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LTC4355CS-PBF Datasheet(PDF) 9 Page - Linear Technology |
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LTC4355CS-PBF Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  LTC4355 4355fa applicaTions inForMaTion System Power Supply Failure The LTC4355 automatically supplies load current from the system input supply with the higher voltage. If this supply shorts to ground, reverse current begins to flow through the pass transistor temporarily and the transis- tor begins to turn off. When this reverse current creates –25mV of voltage drop across the drain and source pins of the pass transistor, a fast pulldown circuit engages to drive the gate low faster. The remaining system power supply delivers the load cur- rent through the body diode of its pass transistor until the channel turns on. The LTC4355 ramps the gate up with 20µA, turning on the N-channel MOSFET to reduce the voltage drop across it. When the capacitances at the inputs and output are very small, large changes in current can cause inductive tran- sients that exceed the 100V Absolute Maximum Ratings of the pins. A surge suppressor (TransZorb) at the output will minimize this ringing. 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 less than 1000pF gate capacitance is chosen, a 1000pF compensation capacitor connected across the gate and source pins might be required. Design Example The following design example demonstrates the calcula- tions involved for selecting components in a 36V to 72V system with 5A maximum load current (see Figure 1). First, choose the N-channel MOSFET. The 100V, FDS3672 in the SO-8 package with RDS(ON) = 22mW(max) offers a good solution. The maximum voltage drop across it is: DV = 5A • 22mW = 110mV The maximum power dissipation in the MOSFET is a mere: P = 5A • 110mV = 0.55W Next, select the resistive dividers that guarantee the PWRFLT pins willl not assert when the input supplies are above 36V. The maximum VMONx(TH) is 1.245V and the maximumIMONx(IN)is1µA.Choosea1%toleranceresistor R1 = 12.7k W. Then, I V R MIN I V R MONx TH MONx TH MAX 2 1 1 245 1 = + = ( ) ( )( ) ( ) . 22 7 1 1 100 . ( %) k A A W − + = µ µ Use IR2 to choose R2. R V V A k 2 36 1 245 100 348 = − = . µ W 4355 F01 LTC4355 GND GATE1 GATE2 IN1 OUT IN2 MON1 MON2 SET R1 12.7k R2 340k R3 12.7k R4 340k R5 33k R7 33k R9 33k R6 33k R8 33k D1 D3 D2 D4 D5 GND M1 FDS3672 M2 FDS3672 TO LOAD VIN1 = +48V VIN2 = +48V VDSFLT FUSEFLT1 FUSEFLT2 PWRFLT1 PWRFLT2 F1 7A F2 7A GREEN LEDs PANASONIC LN1351C Figure 1. 36V to 72V/5A Design Example |
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