LTC4252-2CMSTR datasheet(26/36 Pages) LINER

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LTC4252-1/LTC4252-2

LTC4252A-1/LTC4252A-2

425212fb

APPLICATIO S I FOR ATIO

VTMRH threshold, TIMER is discharged by 5.8µA. In Figure

14b, when TIMER exceeds VTMRH, GATE pulls down

immediately and the LTC4252 shuts down. In Figure 14c,

multiple momentary faults cause the TIMER capacitor to

integrate and reach VTMRH. GATE pull down follows and

the LTC4252 shuts down. During shutdown, the LTC4252-1

latches TIMER high with a 5.8

µA pull-up current source;

the LTC4252-2 activates a shutdown cooling cycle.

Resetting a Fault Latch (LTC4252-1)

The latched circuit breaker fault of LTC4252-1 benefits

from long cooling time. It is reset by pulling the UV pin

below VUVLO with a switch. Reset is also accomplished by

pulling the VIN pin momentarily below (VLKO – VLKH). A

third reset method involves pulling the TIMER pin below

VTMRL as shown in Figure 15. An initial timing cycle is

skipped if TIMER is used for reset. An initial timing cycle

is generated if reset by the UV pin or the VIN pin.

The duration of the TIMER reset pulse should be smaller

than the time taken to reach 0.2V at SS pin. With a single

pole mechanical pushbutton switch, this may not be

feasible. A double pole, single throw pushbutton switch

removes this restriction by connecting the second switch

to the SS pin. With this method, both the SS and TIMER

pins are released at the same time (see Figure 24).

Shutdown Cooling Cycle (LTC4252-2)

Figure 16 shows the timer behavior of the LTC4252-2. At

time point 2, TIMER exceeds VTMRH, GATE pulls down

immediately and the LTC4252 shuts down. TIMER starts

a shutdown cooling cycle by discharging TIMER with

5.8

µA to the VTMRL threshold. TIMER then charges with

5.8

µA to the VTMRH threshold. There are four 5.8µA

discharge phases and three 5.8

µA charge phases in this

shutdown cooling cycle spanning time points 2 and 3. At

time point 3, the LTC4252 automatic retry occurs with a

start-up cycle. Good thermal management techniques are

highly recommended; power and thermal dissipation must

be carefully evaluated when implementing the automatic

retry scheme.

(14a) Momentary Circuit-Breaker Fault

Figure 14. Circuit-Breaker Timing Behavior (All Waveforms are Referenced to VEE)

(14b) Circuit-Breaker Time Out

(14c) Multiple Circuit-Breaker Fault

CB FAULT

TIMER

GATE

SENSE

VOUT

DRAIN

PWRGD

TIMER

GATE

SENSE

VOUT

DRAIN

PWRGD

TIMER

GATE

SENSE

VOUT

DRAIN

PWRGD

CB FAULT

5.8

µA

5.8

µA

4252-1/2 F14

CB TIMES OUT

VACL

VCB

VACL

VDRNCL

VCB

VACL

VTMRH

VCB

230

µA + 8 • IDRN

VDRNCL

230

µA + 8 • IDRN

230

µA + 8 • IDRN

230

µA + 8 • IDRN

LTC4252-2CMSTR Datasheet(PDF) 26 Page - Linear Technology

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