IR3838MPBF datasheet(21/35 Pages) IRF

21 / 35 page

IR3838MPbF

Rev 1.41

Inductor Selection

The inductor is selected based on output power,

operating frequency and efficiency requirements.

A low inductor value causes large ripple current,

resulting in the smaller size, faster response to a

load transient but poor efficiency and high output

noise. Generally, the selection of the inductor

value can be reduced to the desired maximum

ripple current in the inductor

. The optimum

point is usually found between 20% and 50%

ripple of the output current.

For the buck converter, the inductor value for the

desired

operating

ripple

current

can

determined using the following relation:

Where:

Δi ≈ 42.5%(I

o), then the output inductor is

calculated to be 0.6μH. Select MPL104-0R6 from

Delta (L=0.6μH) which provides a compact, low

profile inductor suitable for this application.

Output Capacitor Selection

The voltage ripple and transient requirements

determine the output capacitors type 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

described as

A bootstrap capacitor of value 0.1uF is suitable

for most applications.

Input Capacitor Selection

The ripple current generated during the on time of

the control FET should be provided by the input

capacitor. The RMS value of this ripple is

expressed by:

Where:

D is the Duty Cycle

RMS is the RMS value of the input capacitor

current.

Io is the output current.

For I

o=10A and D = 0.15, the IRMS = 3.6A.

Ceramic capacitors are recommended due to

their peak current capabilities. They also feature

low ESR and ESL at higher frequency which

enables better efficiency. For this application, it is

advisable

have

3x10uF,

16V

ceramic

capacitors, ECJ-3YX1C106K from Panasonic. In

addition to these, although not mandatory, a

1x330uF, 25V SMD capacitor EEV-FK1E331P

may also be used as a bulk capacitor and is

recommended if the input power supply is not

located close to the converter.

....(12)

..........

)

(

RMS







(13)

..........



)

( i



cycle

Duty

time

Turn

frequency

Switching

current

ripple

Inductor

Voltage

Output

voltage

input

Maximum



Δt

Δi



(14)

..........

;





















Fig. 17. Bootstrap circuit to generate

Vc voltage

(15)

.....

..........

)

(

)

ESL

(

)

ESR

(

)

(

)

ESL

(

)

ESR

(

ESL

ESR









































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