Datasheet

C5 x 0.6V
t
SS
> 180 Ps +
70 PA
I
OUT
x t
on
'V
IN
C1 =
www.ti.com Design Procedure
Figure 5. Inductor Selection for V
OUT
= 3.3V
Step 4: Determine the value of other components
C1: The function of C1 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 connecting to the VIN pin has finite output
impedance. If the voltage source’s dynamic impedance is high (effectively a current source), C1 supplies
the average input current, but not the ripple current. At the 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, C1 must be capable
of supplying this average load current during the maximum on-time. C1 is calculated from:
(6)
where I
OUT
is the load current, t
on
is the maximum on-time, and ΔV
IN
is the allowable ripple voltage at V
IN
.
In this demonstration board, a 10 µF capacitor is 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 LM3103 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, i.e. the time for the reference voltage at the
regulation comparator and the output voltage to reach their final value. The soft-start time is affected by
the output capacitor, which can lengthen the time, and the charging of C5. The minimum soft-start time is
determined by the following equation:
(7)
In this demonstration board, a 33 nF capacitor is used for C5, and the corresponding soft-start time is
about 600 µs.
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 1 µF for
stability, and should be a good quality, low ESR, ceramic capacitor.
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.
5
SNVA268A–October 2007–Revised April 2013 AN-1678 LM3103 Demonstration Board Reference Design
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