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LT1640ALCS8 Datasheet(PDF) 9 Page - Linear Technology |
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LT1640ALCS8 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  9 LT1640AL/LT1640AH VEE VDD LT1640AL PWRGD SENSE C1 150nF 25V CL 100 µF 100V Q1 IRF530 R2 10 Ω 5% R3 18k 5% C2 3.3nF 100V R4 562k 1% R5 9.09k 1% R6 10k 1% R1 0.02 Ω 5% 4 3 2 OV = 71V GND – 48V UV = 37V OV UV 5 C3 6 8 1 GATE DRAIN 1640A F08 + 7 GND (SHORT PIN) * * DIODES INC. SMAT70A R7 4 3 2 1 Figure 8. Extending the Short-Circuit Protection Delay Figure 7. Short-Circuit Protection Waveforms 1640A F07 Electronic Circuit Breaker The LT1640A features an electronic circuit breaker func- tion that protects against short circuits or excessive sup- ply currents. By placing a sense resistor between the VEE and SENSE pin, the circuit breaker will be tripped when- ever the voltage across the sense resistor is greater than 50mV for more than 3 µs as shown in Figure 7. Note that the circuit breaker threshold should be set sufficiently high to account for the sum of the load current and the inrush current. If the load current can be controlled by the PWRGD/PWRGD pin (as in Figure 6a), the threshold can be set lower, since it will never need to accommodate inrush current and load current simultaneously. When the circuit breaker trips, the GATE pin is immediately pulled to VEE and the external N-channel turns off. The GATE pin will remain low until the circuit breaker is reset by pulling UV low, then high or cycling power to the part. If more than 3 µs deglitching time is needed to reject current noise, an external resistor and capacitor can be added to the sense circuit as shown in Figure 8. R7 and C3 act as a lowpass filter that will slow down the SENSE pin voltage from rising too fast. Since the SENSE pin will source current, typically 20 µA, there will be a voltage drop on R7. This voltage will be counted into the circuit breaker trip voltage just as the voltage across the sense resistor. A small resistor is recommended for R7. A 100 Ω for R7 will cause a 2mV error. The following equation can be used to estimate the delay time at the SENSE pin: tR C In Vt Vt VV t O iO = –• • – () – ( ) –( ) 1 Where V(t) is the circuit breaker trip voltage, typically 50mV. V(tO) is the voltage drop across the sense resistor before the short or overcurrent condition occurs. Vi is the voltage across the sense resistor when the short current or overcurrent is applied on it. Example: A system has a 1A current load and a 0.02 Ω sense resistor is used. An extended delay circuit needs to be designed for a 50 µs delay time after the load jumps to 5A. In this case: V(t) = 50mV V(tO) = 20mV Vi = 5A • 0.02Ω = 100mV If we choose R = 100 Ω, we will get C = 1µF. APPLICATIO S I FOR ATIO |
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