PCMB065T-2R2MS datasheet(25/47 Pages) IRF

25 / 47 page

Dual output, 4A/Phase, Highly Integrated SupIRBuck

Single-Input Voltage, Synchronous Buck Regulator

IR3891

www.irf.com

May 08, 2013

When the control FET turns on in the next cycle, the

capacitor node connected to SW rises to the bus

voltage Vin. However, if the value of C1 is

appropriately chosen, the voltage Vc across C1

remains approximately unchanged and the voltage at

the Boot pin becomes:

Boot







(13)

VIN

Vcc

Boot

PGnd

+ VD -

IR3891

Cvin

Figure 25: Bootstrap circuit to generate Vc voltage

A bootstrap capacitor of value 0.1uF is suitable for

most applications.

Input Capacitor Selection

The ripple currents generated during the on time of

the control FETs should be provided by the input

capacitor. The RMS value of this ripple for each

channel is expressed by:



RMS







(14)



(15)

Where:

D is the Duty Cycle

IRMS is the RMS value of the input capacitor

current.

Io is the output current.

For channel 1, Io=4A and D = 0.15, the IRMS =

1.43A.

For channel 2, Io=4A and D = 0.1, the IRMS = 1.2A.

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, 25V ceramic capacitors, C3216X5R1E106K

from TDK.

In addition to these, although not

mandatory, a 1x330uF, 25V SMD capacitor EEV-

FK1E331P from Panasonic 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

Inductors are 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 may reduce efficiency and cause higher

output noise. Generally, the selection of the inductor

value can be reduced to the desired maximum ripple

current in the inductor (∆i). 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 be

determined using the following relation:

;

























(16)

Where:

Vin = Maximum input voltage

V0 = Output Voltage

∆i

= Inductor Peak-to-Peak Ripple Current

= Switching Frequency

∆t

= On time for Control FET

= Duty Cycle

If ∆i ≈ 20%*Io, then the channel 1 output inductor is

calculated to be 3.2μH. Select L=2.2μH, PCMB065T-

2R2MS, from Cyntec which provides a compact, low

profile inductor suitable for this application.

For

channel 2, the output inductor is calculated to be

2.25μH. Select L=1.5μH, PCMB065T-1R5MS, from

Cyntec.

Output Capacitor Selection

The voltage ripple and transient requirements

determine the output capacitors type and values. The

criterion is normally based on the value of the

PCMB065T-2R2MS Datasheet(PDF) 25 Page - International Rectifier

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