IR3897MPBF datasheet(29/47 Pages) IRF

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JANUARY 18, 2013 |DATA SHEET | Rev 3.5

IR3897

4A Highly Integrated SupIRBuck

Single‐Input Voltage, Synchronous Buck Regulator

PD‐97663

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:

(12)

Boot





 

Figure 24: Bootstrap circuit to generate Vc voltage

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

control FET should be provided by the input capacitor.

The RMS value of this ripple is expressed by:

)

(13)

RMS

 

 



(14)





Where:

D is the Duty Cycle

IRMS is the RMS value of the input capacitor current.

Io is the output current.

For Io=4A and D = 0.1, the IRMS = 1.8A.

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, 25V

ceramic capacitors, C3216X5R1E106M 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

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 (Δi). 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:



;

(15)

Vi F



 

  











Where:

Vin = Maximum input voltage

V0 = Output Voltage

Δi = Inductor Peak‐to‐Peak Ripple Current

Fs = Switching Frequency

Δt = ON time

D = Duty Cycle

If Δi ≈ 30%*Io, then the output inductor is calculated to be

1.5μH. Select L=1.5μH, PCMB065T‐1R5MS, from Cyntec which

provides a compact, low profile inductor suitable for this

application.

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