LTC4253IGN datasheet(28/32 Pages) LINER

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LTC4253

4253f

APPLICATIO S I FOR ATIO

(Refer to Block Diagram)

delay. Careful selection of CT, RD and MOSFET helps

prevent SOA damage in a low impedance fault condition.

Soft-Start

If the SS pin is not connected, this pin defaults to a linear

voltage ramp, from 0V to 2.2V in about 300

µs at GATE

start-up, as shown in Figure 16a. If a soft-start capacitor,

CSS, is connected to this SS pin, the soft-start response is

modified from a linear ramp to an RC response (Equa-

tion 6), as shown in Figure 16b. This feature allows load

current to slowly ramp-up at GATE start-up. Soft-start is

initiated at time point 3 by a TIMER transition from VTMRH

to VTMRL (time points 1 and 2) or by the OV pin falling

below the VOVLO threshold after an OV condition. When the

SS pin is below 0.2V, the analog current limit amplifier

keeps GATE low. Above 0.2V, GATE is released and 50

µA

ramps up the compensation network and GATE capaci-

tance at time point 4. Meanwhile, the SS pin voltage

continues to ramp up. When GATE reaches the MOSFET’s

threshold, the MOSFET begins to conduct. Due to the

MOSFET’s high gm, the MOSFET current quickly reaches

the soft-start control value of VACL(t) (Equation 7). At time

point 6, the GATE voltage is controlled by the current limit

amplifier. The soft-start control voltage reaches the circuit

breaker voltage, VCB at time point 7 and the circuit breaker

TIMER activates. As the load capacitor nears full charge,

load current begins to decline below VACL(t). The current

limit loop shuts off and GATE releases at time point 8. At

time point 9, SENSE voltage falls below VCB and TIMER

deactivates.

RESET PULSE

WIDTH MUST FULLY

DISCHARGE TIMER

12 34

6 7 8 9

4253 F14

LATCHED TIMER RESET BY RESET PULLING HIGH

RESET PULLS LOW

TIMER

GATE

SENSE

RESET

DRAIN

PWRGD1

VTMRH

VIH

VIL

VACL

VCB

VTMRL

VGATEL

20 • (VACL + VOS)

20 • (VCB + VOS)

20 • VOS

µA

VDRNCL

VDRNL

VIN – VGATEH

200

µA + 8 • IDRN

Figure 14. Reset of LTC4253’s Latched Fault (All Waveforms are Referenced to VEE)

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