within the same range. Therefore, for both output

voltage settings less than 2.4V or both output voltage

settings greater or equal to 2.4V, PWM1 operates out of

phase with PWM2. For one PWM output voltage setting

below 2.4V and the other PWM output voltage setting of

2.4V and above, PWM1 operates in phase with PWM2.

Component Selection Guidelines

Output Capacitor Selection

The output capacitors for each output have unique

requirements. In general the output capacitors should be

selected to meet the dynamic regulation requirements.

Additionally, the PWM converters require an output capacitor

to filter the current ripple. The load transient for the

microprocessor core requires high quality capacitors to

supply the high slew rate (di/dt) current demands.

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

should be selected for transient load regulation.

PWM Output Inductor Selection

Each 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. Additionally, PWM2 output inductor has to

meet the minimum value criteria for loop stability as

described in paragraph ‘PWM2 Controller Feedback

Compensation’. 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

HIP6020 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.

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HIP6020