LTC1430 查看數據表（PDF） - Linear Technology

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LTC1430

High Power Step-Down Switching Regulator Controller

Linear Technology 

LTC1430

APPLICATI 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.

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

will generate a larger overdrive at ILIM, allowing it to pull SS

down more quickly and preventing damage to the output

components.

The ILIM amplifier output is disabled when M1 is OFF to

prevent the low IFB voltage in this condition from activating

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

turns on; this allows for the IFB node to slew back high and

the ILIM amplifier to settle to the correct value. As the

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 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.

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

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.

COMP

LTC1430

FB

VCC

SS

CSS

12µA

LTC1430 • F09

Figure 9. Soft-Start Clamps COMP Pin

0.1µF

RIMAX

PVCC

IMAX

IFB 1k

12µA

ILIM

VCC

12µA

SS

CSS

LTC1430

LTC1430 • F10

Figure 10. Current Limit Operation

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