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LT1640LIN8 Datasheet(PDF) 8 Page - Linear Technology |
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LT1640LIN8 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LT1640L/LT1640H APPLICATIONS INFORMATION The waveforms are shown in Figure 6b. When the power pins make contact, they bounce several times. While the contacts are bouncing, the LT1640 senses an undervoltage condition and the GATE is immediately pulled low when the power pins are disconnected. Once the power pins stop bouncing, the GATE pin starts to ramp up. When Q1 turns on, the GATE voltage is held constant by the feedback network of R3 and C2. When the DRAIN voltage has finished ramping, the GATE pin then ramps to its final value. Electronic Circuit Breaker The LT1640 features an electronic circuit breaker function that protects against short circuits or excessive supply currents. By placing a sense resistor between the VEE and SENSE pin, the circuit breaker will be tripped whenever the voltage across the sense resistor is greater than 50mV for more than 3 µs as shown in Figure 7. 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 over current condition occurs. Vi is the voltage across the sense resistor when the short current or over current 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. VEE VDD LT1640L /LT1640H PWRGD/ PWRGD SENSE C1 0.033 µF 24V CL 100 µF 100V Q1 IRF530 R2 10 Ω 5% R3 10k 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 R7 C3 6 8 7 1 GATE DRAIN 1640 F08 + Figure 8. Extending the Short-Circuit Protection Delay 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 1640 F07 Figure 7. Short-Circuit Protection Waveforms |
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