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LTC4252-2IMSTR Datasheet(PDF) 29 Page - Linear Technology |
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LTC4252-2IMSTR Datasheet(HTML) 29 Page - Linear Technology |
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29 / 36 page  LTC4252-1/LTC4252-2 LTC4252A-1/LTC4252A-2 29 425212fb 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 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: APPLICATIO S I FOR ATIO R R V V CB SUPPLY MAX 6 4 = () (16) If R6 is 27 Ω, R4 is 38.3k. The peak circuit breaker power limit is: POWER VV VV POWER POWER MAX SUPPLY MIN SUPPLY MAX SUPPLY MIN SUPPLY MAX SUPPLY MIN SUPPLY MIN = + () = () ( ) () ( ) () () •• • .• 2 4 1 064 (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: POWER V R VV V R R FAULT SUPPLY S ACL SUPPLY Z = () ⎛ ⎝⎜ ⎞ ⎠⎟ •– – • 6 4 (18) Figure 18. Soft-Start Timing (All Waveforms are Referenced to VEE) (18a) Without External CSS (18b) With External CSS TIMER GATE SENSE SS DRAIN VTMRH VDRNCL VACL VCB VDRNL VGS(th) VIN – VGATEH VTMRL 4252-1/2 F18 PWRGD 5.8 µA 58 µA 58 µA TIMER GATE SENSE SS DRAIN VTMRH VDRNCL VCB VACL VDRNL VGS(th) VIN – VGATEH VTMRL PWRGD 5.8 µA 58 µA 58 µA 12 34 567 7a 8 9 10 11 END OF INTIAL TIMING CYCLE 12 3 4 5 6 7 8 9 10 11 END OF INTIAL TIMING CYCLE 20 • VOS 20 • (VCB + VOS) 20 • (VACL + VOS) 20 • VOS 20 • (VCB + VOS) 20 • (VACL + VOS) 230 µA + 8 • IDRN 230 µA + 8 • IDRN |
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