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LTC1736

APPLICATIO S I FOR ATIO

Why should you defeat overcurrent latchoff? During the

prototyping stage of a design, there may be a problem with

noise pickup or poor layout causing the protection circuit

to latch off. Defeating this feature will easily allow trouble-

shooting of the circuit and PC layout. The internal short-

circuit and foldback current limiting still remains active,

thereby protecting the power supply system from failure.

After the design is complete, a decision can be made

whether to enable the latchoff feature.

The value of the soft-start capacitor CSS will need to be

scaled with output voltage, output capacitance and load

current characteristics. The minimum soft-start capaci-

tance is given by:

CSS > (COUT )(VOUT)(10–4)(RSENSE)

The minimum recommended soft-start capacitor of CSS =

0.1

µF will be sufficient for most applications.

Fault Conditions: Current Limit and Current Foldback

The LTC1736 current comparator has a maximum sense

voltage of 75mV resulting in a maximum MOSFET current

of 75mV/RSENSE.

The LTC1736 includes current foldback to help further

limit load current when the output is shorted to ground.

The foldback circuit is active even when the overload

shutdown latch described above is defeated. If the output

falls by more than half, then the maximum sense voltage

is progressively lowered from 75mV to 30mV. Under

short-circuit conditions with very low duty cycle, the

LTC1736 will begin cycle skipping in order to limit the

short-circuit current. In this situation the bottom MOSFET

will be conducting the peak current. The short-circuit

ripple current is determined by the minimum on-time

tON(MIN) of the LTC1736 (less than 200ns), the input

voltage, and inductor value:

∆IL(SC) = tON(MIN) VIN/L.

The resulting short circuit current is:

SENSE

LSC

∆

()

The current foldback function is always active and is not

effected by the current latchoff function.

Fault Conditions: Output Overvoltage Protection

(Crowbar)

The output overvoltage crowbar is designed to blow a

system fuse in the input lead when the output of the

regulator rises much higher than nominal levels. This

condition causes huge currents to flow, much greater than

in normal operation. This feature is designed to protect

against a shorted top MOSFET; it does not protect against

a failure of the controller itself.

The comparator (OV in the Functional Diagram) detects

overvoltage faults greater than 7.5% above the nominal

output voltage. When this condition is sensed, the top

MOSFET is turned off and the bottom MOSFET is forced

on. The bottom MOSFET remains on continuously for as

long as the OV condition persists; if VOUT returns to a safe

level, normal operation automatically resumes. Note that

VID controlled output voltage decreases may cause the

overvoltage protection to be momentarily activated. This

will not cause permanent latchoff nor will it disrupt the

desired voltage change.

With soft-latch overvoltage protection, dynamic VID code

changes are allowed and the overvoltage protection tracks

the new VID code, always protecting the load (CPU). If

dynamic VID code changes are anticipated and the mini-

mum load current is light, it may be necessary to either

force continuous operation by pulling FCB low during the

transition to maximize current sinking capability or con-

nect PGOOD to FCB to automatically force continuous

operation during VID transitions.

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