IR3638SPBF 查看數據表（PDF） - International Rectifier

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IR3638SPBF

HIGH FREQUENCY SYNCHRONOUS PWM BUCK CONTROLLER FOR TRACKING APPLIACTIONS

International Rectifier 

Inductor Selection (Cont..)

For the buck converter, the inductor value for

desired operating ripple current can be

determined using the following relation:

Vin

−Vo

=

L∗

Δi

Δt

;

Δt = D ∗ 1

Fs

( ) L =

Vin −Vo

∗ Vo

Vin ∗ Δi * Fs

--(5 )

Vin = Maximum input voltage

Vo = Output Voltage

Δi = Inductor ripple current

F s= Switching frequency

Δt = Turn on time

D = Duty cycle

If Δi ≈ 40%(Io ) , then the output inductor will be:

L = 0.95uH

The coilcraft DO3316H-102ML (1uH, 10A,

6mOhm) is suitable for this application.

Output Capacitor Selection

The voltage ripple and transient requirements

determines the output capacitors types and

values. The criteria is normally based on the

value of the Effective Series Resistance (ESR).

However the actual capacitance value and the

Equivalent Series Inductance (ESL) are other

contributing components, these components can

be described as:

Δ Δ Δ Δ V = V + V + V o

o(ESR)

o(ESL)

o(C )

ΔVo(ESR) = ΔIL * ESR - -(6)

ΔVo(ESL)

=

⎜⎛Vin

⎝L

⎟⎞

⎠

*

ESL

ΔVo(C)

=

8

ΔIL

* Co *

Fs

ΔVo = Output voltage ripple

ΔIL = Inductor ripple current

3/19/07

IR3638SPbF

It is recommended to select output capacitor with

low enough ESR to meet output voltage ripple

and step load transient requirements.

PosCap capacitors offer low ESR with large

storage capacity per unit volume. These

capacitors offer a cost effective output capacitor

solution.

Sanyo 2R5TPF470M (2.5V, 470uF, 10mOhm) is

selected for this design.

Equation (6) can be used to calculate the

required ESR for the specific voltage ripple.

One Capacitor would meet the voltage ripple

requirement.

Power MOSFET Selection

The IR3638 uses two N-Channel MOSFETs. The

selections 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 caution should be taken with devices at

very low Vgs to prevent undesired turn-on of the

complementary MOSFET, which results a shoot-

through current.

The total power dissipation for MOSFETs

includes conduction and switching losses. For

the Buck converter the average inductor current

is equal to the DC load current. The conduction

loss is defined as:

Pcond

=

(upper switch)=

I2

load

∗ Rds(on)

∗D

∗ϑ

Pcond

=

(lower switch)=

I2

load

∗ Rds(on)

∗ (1 −

D)∗ϑ

ϑ = Rds(on) temperatuer dependency

The RDS(on) temperature dependency should be

considered 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.

For this design, IRF8910 is a good choice. The

device provides two N-MOSFETs in a compact

SO-8 package.

10

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