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LTC4218IGN-TRPBF Datasheet(PDF) 10 Page - Linear Technology |
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LTC4218IGN-TRPBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  LTC4218 10 4218fb R6 150k R7 20k ADC R2 224k C1 0.1 μF R3 20k 12V 12V 4218 F01 R8 10k R1 10 Ω CT 0.1 μF CL 330 μF VOUT 12V 3A VDD UV FB PG GND IMON RSET 20k RS 2m Ω Q1 Si7108DN RMON 20k ISET CGATE 0.01 μF RGATE 1k GATE SOURCE SENSE– SENSE+ LTC4218GN OV INTVCC TIMER FLT + R4 140k R5 20k APPLICATIONS INFORMATION The typical LTC4218 application is in a high availability system that uses a positive voltage supply to distribute power to individual cards. The basic application circuit is shown in Figure 1. External component selection is discussed in detail in the following sections. Figure 2. Supply Turn-On Figure 1. 3A, 12V Card Resident Application Turn-On Sequence The power supply on a board is controlled by placing an external N-channel pass transistor (Q1) in the power path. Note the sense resistor (RS) detects current and the capacitor (CGATE) controls gate slew rate. Resistor R1 prevents high frequency oscillations in Q1 and resistor RGATE isolates CGATE during fast turn-off. Several conditions must be present before the external pass transistor can be turned on. First, the supply VDD must exceed its undervoltage lockout level. Next, the internally generated supply INTVCC must cross its 2.65V undervoltage threshold. This generates a 25μs power- on-reset pulse which clears the logic’s fault register and initializes internal latches. After the power-on-reset pulse, the LTC4218 will go through the following sequence. First, the UV and OV pins must indicate that the input power is within the acceptable range. All of these conditions must be satisfied for a duration of 100ms to ensure that any contact bounce during the insertion has ended. The pass transistor is turned on by charging up the GATE with a 24μA charge pump generated current source (Figure 2). The voltage at the GATE pin rises with a slope equal to 24μA/CGATE and the supply inrush current is set at: I C C µA INRUSH L GATE = •24 When the GATE voltage reaches the MOSFET threshold voltage, the switch begins to turn on and the SOURCE voltage follows the GATE voltage as it increases. Once SOURCE reaches VDD, the GATE will ramp up until clamped by the 6.15V zener between GATE and SOURCE. As the SOURCE pin voltage rises, so will the FB pin which is monitoring it. If the voltage across the current sense resistor (RS)getstoohigh,theinrushcurrentwillbelimited by the internal current limiting circuitry. Once the FB pin crosses its 1.235V threshold and the GATE to SOURCE voltage exceeds 4.2V, the PG pin will cease to pull low and indicate that the power is good. Turn-Off Sequence The switch can be turned off by a variety of conditions. A normal turn-off is initiated by the UV pin going below its 1.235V threshold. Additionally, several fault conditions will turn off the switch. These include an input overvoltage (OV pin) and overcurrent circuit breaker (SENSE pin). Normally, the switch is turned off with a 250μA current pulling down the GATE pin to ground. With the switch turned off, the SOURCE pin voltage drops which pulls the FB pin below t1 t2 SLOPE = 24 μA/CGATE GATE SOURCE VDD + 6.15 VDD 4218 F02 |
Similar Part No. - LTC4218IGN-TRPBF |
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Similar Description - LTC4218IGN-TRPBF |
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