IR3840WM datasheet(19/34 Pages) IRF

19 / 34 page

Rev 12.0

PD-97657

IR3840WMPbF

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 ≈ 35%(I

o), then the output inductor is

calculated to be 0.607μH. Select L=0.6 μH.

The MPL104-0R6 from Delta provides a compact,

low profile inductor suitable for this application.

the voltage V

c across C6 remains approximately

unchanged and the voltage at the Boot pin

becomes

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 upper MOSFET 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=12A and D = 0.15, the IRMS = 4.28A.

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 to have 4x10uF 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)

..........

)

( D

RMS

−

∗

(13)

..........

)

( i

cycle

Duty

time

Turn

frequency

Switching

current

ripple

Inductor

Voltage

Output

voltage

input

Maximum

Δt

Δi

()

(14)

..........

;

∗

−

∗

−

Fig. 12. Bootstrap circuit to generate

Vc voltage

(11)

..........

Boot

−

≅

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