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LT4256-1CS8 Datasheet(PDF) 10 Page - Linear Technology |
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LT4256-1CS8 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  LT4256-1/LT4256-2 10 425612f APPLICATIO S I FOR ATIO Figure 7. Current Limit Sense Voltage vs Feedback Pin Voltage Figure 8. Response Time to Overcurrent Figure 11 shows how the LT4256-1/LT4256-2 are com- manded to shut off with a logic signal. This is accom- plished by pulling the gate of the open-drain MOSFET, Q2, (tied to the UV pin) high. Short-Circuit Protection The LT4256-1/LT4256-2 features a programmable foldback current limit with an electronic circuit breaker that protects against short circuits or excessive load currents. The current limit is set by placing a sense resistor (R5) between VCC and SENSE. The current limit threshold is calculated as: ILIMIT = 55mV/R5 (5) where R5 is the sense resistor. To limit excessive power dissipation in the pass transistor and to reduce 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 internally on FB. If the LT4256-1/LT4256-2 go into current limit when the voltage on FB is 0V, the current limit circuit drives the GATE pin to force a constant 14mV drop across the sense resistor. As the output at FB increases, the voltage across the sense resistor increases until the FB pin reaches 2V, at which point the voltage across the sense resistor is held constant at 55mV (see Figure 7). For a 0.025 Ω sense resistor, the current limit is set at 2200mA and folds back to 560mA when the output is shorted to ground. Thus, MOSFET peak power dissipation under short-circuit conditions is reduced from 105.6W to 26.5W. See the Layout Considerations section for impor- tant information about board layout to minimize current limit threshold error. The LT4256-1/LT4256-2 also features a variable overcurrent response time. The time required for the part to regulate the GATE voltage is a function of the voltage across the sense resistor connected between VCC and SENSE. This helps to eliminate sensitivity to current spikes and transients that might otherwise unnecessarily trigger a current limit response and increase MOSFET dissipation. Figure 8 shows the response time as a func- tion of the overdrive at SENSE. TIMER TIMER provides a method for programming the maximum time the part 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. When the current limit circuitry becomes active, a 118 µA pull-up current source is connected to TIMER and the voltage will rise with a slope equal to 115 µA/CTIMER as long as the circuitry stays active. Once the desired maximum current limit time is known, the capacitor value is: C nF t ms C A V t [] • [ ]; . • == µ 25 115 465 (6) 14mV 0V 2V FB 4256 F07 55mV VCC – VSENSE 50 100 150 200 4256 F08 12 10 8 6 4 2 VCC – VSENSE (mV) 0 |
Similar Part No. - LT4256-1CS8 |
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Similar Description - LT4256-1CS8 |
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