LTM4656 datasheet(11/28 Pages) AD

11 / 28 page

LTM4656/LTM4656-1

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

When the TMR pin reaches 1.275V, the FLT pin is latched

low as an early warning of impending shutdown, then it

continues unabated until the TMR reaches 1.375V, pro-

ducing an early warning period given by:

tFLT = 0.01µF •

1.275V – 0.5V

ITMR

tWARN = 0.01µF •

1.375V – 1.275V

ITMR

ITMR taken from Figure 1.

Because ITMR is a function of VIN–SENSE1, the exact time

in current limit depends upon the input waveform and the

time required for the output current to come into regula-

tion. Testing of the overall solution should be verified, and

compared to the MOSFET SOA curves.

COOLDOWN PHASE

Cooldown behavior is initiated by overcurrent. During

the cooldown phase, the timer continues to charge from

1.375V to 4.3V with 2µA, and then discharges back down

to 0.5V with 2µA, for a total equivalent voltage swing of

6.725V. The cooldown time is given by:

tCOOL = 0.01µF •

6.725V

2µA

= 33.6ms

This long cool time assures that during retry the MOSFET

does not overheat.

The LTM4656 will auto-retry at the end of the cooldown

phase.Retryisautomaticallyinitiated.Thecooldownphase

may be interrupted in the LTM4656 by pulling SHDN low

for at least 10ms.

The FLT pin goes high in shutdown and is cleared high

when power is first applied to VIN.

Brief overcurrent conditions interrupt the operation of

the timer. If the TMR pin has not yet reached 1.275V

when fault drops out of current limit, the timer capacitor

is discharged back to 0.5V with a 2µA current sink. If the

TMR voltage crosses 1.275V, then FLT is set low. If the

overcurrent abates before reaching 1.375V, the timer

capacitor discharges with 2µA back to 0.5V, whereupon

FLT resets high. If several short overcurrent events occur

in rapid succession, the timer capacitor will integrate the

charging and discharging currents. Figure 20 shows an

overcurrent fault wave from and a retry cycle.

MIN MOSFET SOA CURVE

Figure 3 shows the MIN MOSFET SOA curve. This curve

can be compared to the period of time the MIN Power

MOSFET stays on during a fault condition with an over-

current flowing through MIN, and the worse-case voltage

across the MIN MOSFET.

Figure 3. MIN Internal MOSFET SOA Curves

10s

100ms

10ms

1ms

TA = 25°C

SINGLE PULSE

TJ = MAX RATED

THIS AREA IS

LIMITED BY

RDS(ON)

RθJA = 125°C/W

VDS, DRAIN TO SOURCE VOLTAGE (V)

0.01

0.1

100 200

0.01

0.1

100

400

4656 F03

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