PCMB065T-2R2MS datasheet(19/33 Pages) IRF

19 / 33 page

Rev 16.0

PD-97509

IR3843AMPbF

the voltage Vc 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.

For applications with 21V input voltage, the switch

node may ring above the 25V absolute maximum

voltage rating. To prevent

this, in addition to

using best layout practices, it may be necessary

to provide a 10 ohm resistor in series with the

boot capacitor.

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=3A and D = 0.15, the IRMS = 1.07A.

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 1x10uF 25V ceramic capacitors

C3216X5R1E106M from TDK. 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.

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 ≈ 40%(I

o), then the output inductor is

calculated to be 2.13μH. Select L=2.2 μH.

The PCMB065T-2R2MS from Vishay provides a

compact, low profile inductor suitable for this

application

....(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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