SL40307A-R19KHF datasheet(30/53 Pages) IRF

30 / 53 page

IR3846

30 www.irf.com

August 01, 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







(15)

Figure 33: 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







(16)



(17)

Where:

= Duty Cycle

IRMS = RMS value of the input capacitor current

= output current.

Vin = Power Stage input voltage

Io=35A and D = 0.1, the IRMS = 10.5A.

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

7x22uF,

25V

ceramic

capacitors,

GRM31CR61E226KE15L from Murata. 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 also result in reduced efficiency and

high 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:

;

























(18)

Where:

Vin = Maximum input voltage

V0 = Output Voltage

∆i

= Inductor Ripple Current

= Switching Frequency

∆t

= On time for Control FET

= Duty Cycle

If ∆i ≈ 30%*Io, then the inductor is calculated to be

0.24μH. Select L=0.25μH, 744309025, from Wurth

Electronik which provides an inductor suitable for this

application.

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

Effective Series Resistance (ESR). However the

actual capacitance value and the Equivalent Series

Inductance (ESL) are other contributing components.

These components can be described as:

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