Datasheet

t
SS
=
8 PA
C
SS
x 0.8V
C
IN
=
'V
IN
I
OUT
x t
on
LM3102
SNVS515G –SEPTEMBER 2007–REVISED JANUARY 2012 www.ti.com
Figure 29. Inductor selection for V
OUT
= 0.8V
Figure 28 and Figure 29 show curves on inductor selection for various V
OUT
and R
ON
. For small R
ON
, according
to Equation 8, V
IN
is limited. Some curves are therefore limited as shown in the figures.
C
VCC
: 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. C
VCC
should be no smaller than 680 nF for
stability, and should be a good quality, low ESR, ceramic capacitor.
C
OUT
and C
OUT3
: C
OUT
should generally be no smaller than 10 µF. Experimentation is usually necessary to
determine the minimum value for C
OUT
, as the nature of the load may require a larger value. A load which
creates significant transients requires a larger C
OUT
than a fixed load.
C
OUT3
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 recommended.
C
IN
and C
IN3
: The function of C
IN
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), C
IN
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, C
IN
must be capable of supplying this average load current during the maximum on-time. C
IN
is calculated from:
where
• I
OUT
is the load current
• t
on
is the maximum on-time
• ΔV
IN
is the allowable ripple voltage at V
IN
(10)
C
IN3
’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 recommended.
C
BST
: A 33 nF high quality ceramic capacitor with low ESR is recommended for C
BST
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.
C
SS
: 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 time is determined from the following
equation:
(11)
C
FB
: If the output voltage is higher than 1.6V, C
FB
is needed in the Discontinuous Conduction Mode to reduce the
output ripple. The recommended value for C
FB
is 10 nF.
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