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IR3841MPbF
the voltage Vc across C6 remains approximately
unchanged and the voltage at the Boot pin
becomes
V Boot ? V in + V cc ? V D .......... .......... .......... .......... (11)
V IN
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
V cc
Boot
C6
+
Vc
-
value can be reduced to the desired maximum
ripple current in the inductor ( Δ i ) . 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
V in ? V o = L ?
; Δ t = D ?
Δ t F s
V o
L = ( V in ? V o ) ?
V o
SW L
IR3841
PGnd
Fig. 12. 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 upper MOSFET should be provided by the
input capacitor. The RMS value of this ripple is
expressed by:
I RMS = I o ? D ? ( 1 ? D ) .......... .......... ....(12)
D = .......... .......... .......... .. (13)
V in
determined using the following relation:
Δ i 1
.......... .......... .......... . (14)
V in ? Δ i * F s
Where:
V in = Maximum input voltage
V o = Output Voltage
Δ i = Inductor ripple current
F s = Switching frequency
Δ t = Turn on time
D = Duty cycle
If Δ i ≈ 35%( I o ), then the output inductor is
calculated to be 0.91 μ H. Select L =1 μ H.
The MPL105-1R0 from Delta provides a compact,
low profile inductor suitable for this application.
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 =8A and D = 0.15, the I RMS = 2.86A.
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 4x10uF 16V ceramic capacitors
ECJ-3YX1C106K from Panasonic. In addition to
these, although not mandatory, a 1X330uF, 25V
SMD capacitor EEV-FK1E331P may also be
used as a bulk capacitor and is recommended if
the input power supply is not located close to the
converter.
.
06/18/09
18
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