PCMB065T-R68MS datasheet(28/46 Pages) IRF

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JANURARY 18, 2013 | DATA SHEET | Rev 3.5

IR3898

6A Highly Integrated SupIRBuck

Single-Input Voltage, Synchronous Buck Regulator

PD-97662

(9)

ref





 







When an external resistor divider is connected to the

output as shown in Fig. 23.

(10)

ref















For the calculated values of R5 and R6, see feedback

compensation section.

IR3898

Vout

Figure 23: Typical application of the IR3898

for programming the output voltage

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 to 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 (C1). The operation of the

circuit is as follows: When the sync FET is turned on, the

capacitor node connected to SW is pulled down to

ground. The capacitor charges towards Vcc through the

internal bootstrap diode (Fig.24), which has a forward

voltage drop VD. The voltage Vc across the bootstrap

capacitor C1 is approximately given as:

(11)



 

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





 

IR3898

VIN

Vcc

Boot

PGnd

+ VD -

Cvin

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=6A 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

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