IR3621 datasheet(12/29 Pages) IRF

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IR3621&(PbF)

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For higher efficiency, low ESR capacitors are recom-

mended.

Choose two Poscap from Sanyo 16TPB47M (16V, 47

µF,

70m

Ω ) with a maximum allowable ripple current of 1.4A

for inputs of each channel.

Inductor Selection

The inductor is selected based on operating frequency,

transient performance and allowable output voltage ripple.

Low inductor values result in faster response to step

load (high

∆i/∆t) and smaller size but will cause larger

output ripple due to increased 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:

VIN - VOUT = L

;

∆t = D×

; D =

VOUT

VIN

∆i

∆t

L = (VIN - VOUT)

---(7)

VOUT

VIN

×∆i×fS

Where:

VIN = Maximum Input Voltage

VOUT = Output Voltage

∆i = Inductor Ripple Current

fS = Switching Frequency

∆t = Turn On Time

D = Duty Cycle

Where:

∆VO = Output Voltage Ripple

∆i = Inductor Ripple Current

∆VO = 3% of VO will result to ESR(2.5V) =16.6mΩ and

ESR(1.8V) =16m

Ω

ESR

≤

---(8)

∆VO

∆IO

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

The 6TPC330M, 330

µF, 6.3V has an ESR 40mΩ. Se-

lecting three of these capacitors in parallel for 2.5V out-

put, results to an ESR of

≅ 13.3mΩ which achieves our

low ESR goal. And selecting three of these capacitors in

parallel for 1.8V output, results in an ESR of

≅ 13.3mΩ

which achieves our low ESR goal.

The capacitors value must be high enough to absorb the

inductor's ripple current.

Power MOSFET Selection

The IR3621 uses four N-Channel MOSFETs. The selec-

tion criteria to meet power transfer requirements is based

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

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

tance RDS(ON) and thermal management.

The both control and synchronous MOSFETs must have

a maximum operating voltage (VDSS) that exceeds the

maximum input voltage (VIN).

Input Capacitor Selection

The 1800 out of phase will reduce the RMS value of the

ripple current seen by input capacitors. This reduces

numbers of input capacitors. The input capacitors must

be selected that can handle both the maximum ripple

RMS at highest ambient temperature as well as the

maximum input voltage. The RMS value of current ripple

for duty cycles under 50% is expressed by:

For

∆i(2.5V) = 45%(IO(2.5V) ), then the output inductor will

be:

L4 = 1.1

µH

For

∆i(1.8V) = 35%(IO(1.8V) ), then the output inductor will

be:

L3 = 1.1

µH

Panasonic provides a range of inductors in different val-

ues and low profile for large currents.

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

the

following

relationship:

RMS=

1(1-D1)+I2

2(1-D2)-2I1I2D1D2)

--- (6)

Where:

RMS is the RMS value of the input capacitor current

1 and D2 are the duty cycle for each output

1 and I2 are the current for each output

For this application the I

RMS =4.8A

(ESL, Equivalent Series Inductance is neglected)

Choose ETQP6F1R1BFA (1.1

µH, 16A, 2.2mΩ) both for

L3 and L4.

For 2-phase application, equation (7) can be used for

calculating the inductors value. In such case the induc-

tor ripple current is usually chosen to be between 10-

40% of maximum phase current.

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