MCR03EZPFX1002 datasheet(18/32 Pages) IRF

18 / 32 page

Rev 16.0

PD-97660

IR3831WMPbF

Where:

D is the Duty Cycle

RMS is the RMS value of the input capacitor

current.

o is the output current.

For I

o=8 A and D = 0.0625, the IRMS = 1.94 A

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 3x10uF 16V ceramic capacitors

ECJ-3YX1C106K from Panasonic. In addition to

these, although not mandatory, a 330uF 25V

SMD capacitor EEV-FK1E331P from Panasonic

may 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 results in a smaller size and

faster response to a load transient but poor

efficiency and high output noise due to large

ripple current. 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 0.63μH. Select L=0.6 μH.

The MPL104-0R6 from Delta provides a compact,

low profile inductor suitable for this application.

Bootstrap Capacitor Selection

To drive the Control FET, it is necessary to

supply a gate voltage at least 4V greater than the

voltage at the SW pin, which is connected the

source of the Control FET . This is achieved by

using a bootstrap configuration, which comprises

the internal bootstrap diode and an external

bootstrap capacitor (C6), as shown in Fig. 11.

The operation of the circuit is as follows: When

the lower MOSFET is turned on, the capacitor

node connected to SW is pulled down to ground.

The capacitor charges towards V

cc through the

internal bootstrap diode, which has a forward

voltage drop V

D. The voltage Vc across the

bootstrap capacitor C6 is approximately given as

When the upper MOSFET turns on in the next

cycle, the capacitor node connected to SW rises

to the bus voltage V

in. However, if the value of C6

is appropriately chosen, 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:

....(12)

..........

)

( D

RMS

−

∗

(13)

..........

)

( i

cycle

Duty

time

Turn

frequency

Switching

current

ripple

Inductor

Voltage

Output

voltage

input

Maximum

Δt

Δi

()

(14)

...

..........

;

∗

−

∗

−

Fig. 11. Bootstrap circuit to generate

Vc voltage

(11)

..........

Boot

−

≅

(10)

..........

−

≅

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