8

Rev. 1.0

01/29/04

IRU3138

www.irf.com

For higher efficiency, a low ESR capacitor is recom-

mended. Choose three Poscap from Sanyo 6TPC150M

(6.3V, 150

mF, 40mV) with a maximum allowable ripple

current of 5.7A.

Inductor Selection

The inductor is selected based on operating frequency,

transient performance and allowable output voltage ripple.

Low inductor value results to faster response to step

load (high

Di/Dt) and smaller size but will cause larger

output ripple due to increase of inductor ripple current.

As a rule of thumb, select an inductor that produces a

ripple current of 10-40% of full load DC.

For the buck converter, the inductor value for desired

operating ripple current can be determined using the fol-

lowing relation:

If

The Panasonic PCCN6B series provides a range of in-

ductors in different values, low profile suitable for large

currents, 1.1

mH, 16A is a good choice for this applica-

tion. This will result to a ripple approximately 22% of

output current.

Output Capacitor Selection

The criteria to select the output capacitor is normally

based on the value of the Effective Series Resistance

(ESR). In general, the output capacitor must have low

enough ESR to meet output ripple and load transient

L = 0.91

mH

D

3(1-D)

---(9)

Where:

3

;

Dt = D3

; D =

1

Di

Dt

3

---(11)

Where:

∆i = Inductor Ripple Current

∆t = Turn On Time

D = Duty Cycle

Where:

Di = Inductor Ripple Current

≅ 22% of 12A = 2.64A

This results to: ESR=18.9m

V

ESR

[

---(10)

requirements, yet have high enough ESR to satisfy sta-

bility requirements. The ESR of the output capacitor is

calculated by the following relationship:

The Sanyo TPC series, Poscap capacitor is a good choice.

The 6TPC330M, 330

mF, 6.3V has an ESR 40mV. Se-

lecting three of these capacitors in parallel, results to an

ESR of

≅ 13.3mV which achieves our low ESR goal.

The capacitor value must be high enough to absorb the

inductor's ripple current. The larger the value of capaci-

tor, the lower will be the output ripple voltage.

Power MOSFET Selection

The IRU3138 uses two N-Channel MOSFETs. The se-

lections criteria to meet power transfer requirements is

The MOSFET must have a maximum operating voltage

The gate drive requirement is almost the same for both

MOSFETs. Logic-level transistor can be used and cau-

vent undesired turn-on of the complementary MOSFET,

which results a shoot-through current.

The total power dissipation for MOSFETs includes con-

duction and switching losses. For the Buck converter,

the average inductor current is equal to the DC load cur-

rent. The conduction loss is defined as:

ered for the worst case operation. This is typically given

in the MOSFET data sheet. Ensure that the conduction

losses and switching losses do not exceed the package

ratings or violate the overall thermal budget.