12
IR3621 & (PbF)
www.irf.com
For higher efficiency, low ESR capacitors are recom-
mended.
Choose two Poscap from Sanyo 16TPB47M (16V, 47µF,
70mΩ ) with a maximum allowable ripple current of 1.4A
for inputs of each channel.
Inductor Selection
The inductor is selected based on operating frequency,
transient performance and allowable output voltage ripple.
Low inductor values result in faster response to step
load (high ∆i/∆t) and smaller size but will cause larger
output ripple due to increased inductor ripple current. As
a rule of thumb, select an inductor that produces a ripple
current of 10-40% of full load DC.
For the buck converter, the inductor value for desired
operating ripple current can be determined using the fol-
lowing relation:
VIN - VOUT = L× ; ∆t = D× ; D =
1
fS
VOUT
VIN
∆i
∆t
L = (VIN - VOUT)× ---(7)
VOUT
VIN×∆i×fS
Where:
VIN = Maximum Input Voltage
VOUT = Output Voltage
∆i = Inductor Ripple Current
fS = Switching Frequency
∆t = Turn On Time
D = Duty Cycle
Where:
∆VO = Output Voltage Ripple
∆i = Inductor Ripple Current
∆VO = 3% of VO will result to ESR(2.5V) =16.6mΩ and
ESR(1.8V) =16mΩ
ESR ≤ ---(8)
∆VO
∆IO
The Sanyo TPC series, Poscap capacitor is a good choice.
The 6TPC330M, 330µF, 6.3V has an ESR 40mΩ. Se-
lecting three of these capacitors in parallel for 2.5V out-
put, results to an ESR of ≅ 13.3mΩ which achieves our
low ESR goal. And selecting three of these capacitors in
parallel for 1.8V output, results in an ESR of ≅ 13.3mΩ
which achieves our low ESR goal.
The capacitors value must be high enough to absorb the
inductor's ripple current.
Power MOSFET Selection
The IR3621 uses four N-Channel MOSFETs. The selec-
tion criteria to meet power transfer requirements is based
on maximum drain-source voltage (VDSS), gate-source
drive voltage (VGS), maximum output current, On-resis-
tance RDS(ON) and thermal management.
The both control and synchronous MOSFETs must have
a maximum operating voltage (VDSS) that exceeds the
maximum input voltage (VIN).
Input Capacitor Selection
The 180
0
out of phase will reduce the RMS value of the
ripple current seen by input capacitors. This reduces
numbers of input capacitors. The input capacitors must
be selected that can handle both the maximum ripple
RMS at highest ambient temperature as well as the
maximum input voltage. The RMS value of current ripple
for duty cycles under 50% is expressed by:
For ∆i(2.5V) = 45%(IO(2.5V) ), then the output inductor will
be:
L4 = 1.1µH
For ∆i(1.8V) = 35%(IO(1.8V) ), then the output inductor will
be:
L3 = 1.1µH
Panasonic provides a range of inductors in different val-
ues and low profile for large currents.
Output Capacitor Selection
The criteria to select the output capacitor is normally
based on the value of the Effective Series Resistance
(ESR). In general, the output capacitor must have low
enough ESR to meet output ripple and load transient
requirements, yet have high enough ESR to satisfy sta-
bility requirements. The ESR of the output capacitor is
calculated by the following relationship:
I
RMS
= (I
1
2
D
1
(1-D
1
)+I
2
2
D
2
(1-D
2
)-2I
1
I
2
D
1
D
2
) --- (6)
Where:
I
RMS
is the RMS value of the input capacitor current
D
1
and D
2
are the duty cycle for each output
I
1
and I
2
are the current for each output
For this application the I
RMS
=4.8A
(ESL, Equivalent Series Inductance is neglected)
Choose ETQP6F1R1BFA (1.1µH, 16A, 2.2mΩ) both for
L3 and L4.
For 2-phase application, equation (7) can be used for
calculating the inductors value. In such case the induc-
tor ripple current is usually chosen to be between 10-
40% of maximum phase current.
