IR3897 IRF [International Rectifier], IR3897 Datasheet - Page 27

• Manufacturer Part Number: IR3897
• Description: 4A HIGHLY INTEGRATED SUPLRBUCK
• Manufacturer: IRF [International Rectifier]

When the control FET turns on in the next cycle, the
capacitor node connected to SW rises to the bus voltage
V
the voltage V
unchanged and the voltage at the Boot pin becomes:
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:
Where:
in
. However, if the value of C1 is appropriately chosen,
D is the Duty Cycle
I
Io is the output current.
For I
I
D
RMS
RMS
Figure 24: Bootstrap circuit to generate Vc voltage

is the RMS value of the input capacitor current.
o
V
V

=4A and D = 0.1, the I
in
o
I

o
27
V
c

across C1 remains approximately
Boot
D

 
AUGUST 08, 2012 |DATA SHEET | Rev 3.3
V
(1
in

D
V
cc
)

 
V
RMS
D
= 1.8A.
(13)
(14)
Single‐Input Voltage, Synchronous Buck Regulator
(12)
- 27 -
4A Highly Integrated SupIRBuck
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:
Where:
If Δi ≈ 30%*I
1.5μH. Select L=1.5μH, PCMB065T‐1R5MS, from Cyntec which
provides a compact, low profile inductor suitable for this
application.
V
V
Δi = Inductor Peak‐to‐Peak Ripple Current
F
Δt = ON time
D = Duty Cycle
s
in
0
= Switching Frequency
= Output Voltage
= Maximum input voltage
V
in
L

o

, then the output inductor is calculated to be
V
o

V
 
in
L

V

o


i
t

;
V
in
 
 
t
V
o
i F
D
*
TM

s
F
1
s

IR3897
(15)
PD‐97663