Datasheet

LT3800
12
3800fc
APPLICATIONS INFORMATION
The soft-start cycle should be evaluated to verify that the
reduced R
SS
value allows operation without excessive
modulation of the V
C
pin before fi nalizing the design.
If the V
C
pin has an excessive ripple component during the
soft-start cycle, converter output ripple should be reduced
or R
SS
increased. Reduction in converter output ripple is
typically accomplished by increasing output capacitance
and/or reducing output capacitor ESR.
External Current Limit Foldback Circuit
An additional start-up voltage offset can occur during the
period before the LT3800 soft-start circuit becomes active.
Before the soft-start circuit throttles back the V
C
pin in
response to the rising output voltage, current as high as
the peak programmed current limit (I
MAX
) can fl ow in the
inductor. Switching will stop once the soft-start circuit takes
hold and reduces the voltage on the V
C
pin, but the output
voltage will continue to increase as the stored energy in
the inductor is transferred to the output capacitor. With
I
MAX
fl owing in the inductor, the resulting leading-edge
rise on V
OUT
due to energy stored in the inductor follows
the relationship:
V
OUT
= I
MAX
•
L
C
OUT






1/ 2
Inductor current typically doesn’t reach I
MAX
in the few
cycles that occur before soft-start becomes active, but can
with high input voltages or small inductors, so the above
relation is useful as a worst-case scenario.
This energy transfer increase in output voltage is typically
small, but for some low voltage applications with relatively
small output capacitors, it can become signifi cant. The volt-
age rise can be reduced by increasing output capacitance,
which puts additional limitations on C
OUT
for these low
voltage supplies. Another approach is to add an external
current limit foldback circuit which reduces the value of
I
MAX
during start-up.
An external current limit foldback circuit can be easily
incorporated into an LT3800 DC/DC converter application
by placing a 1N4148 diode and a 47k resistor from the
converter output (V
OUT
) to the LT3800’s V
C
pin. This limits
the peak current to 0.25 • I
MAX
when V
OUT
= 0V. A current
limit foldback circuit also has the added advantage of
providing a reduced output current in the DC/DC converter
during short-circuit fault conditions, so a foldback circuit
may be useful even if the soft-start function is disabled.
If the soft-start circuit is disabled by shorting the C
SS
pin
to ground, the external current limit fold-back circuit must
be modifi ed by adding an additional diode and resistor.
The 2-diode, 2-resistor network shown also provides
0.25 • I
MAX
when V
OUT
= 0V.
Soft-Start Characteristic Showing Excessive Ripple Component Desirable Soft-Start Characteristic
V
OUT
V
OUT(SS)
V(V
C
)
250µs/DIV
3800 AI07
V
OUT
V
OUT(SS)
V(V
C
)
250µs/DIV
3800 AI08