12

LTC1430

Figure 10. Current Limit Operation

–

+

LTC1430

LTC1430 • F10

SS

12

12

µA

1k

0.1

µF

S

APPLICATI

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

state until the overcurrent condition disappears. At this

internal pull-up will recharge the soft-start capacitor,

an external 1k series resistor to prevent voltage transients

at the drain of M2 from damaging internal structures.

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

down more quickly and preventing damage to the output

components.

the current limit. It is re-enabled a fixed 170ns after M1

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-

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.

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.

LTC1430

LTC1430 • F09

12

µA

COMP

SS

FB

Figure 9. Soft-Start Clamps COMP Pin