ISL6532B

FN9120 Rev 3.00

Page 13 of 15

Jul 2004

ratings above the maximum input voltage and largest RMS

current required by the circuit. Their voltage rating should be at

least 1.25 times greater than the maximum input voltage, while

a voltage rating of 1.5 times is a conservative guideline. For

worst cases, the RMS current rating requirement for the input

capacitor of a buck regulator is approximately 1/2 the DC

output load current.

The maximum RMS current required by the regulator may be

closely approximated through the following equation:

For a through hole design, several electrolytic capacitors 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.

Some capacitor series available from reputable manufacturers

are surge current tested.

MOSFET Selection - PWM Buck Converter

The ISL6532B requires 2 N-Channel power MOSFETs for

switching power and a third MOSFET to block backfeed from

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. The

switching losses seen when sourcing current will be different

from the switching losses seen when sinking current. When

sourcing current, the upper MOSFET realizes most of the

switching losses. The lower switch realizes most of the

switching losses when the converter is sinking current (see the

equations below). These equations assume linear voltage-

current transitions and do not adequately model power loss

due the reverse-recovery of the upper and lower MOSFET’s

body diode. The gate-charge losses are dissipated in part by

the ISL6532B and do not significantly heat the MOSFETs.

However, large gate-charge increases the switching interval,

tSW which increases the MOSFET switching losses. Ensure

that both MOSFETs are within their maximum junction

temperature at high ambient temperature by calculating the

temperature rise according to package thermal-resistance

specifications. A separate heatsink may be necessary

depending upon MOSFET power, package type, ambient

temperature and air flow.

MAX

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

MAX

2

1

12

------

–



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

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









+









=

Approximate Losses while Sourcing current

Approximate Losses while Sinking current

Io

2





1D

–





1

2

--- Io









+

=

Io

2





D



1

2

--- Io









+

=