LTC1430AIGN datasheet(15/24 Pages) LINER

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LTC1430A

APPLICATI

I FOR ATIO

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 Q1 is OFF to

prevent the low IFB voltage in this condition from activating

the current limit. It is re-enabled a fixed 170ns after Q1

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

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

LTC1430A goes deeper into current limit, it will reach a

point where the Q1 on-time needs to be cut to below 170ns

to control the output current. This conflicts with the

minimum 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,

lengthening the off-time of Q1 while the on-time remains

constant at 170ns. This further reduces the duty cycle,

allowing the LTC1430A 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 Q2 and the LTC1430A 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.

Oscillator Frequency

The LTC1430A includes an onboard current controlled

oscillator which will typically free-run at 200kHz. An

internal 20

µA current is summed with any current in or out

of the FREQSET pin (Pin 11), setting the oscillator fre-

quency to approximately 10kHz/

µA. FREQSET is internally

servoed to the LTC1430A reference voltage (1.265V).

With FREQSET floating, the oscillator is biased from the

LTC1430A will be in full operation. An internal switch

shorts the SS pin to GND during shutdown.

The LTC1430A detects the output current by watching the

voltage at IFB while Q1 is ON. The ILIM amplifier compares

this voltage to the voltage at IMAX (Figure 13). In the ON

state, Q1 has a known resistance; by calculating back-

wards, the voltage generated at IFB by the maximum

output current in Q1 can be determined. As IFB falls below

IMAX, ILIM will begin to sink current from the soft start pin,

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 LTC1430A 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 Q2 from damaging internal structures.

–

ILIM

LTC1430A

1430 F13

RIMAX

PVCC

IMAX

IFB

CSS

µA

VCC

0.1

µF

Figure 13. Current Limit Operation

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.

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