IRU3047CW datasheet(7/19 Pages) IRF

Part #	IRU3047CW

Description	DUAL SYNCHRONOUS PWM CONTROLLER WITH CURRENT SHARING CIRCUITRY AND LDO CONTROLLER

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Manufacturer	IRF [International Rectifier]

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7 / 19 page

IRU3047

7

Rev. 1.0

09/09/02

www.irf.com

For higher efficiency, a low ESR capacitor is recom-

mended.

For VIN1=12V, choose two Poscap from Sanyo

16TPB47M (16V, 47

mF, 70mV, 1.4A)

For VIN2=5V, choose two 6TPB330M (6.3V, 330

mF,

40m

V, 3A).

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 6TPB470M 470

mF, 6.3V has an ESR 40mV.

Selecting 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.

The resulting output ripple current is smaller then each

channel ripple current due to the 180

8 phase shift. These

currents cancel each other. The cancellation is not the

maximum because of the different duty cycle for each

channel.

Inductor Selection

The inductor is selected based on output power, operat-

ing frequency and efficiency requirements. Low induc-

tor value causes large ripple current, resulting in the

smaller size, but poor efficiency and high output noise.

Generally, 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.

Where:

VIN = Maximum Input Voltage

VOUT = Output Voltage

∆i = Inductor Ripple Current

fS = Switching Frequency

∆t = Turn On Time

D = Duty Cycle

For the buck converter, the inductor value for desired

operating ripple current can be determined using the fol-

lowing relation:

For

Di1=30% of I1, we get L3=5.46mH

For

Di2=30% of I2, we get: L4=3.47mH

The Coilcraft DO5022HC series provides a range of in-

ductors in different values and low profile for large cur-

rents.

For L3 choose DO5022P-602HC (6

mH, 7.5A)

For L4 choose DO5022P-472HC (4.7

mH, 8.4A)

Power MOSFET Selection

The selections criteria to meet power transfer require-

ments is based on maximum drain-source voltage (VDSS),

gate-source drive voltage (VGS), maximum output cur-

rent, 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. Caution should be taken with devices at very

low VGS to prevent undesired turn-on of the complemen-

tary 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 total conduction loss is defined as:

ESR

[

---(5)

DVO

DIO

Where:

DVO = Output Voltage Ripple

DIO = Output Current

DVO=100mV and DIO=5A, results to ESR=20mV

VIN - VOUT = L

3

;

Dt = D3

; D =

1

fS

VOUT

VIN

Di

Dt

L = (VIN - VOUT)

3

---(6)

VOUT

VIN

3Di3fS

2

PCOND(Upper Switch) = ILOAD

3RDS(ON)3D3q

PCOND(Lower Switch) = ILOAD

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

q = RDS(ON) Temperature Dependency

PCON(TOTAL)=PCON(Upper Switch)

q+PCON(LowerSwitch)q

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