IRU3038CS datasheet(6/18 Pages) IRF

6 / 18 page

Rev. 2.0

09/12/02

IRU3038

www.irf.com

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

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 6TPC150M 150

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

lecting two of these capacitors in parallel, results to an

ESR of

≅ 20mV 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.

Inductor Selection

The inductor is selected based on output power, operat-

ing frequency and efficiency requirements. Low inductor

value causes large ripple current, resulting in the smaller

size, but poor efficiency and high output noise. Gener-

ally, the selection of inductor value can be reduced to

desired maximum ripple current in the inductor (

∆i). The

optimum point is usually found between 20% and 50%

ripple of the output current.

For the buck converter, the inductor value for desired

operating ripple current can be determined using the fol-

lowing relation:

Di = 30%(IO), then the output inductor will be:

The Coilcraft DO5022HC series provides a range of in-

ductors in different values, low profile suitable for large

currents, 3.3

mH, 10A is a good choice for this applica-

tion. This will result to a ripple approximately 26.5% of

output current.

Power MOSFET Selection

The IRU3038 uses two N-Channel MOSFETs. The se-

lections criteria to meet power transfer requirements is

based on maximum drain-source voltage (VDSS), gate-

source drive voltage (VGS), maximum output current, On-

resistance RDS(ON) and thermal management.

The MOSFET must have a maximum operating voltage

(VDSS) exceeding the maximum input voltage (VIN).

The gate drive requirement is almost the same for both

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

tion should be taken with devices at very low VGS to pre-

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:

The RDS(ON) temperature dependency should be consid-

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.

Choose IRF7460 for both control MOSFET and synchro-

nous MOSFET. This device provides low on-resistance

in a compact SOIC 8-Pin package.

Where:

DVO = Output Voltage Ripple

DIO = Output Current

DVO=100mV and DIO=4A

This results to: ESR=25m

Where:

VIN = Maximum Input Voltage

VOUT = Output Voltage

∆i = Inductor Ripple Current

fS = Switching Frequency

∆t = Turn On Time

D = Duty Cycle

PCOND (Upper Switch) = ILOAD

3 RDS(ON) 3 D 3 q

PCOND (Lower Switch) = ILOAD

3 RDS(ON) 3 (1 - D) 3 q

q = RDS(ON) Temperature Dependency

L = 2.6

ESR

[

---(4)

DVO

DIO

VIN - VOUT = L

;

Dt = D3

; D =

VOUT

VIN

L = (VIN - VOUT)

---(5)

VOUT

VIN

3Di3fS

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