2-207

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 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 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 suitable

components. In most cases, multiple electrolytic capacitors

of small case size perform better than a single large case

capacitor. For a given transient load magnitude, the output

voltage transient response due to the output capacitor

characteristics can be approximated by the following

equation:

Linear Output Capacitors

The output capacitors for the linear regulator and the linear

controller provide dynamic load current. The linear controller

uses dominant pole compensation integrated in the error

amplifier and is insensitive to output capacitor selection.

regulation.

The output capacitor for the linear regulator provides loop

stability. The linear regulator (OUT2) requires an output

capacitor characteristic shown in Figure 13. The upper line

plots the 45 phase margin with 150mA load and the lower

line is the 45 phase margin limit with a 10mA load. Select a

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 ntransient. 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

HIP6016 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

capacitors. 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

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.

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 should be

V

TRAN

ESL

dI

TRAN

dt

-----------------------

×

ESR

I

TRAN

×

+

=

10

1000

100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

CAPACITANCE (

µF)

ESR (

Ω)

ST

ABLE

OPERA

TION

∆I

V

IN

V

OUT

–

F

S

L

O

×

----------------------------------

V

OUT

V

IN

-----------------

×

=

∆V

OUT

∆I ESR

×

=

t

RISE

L

O

I

TRAN

×

V

IN

V

OUT

–

----------------------------------

=

t

FALL

L

O

I

TRAN

×

V

OUT

----------------------------------

=

HIP6016