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LT1641-1CS8 Datasheet(PDF) 8 Page - Linear Technology |
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LT1641-1CS8 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LT1641-1/LT1641-2 164112fc Short-Circuit Protection The chip features a programmable foldback current limit with an electronic circuit breaker that protects against short-circuits or excessive supply currents. The current limit is set by placing a sense resistor between VCC (Pin 8) and SENSE (Pin 7). To prevent excessive power dissipation in the pass tran- sistor and to prevent voltage spikes on the input supply during short-circuit conditions at the output, the current folds back as a function of the output voltage, which is sensed at the FB pin (Figure 7). When the voltage at the FB pin is 0V, the current limit circuit drives the GATE pin to force a constant 12mV drop across the sense resistor. As the output voltage at the FB pin increases, the voltage across the sense resistor in- creases until the FB pin reaches 0.5V, at which point the voltage across the sense resistor is held constant at 47mV. The maximum current limit is calculated as: ILIMIT = 47mV/RSENSE For a 0.025 Ω sense resistor, the current limit is set at 1.88A and folds back to 480mA when the output is shorted to ground. The IC also features a variable overcurrent response time. The time required to regulate Q1’s drain current depends on: Q1’s input capacitance; gate capacitor C1 and com- pensation resistor R6; and the internal delay from the SENSE to the GATE pin. Figure 8 shows the delay from a voltage step at the SENSE pin until the GATE voltage starts falling, as a function of overdrive. TIMER The TIMER pin (Pin 5) provides a method for program- ming the maximum time the chip is allowed to operate in current limit. When the current limit circuitry is not active, the TIMER pin is pulled to GND by a 3 µA current source. After the current limit circuit becomes active, an 80 µApull- up current source is connected to the TIMER pin and the voltage will rise with a slope equal to 77 µA/CTIMER as long as the current limit circuit remains active. Once the desired maximum current limit time is set, the capacitor value is: C(nF) = 62 • t(ms). If the current limit circuit turns off, the TIMER pin will be discharged to GND by the 3 µA current source. Whenever the TIMER pin reaches 1.233V, either the inter- nal fault latch is set (LT1641-1) or the autorestart latch is set (LT1641-2). The GATE pin is immediately pulled to GND and the TIMER pin is pulled back to GND by the 3 µA APPLICATIO S I FOR ATIO SHORT PIN VCC TIMER SENSE GATE 87 6 54 2 1 3 D1 CMPZ 5248B GND PWRGD PWRGD FB R5 10 Ω 5% ON R1 49.9k 1% VIN 24V GND 1641-1 F05 R2 3.4k 1% C2 0.68 µF R6, 1k, 5% RS 0.025 Ω C1 10nF R3 59k 1% R4 3.57k 1% R7 24k 5% + CL VOUT LT1641-1 Q1 IRF530 Figure 5. Typical Application Figure 6. Power-Up Waveforms |
Similar Part No. - LT1641-1CS8 |
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Similar Description - LT1641-1CS8 |
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