LTC1430
PPLICATI
O U
W
U
A
S I FOR ATIO
causing the voltage at SS to fall. As SS falls, it will limit the
output duty cycle, limiting the current at the output.
Eventually the system will reach equilibrium, where the
pull-up current at the SS pin matches the pull-down
current in the ILIM amplifier; the LTC1430 will stay in this
state until the overcurrent condition disappears. At this
time IFB will rise, ILIM will stop sinking current and the
internal pull-up will recharge the soft-start capacitor,
restoring normal operation. Note that the IFB pin requires
an external 1k series resistor to prevent voltage transients
at the drain of M2 from damaging internal structures.
turns on; this allows for the IFB node to slew back high and
the ILIM amplifier to settle to the correct value. As the
LTC1430goesdeeperintocurrentlimit,itwillreachapoint
where the M1 on-time needs to be cut to below 170ns to
control the output current. This conflicts with the mini-
mum settling time needed for proper operation of the ILIM
amplifier. At this point, a secondary current limit circuit
begins to reduce the internal oscillator frequency, length-
ening the off-time of M1 while the on-time remains con-
stant at 170ns. This further reduces the duty cycle, allow-
ing the LTC1430 to maintain control over the output
current.
The ILIM amplifier pulls current out of SS in proportion to
the difference between IFB and IMAX. Under mild overload
conditions, the SS pin will fall gradually, creating a time
delay before current limit takes effect. Very short, mild
overloads may not trip the current limit circuit at all.
Longer overload conditions will allow the SS pin to reach
a steady level, and the output will remain at a reduced
voltage until the overload is removed. Serious overloads
willgeneratealargeroverdriveatILIM,allowingittopullSS
down more quickly and preventing damage to the output
components.
Under extreme output overloads or short circuits, the ILIM
amplifier will pull the SS pin more than 2V below VCC in a
single switching cycle, cutting the duty cycle to zero. At
this point all switching stops, the output current decays
throughM2andtheLTC1430runsapartialsoft-startcycle
and restarts. If the short is still present the cycle will
repeat. Peak currents can be quite high in this condition,
but the average current is controlled and a properly
designed circuit can withstand short circuits indefinitely
with only moderate heat rise in the output FETs. In addi-
tion, thesoft-startcyclerepeatfrequencycandropintothe
low kHz range, causing vibrations in the inductor which
provide an audible alarm that something is wrong.
The ILIM amplifier output is disabled when M1 is OFF to
preventthelowIFBvoltageinthisconditionfromactivating
the current limit. It is re-enabled a fixed 170ns after M1
0.1µF
PV
CC
R
IMAX
LTC1430
1k
I
I
FB
MAX
COMP
FB
V
CC
+
–
12µA
I
LIM
12µA
SS
V
CC
C
SS
12µA
SS
LTC1430 • F09
C
SS
Figure 9. Soft-Start Clamps COMP Pin
LTC1430
LTC1430 • F10
Figure 10. Current Limit Operation
12
Linear [ Linear ]
