IR3898 Datasheet, PDF (27/45 Pages) International Rectifier – 6A HIGHLY INTERGRATED SUPLRBUCK

6A Highly Integrated SupIRBuck^TM
Single-Input V_-o₂lt₇a_-ge, Synchronous Buck Regulator
PD-97662
IR3898
When the control FET turns on in the next cycle, the
capacitor node connected to SW rises to the bus voltage
V_in. However, if the value of C1 is appropriately chosen,
the voltage V_c across C1 remains approximately
unchanged and the voltage at the Boot pin becomes:
V_Boot ≅ V_in + V_cc −V_D
(12)
Cvin
+ V_D -
V_cc
IR3898
VIN
Boot
+
C1 Vc
-
SW
L
PGnd
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:
I_RMS= I_o ∗ D ∗ (1− D )
(13)
D= ^Vo
(14)
V_in
Where:
D is the Duty Cycle
I_RMS is the RMS value of the input capacitor current.
Io is the output current.
For I_o=6A and D = 0.1, the I_RMS = 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:
V_in −V_o =
L ∗ ^∆i ;
∆t
∆t = D ∗ ¹
F_s
(15)
L=
(V_in
− V_o
)
∗
V_in
V_o
∗ ∆i
*
F_s
Where:
V_in = Maximum input voltage
V₀ = Output Voltage
Δi = Inductor Peak-to-Peak Ripple Current
F_s = Switching Frequency
Δt = On time
D = Duty Cycle
If Δi ≈ 30%*I_o, then the output inductor is calculated to be
1.0μH. Select L=1.0μH, SPM6550T-1R0M, from TDK which
provides a compact, low profile inductor suitable for this
application.
27 AUGUST 08, 2012 | DATA SHEET | Rev 3.3