IR3637SPBF datasheet(7/21 Pages) IRF

7 / 21 page

IR3637SPBF

Rev. 1.1

06/16/05

www.irf.com

For higher efficiency, low ESR capacitor is recommended.

Two capacitors of Sanyo's TPB series PosCap with

150

µF, 6.3V, 40mΩ ESR and 1.4A ripple current will

meet the ripple current requirement.

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, faster response to a load transient but poor effi-

ciency 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 cur-

rent.

For the buck converter, the inductor value for desired

operating ripple current can be determined using the fol-

lowing relation:

The ESR of the output capacitor is calculated by the

following relationship:

The Sanyo TPB series, PosCap capacitor is a good

choice. The 6TPB150M 150

µF, 6.3V has an ESR 40mΩ.

Selecting two of these capacitors in parallel, results to

an ESR of

≅ 20mΩ which achieves our low ESR goal.

Power MOSFET Selection

The IR3637 uses two N-Channel MOSFETs. The selec-

tions 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.

Where:

VIN = Maximum Input Voltage

VOUT = Output Voltage

∆i = Inductor Ripple Current

fS = Switching Frequency

∆t = Turn On Time

D = Duty Cycle

VIN - VOUT = L

;

∆t = D×

; D =

∆i

∆t

VOUT

VIN

L = (VIN - VOUT)

---(5)

VOUT

VIN

×∆i×fS

For IOUT=6A and D=0.36, the IRMS=2.8A

IRMS = IOUT

×(1-D)

---(3)

Where:

D is the Duty Cycle, D=VOUT/VIN.

IRMS is the RMS value of the input capacitor current.

IOUT is the output current for each channel.

∆i = 40%(IO), then the output inductor will be:

L = 1.2

µH

The Coilcraft DO3316 series provides a range of induc-

tors in different values, low profile suitable for large cur-

rents, 1.5

µH, 8A(Isat) is a good choice for this applica-

tion.

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.

Where:

∆VO = Output Voltage Ripple

∆IO = Inductor Ripple Current

∆VO=50mV and ∆IO=2.4A

Results to ESR=20.8m

Ω

ESR

≤

---(4)

∆VO

∆IO

PCOND (Upper Switch) = ILOAD

× RDS(on) × D × ϑ

PCOND (Lower Switch) = ILOAD

× RDS(on) × (1 - D) × ϑ

ϑ = RDS(ON) Temperature Dependency

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