C5 x 0.8V
t
SS
=
8
P
A
I
OUT
x t
on
'
V
IN
C
IN
=
Design Procedure
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Step 4: Determine the value of other components
C1 and C2: The function of the input capacitor is to supply most of the main MOSFET current during the
on-time, and limit the voltage ripple at the VIN pin, assuming that the voltage source feeding to the VIN pin
has finite output impedance. If the voltage source’s dynamic impedance is high (effectively a current
source), the input capacitor supplies the average input current, but not the ripple current. At maximum
load current, when the main MOSFET turns on, the current to the VIN pin suddenly increases from zero to
the lower peak of the inductor’s ripple current and ramps up to the higher peak value. It then drops to zero
at turn-off. The average current during the on-time is the load current. For a worst case calculation, the
input capacitor must be capable of supplying this average load current during the maximum on-time. The
input capacitor is calculated from:
(6)
where:
C
IN
= C1 + C2 is the input capacitor
I
OUT
is the load current
t
on
is the maximum on-time
Δ
V
IN
is the allowable ripple voltage at V
IN
In this demonstration board, two 10 µF capacitors connecting in parallel are used.
C3: C3’s purpose is to help avoid transients and ringing due to long lead inductance at the VIN pin. A low
ESR 0.1 µF ceramic chip capacitor located close to the LM3102 is used in this demonstration board.
C4: A 33 nF high quality ceramic capacitor with low ESR is used for C4 since it supplies a surge current to
charge the main MOSFET gate driver at turn-on. Low ESR also helps ensure a complete recharge during
each off-time.
C5: The capacitor at the SS pin determines the soft-start time, that is, the time for the reference voltage at
the regulation comparator and the output voltage to reach their final value. The time is determined from
the following equation:
(7)
In this demonstration board, a 10 nF capacitor is used, and the corresponding soft-start time is about
1 ms.
C8: The capacitor on the V
CC
output provides not only noise filtering and stability, but also prevents false
triggering of the V
CC
UVLO at the main MOSFET on/off transitions. C8 should be no smaller than 680 nF
for stability, and should be a good quality, low ESR, ceramic capacitor. In this demonstration board, a 1 µF
capacitor is used.
C9: If the output voltage is higher than 1.6V, C9 is needed in the Discontinuous Conduction Mode to
reduce the output ripple. In this demonstration board, a 10 nF capacitor is used.
C10 and C11: The output capacitor should generally be no smaller than 10 µF. Experimentation is usually
necessary to determine the minimum value for the output capacitor, as the nature of the load may require
a larger value. A load which creates significant transients requires a larger output capacitor than a fixed
load. In this demonstration board, two 47 µF capacitors are connected in parallel to provide a low output
ripple.
C12: C12 is a small value ceramic capacitor located close to the LM3102 to further suppress high
frequency noise at V
OUT
. A 100 nF capacitor is used in this demonstration board.
4
AN-1646 LM3102 Demonstration Board Reference Design
SNVA248 – October 2007
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