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LTC4354CDDB Datasheet(PDF) 8 Page - Linear Technology |
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LTC4354CDDB Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page LTC4354 8 4354f APPLICATIO S I FOR ATIO Figure 1. –36V to –72V/5A Design Example LTC4354 DB GA DA GB VSS VB VA –48V_RTN FAULT M2 IRF3710S M1 IRF3710S 4354 F01 VCC R3 33k TO MODULE INPUT RIN 12k 0.5W R2 2k R1 2k D1 LED CIN 1µF 2, 5 6 14 8 3 7 System Power Supply Failure LTC4354 automatically supplies load current from the system supply with the more negative input potential. If this supply is shorted to the return side, a large reverse current flows from its pass transistor. When this reverse current creates –60mV of voltage drop across the drain and source pins of the pass transistor, the LTC4354 drives the gate low fast and turns it off. The remaining system power supply will deliver the load current through the body diode of its pass transistor until the channel turns on. The LTC4354 ramps the gate up and turns on the N-Channel MOSFET to reduce the voltage drop across it, a process that takes less than 1ms depend- ing on the gate charge of the MOSFET. Drain Resistor Two resistors are required to protect the DA and DB pins from transient voltages higher than 80V. In the case when the supply with the lower potential is shorted to the return side due to supply failure, a reverse current flows briefly through the pass transistor to the other supply to dis- charge the output capacitor. This current stores energy in the stray inductance along the current path. Once the pass transistor is turned off, this energy forces the drain termi- nal of the FET high until it reaches the breakdown voltage. If this voltage is higher than 80V, the internal ESD devices at the DA and DB pins might break down and become damaged. The external drain resistors limit the current into the pins and protect the ESD devices. A 2k resistor is recommended for 48V applications. Larger resistor values increase the source drain sense threshold voltage due to the input current at the drain pins. Loop Stability The servo loop is compensated by the parasitic capaci- tance of the power N-channel MOSFET. No further com- pensation components are normally required. In the case when a MOSFET with very small parasitic capacitance is chosen, a 1000pF compensation capacitor connected across the gate and source pins might be required. Design Example The following demonstrates the calculations involved for selecting components in a –36V to –72V system with 5A maximum load current, see Figure 1. First, select the input dropping resistor. The resistor should allow 2mA of current with the supply at –36V. R VV mA k IN ≤ − = (. ) . 36 11 5 2 12 25 The nearest lower 5% value is 12k. |
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