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LT4256-3IGN датащи(PDF) 14 Page - Linear Technology |
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LT4256-3IGN датащи(HTML) 14 Page - Linear Technology |
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14 / 20 page ![]() LT4256-3 14 42563f APPLICATIO S I FOR ATIO Therefore, using RETRY only, the LT4256-3 will either latch off after an overcurrent fault condition or it will go into a hiccup mode. Power Good Detection The LT4256-3 includes a comparator for monitoring the output voltage. The output voltage is sensed through the FB pin via an external resistor string. The comparator’s output (PWRGD) is an open collector capable of operating from a pull-up as high as 80V. PWRGD can be used to directly enable/disable a power module with an active high enable input. Figure 12 shows how to use PWRGD to control an active low enable input power module. Signal inversion is accomplished by tran- sistor Q2 and R10. The thresholds for the FB pin are 4.45V (low to high) and 3.99V (high to low). To calculate the PWRGD thresholds, use the following equations: R V kR R k V 8= V • R9, high to low (7) (8a) = 4.45V 1+ R8 R9 , low to high (8b) THPWRGD THPWRGD 399 1 20 8 9 200 . – Ω≤ + ≤ Ω OPEN Pin/Open FET Detection OPEN is an output which signals abnormally low load currents. When the voltage across the sense resistor is less than 3mV, the open collector pull-down device is shut off allowing OPEN to be externally pulled high. OPEN is always active when VCC is above 9.8V. If VCC is below 9.8V (the internal UVLO threshold), OPEN is pulled low. Open-circuit MOSFETs are detected with the LT4256-3 by monitoring the voltage across R5 with OPEN while moni- toring the output voltage with PWRGD. An open FET condition is signalled when OPEN is high and PWRGD is low (after the part has completed its start-up cycle). Figure 12. Active Low Enable PWRGD Application 4256 F12 R5 100m Ω LT4256-3 SENSE 13 10 5 7 8 16 15 1 2 4 9 VCC GATE FB 12 VOUT PWRGD RETRY UV OV TIMER GND VCC 24V (SHORT PIN) Q1 IRFZ34VS D1 CMPZ5241BS 11V R3 4.02k R2 4.02k R1 32.4k R7 100 Ω R6 10 Ω VOUT 24V 400mA VLOGIC R4 27k R8 14k CL R10 51k C2 33nF C3 0.01 µF C1 10nF OPEN UV = 20V OV = 40V PWRGD = 18V R9 4.02k Q2 ZN3904 PWRGD GND D2 SMAT70A + |
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