IR3895 Datasheet, PDF(27/43 Page) International Rectifier

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

IR3895 Datasheet, PDF (27/43 Pages) International Rectifier – 16A HIGHLY INTERGRATED SUPLRBUCK

◁

16A Highly Integrated SupIRBuck

SingleâInput V-o2l7ta-Pge, Synchronous Buck Regulator

PDâ97746

IR3895

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:

VBoot ï Vin ï«Vcc ïVDï ï ï ï ï ï ï ï ï ï (12)

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:

IRMS ï½ Io ïª D ïª (1ï D)ï ï ï ï ï ï ï ï ï ï ï ï (13)

ï½

Vin

ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï (14)

Where:

D is the Duty Cycle

IRMS is the RMS value of the input capacitor current.

Io is the output current.

For Io=16A and D = 0.1, the IRMS = 4.8A.

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 5x10uF, 25V

ceramic capacitors, C3216X5R1E106M from TDK.

In addition to these, although not mandatory,

a 1x330uF, 25V SMD capacitor EEVâFK1E331P 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 (Îi). 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:

Vin

ï Vo

ï½

Lïª

ïi

ït

;

ït

ï½

ïª

ï½

ï¨Vin

ï Vo

ï©

ïª

Vin

ïª ïi

ï ï ï ï ï ï ï ï ï ï ï ï (15)

Where:

Vin = Maximum input voltage

V0 = Output Voltage

Îi = Inductor PeakâtoâPeak Ripple Current

Fs = Switching Frequency

Ît = On time for Control FET

D = Duty Cycle

If Îi â 30%*Io, then the output inductor is calculated to

be 0.38Î¼H. Select L=0.4Î¼H, 59PR9875N, from VITEC

which provides a compact, low profile inductor suitable

for this application.

27 AUGUST 08, 2012 | DATA SHEET| Rev 3.1

▷