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IR3865MPBF
DESIGN EXAMPLE
Design Criteria:
2 Δ I ?
1.5 V ? ? 1 6 V - 1.5 V ?
1 6 V ? 2 . 2 ? H ? 300k Hz
? 2.1 A
Input Voltage, VIN = 7V to 16V
Output Voltage, VOUT = 1.5V
Choose an input capacitor:
1 ? 2 . 1 A / 2 ?
Switching Frequency, Fs = 300kHz
Inductor Ripple Current, 2ΔI = 2A
Maximum Output Current, IOUT = 10A
I IN_RMS ? 10 A ?
1.5 V
1 6 V
? 1 ? ? ? ? ? 3 . 1 A
3 ? 10 A ?
2
Over Current Trip, IOC = 15A
Overshoot Allowance for 5A Load Step Down,
A
Panasonic
10μF
(ECJ3YB1E106M)
VOS = VOUT + 75mV
Undershoot Allowance for 5A Load Step Up,
VDROP = 75mV
accommodates 6 Arms of ripple current at
300kHz. Due to the chemistry of multilayer
ceramic capacitors, the capacitance varies over
temperature and operating voltage, both AC and
Find R FF :
R FF ?
1.5 V
1 V ? 20 pF ? 300k Hz
? 2 50 k ?
DC. One 10μF capacitor is recommended. In a
practical solution, one 1μF capacitor is required
along with 10μF. The purpose of the 1μF
capacitor is to suppress the switching noise and
R SET ?
? 8.4k ?
Pick a standard value 255 kΩ, 1% resistor.
Find R SET :
10.7m ? ? 15 A
1 9 ? A
Pick a 8.45 kΩ, 1% standard resistor.
Find a resistive voltage divider for V OUT = 1.5V:
deliver high frequency current.
Choose an output capacitor:
To meet the undershoot and overshoot
specification, equation 7b and 8 will be used to
calculate the minimum output capacitance. As a
result, 240μF will be needed for 5A load
removal. To meet the stability requirement,
V FB ?
R 2
R 2 ? R 1
? V OUT ? 0.5 V
choose an output capacitors with ESR larger
than 10 mΩ . Combine those two requirements,
one can choose a set of output capacitors from
V OUT ? ? V IN ? V OUT ?
1.5 V ? ? 16 V - 1.5 V ?
R 6 ?
2 . 2 ? H
R 2 = 1.40k Ω , R 1 = 2.80 kΩ, both 1% standard
resistors .
Choose the soft start capacitor:
Once the soft start time has chosen, such as
1000us to reach to the reference voltage, a
22nF for C SS is used to meet 1000μs.
Choose an inductor to meet the design
specification:
L ?
V IN ? 2 ΔI ? F s
?
16 V ? 2 A ? 300k Hz
? 2.3 ? H
Choose an inductor with the lowest DCR and
AC power loss as possible to increase the
overall system efficiency. For instance, choose
a PIMB104T-2R2MS-39 manufactured by
CYNTEC. The inductance of this part is 2.2μH
and has 6.0 mΩ DCR. Ripple current needs to
be recalculated using the chosen inductor.
8/8/2012 Rev3.1
manufactures such as SP-Cap (Specialty
Polymer Capacitor) from Panasonic or POSCAP
from Sanyo. A 330μF (EEFUE0E331XR) from
Panasonic with 10 mΩ ESR will meet both
requirements.
If an all ceramic output capacitor solution is
desired, the external slope injection circuit
composed of R6, C13, and C14 is required as
explained in the Stability Consideration Section.
In this design example, we can choose C14 =
1nF and C13 = 100nF. To calculate the value of
R6 with PIMB104T-2R2MS-39 as our inductor:
L
DCR ? C 13
?
6 . 0 m ? ? 100 nF
? 3 . 67 k ?
Pick a standard value for R6 = 3.65 kΩ .
16
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