IR3842AM_15 Datasheet, PDF (19/34 Pages) International Rectifier – HIGHLY EFFICIENT INTEGRATED 6A SYNCHRONOUS BUCK REGULATOR

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)
Fig. 12. Bootstrap circuit to generate
Vc voltage
A bootstrap capacitor of value 0.1uF is suitable
for most applications.
For applications with 21V input voltage, the switch
node may ring above the 25V absolute maximum
voltage rating. To prevent this, in addition to
using best layout practices, it may be necessary
to provide a 10ohm resistor in series with the boot
capacitor.
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 = ^Vo .............................(13)
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 = 0.15, the I_RMS = 2.14 A.
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
Rev 13.0
PD-97510
IR3842AMPbF
advisable to have 2x10uF 25V ceramic capacitors
C3216X5R1E106M from TDK. 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.
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∗
Δi
Δt
;
Δt = D ∗ ¹
F_s
L
=
(V_in
− V_o ) ∗ _Vin
V_o
∗ Δi
* F_s
...............................
(14)
Where:
V_in = Maximum input voltage
V_o = Output Voltage
Δi = Inductor ripple current
F _s= Switching frequency
Δt = Turn on time
D = Duty cycle
If Δi ≈ 42%(I_o), then the output inductor is
calculated to be 1.01μH. Select L=1 μH.
The MPL105-1R0 from Delta provides a compact,
low profile inductor suitable for this application
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