LTC4253 datasheet(29/36 Pages) LINER

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LTC4252-1/LTC4252-2

LTC4252A-1/LTC4252A-2

425212fd

APPLICATIONS INFORMATION

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

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

point 9, the SENSE voltage falls below VCB and TIMER

deactivates.

Large values of CSS can cause premature circuit breaker

time out as VACL(t) may exceed the VCB potential during

the circuit breaker delay. The load capacitor is unable to

achieve full charge in one GATE start-up cycle. A more

serious side effect of large CSS values is SOA duration

may be exceeded during soft-start into a low impedance

load. A soft-start voltage below VCB will not activate the

circuit breaker TIMER.

Power Limit Circuit Breaker

Figure 19 shows the LTC4252A-1 in a power limit circuit

breaking application. The SENSE pin is modulated by the

board supply voltage, VSUPPLY. The D1 Zener voltage, VZ

is set to be the same as the low supply operating voltage,

VSUPPLY(MIN) = 43V. If the goal is to have the high supply

operating voltage, VSUPPLY(MAX) = 71V giving the same

power at VSUPPLY(MIN), then resistors R4 and R6 are

selected using the ratio:

VCB

VSUPPLY(MAX)

(16)

If R6 is 27Ω, R4 is 38.3k. The peak circuit breaker power

limit is:

POWERMAX =

VSUPPLY(MIN) +VSUPPLY(MAX)

()2

4• VSUPPLY(MIN) •VSUPPLY(MAX)

•POWERSUPPLY(MIN)

=1.064 •POWERSUPPLY(MIN)

(17)

when

VSUPPLY = 0.5 • (VSUPPLY(MIN) + VSUPPLY(MAX)) = 57V.

The peak power at the fault current limit occurs at the supply

overvoltage threshold. The fault current limited power is:

POWERFAULT =

VSUPPLY

•VACL –VSUPPLY –VZ

()•R6

⎛

⎝⎜

⎞

⎠⎟

(18)

TIMER

GATE

SENSE

DRAIN

VTMRH

VDRNCL

VACL

VCB

VDRNL

VGS(th)

VIN – VGATEH

VTMRL

425212 F18

PWRGD

5.8μA

58μA

TIMER

GATE

SENSE

DRAIN

VTMRH

VDRNCL

VCB

VACL

VDRNL

VGS(th)

VIN – VGATEH

VTMRL

PWRGD

5.8μA

58μA

12 34 567

8 9

END OF INTIAL TIMING CYCLE

12 3 4 5 6

8 9

END OF INTIAL TIMING CYCLE

CB + VOS)

ACL + VOS)

CB + VOS)

ACL + VOS)

DRN

Figure 18. Soft-Start Timing (All Waveforms Are Referenced to VEE)

(18a) Without External CSS

(18b) With External CSS

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