LTC1430 LINER [Linear Technology], LTC1430 Datasheet - Page 12

• Manufacturer Part Number: LTC1430
• Description: High Power Step-Down Switching Regulator Controller
• Manufacturer: LINER [Linear Technology]

LTC1430
A
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 I
state until the overcurrent condition disappears. At this
time I
internal pull-up will recharge the soft-start capacitor,
restoring normal operation. Note that the I
an external 1k series resistor to prevent voltage transients
at the drain of M2 from damaging internal structures.
The I
the difference between I
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
will generate a larger overdrive at I
down more quickly and preventing damage to the output
components.
The I
prevent the low I
the current limit. It is re-enabled a fixed 170ns after M1
12
PPLICATI
LIM
LIM
FB
amplifier pulls current out of SS in proportion to
will rise, I
amplifier output is disabled when M1 is OFF to
Figure 9. Soft-Start Clamps COMP Pin
LIM
FB
O
voltage in this condition from activating
amplifier; the LTC1430 will stay in this
LIM
U
C
COMP
SS
S
SS
will stop sinking current and the
FB
I FOR ATIO
and I
U
V
CC
LTC1430
12 A
MAX
LIM
LTC1430 • F09
. Under mild overload
, allowing it to pull SS
W
FB
FB
pin requires
U
turns on; this allows for the I
the I
LTC1430 goes deeper into current limit, it will reach a point
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 I
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.
Under extreme output overloads or short circuits, the I
amplifier will pull the SS pin more than 2V below V
single switching cycle, cutting the duty cycle to zero. At
this point all switching stops, the output current decays
through M2 and the LTC1430 runs a partial soft-start cycle
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, the soft-start cycle repeat frequency can drop into the
low kHz range, causing vibrations in the inductor which
provide an audible alarm that something is wrong.
LIM
amplifier to settle to the correct value. As the
I
MAX
C
SS
SS
Figure 10. Current Limit Operation
12 A
V
CC
12 A
+
R
0.1 F
I
IMAX
LIM
FB
LTC1430
–
LTC1430 • F10
node to slew back high and
I
FB
1k
PV
CC
CC
in a
LIM
LIM