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LTC1735 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1735 Datasheet(HTML) 9 Page - Linear Technology |
9 / 16 page LTC4365 9 4365f Overvoltage and Undervoltage Protection The LTC4365 provides two accurate comparators to moni- tor for overvoltage (OV) and undervoltage (UV) conditions at VIN. If the input supply rises above the user adjustable OV threshold, the gate of the external MOSFET is quickly turned off, thus disconnecting the load from the input. Similarly, if the input supply falls below the user adjust- able UV threshold, the gate of the external MOSFET also is quickly turned off. Figure 3 shows a UV/OV application for an input supply of 12V. Figure 2. Gate Drive (GATE – VOUT) vs VOUT APPLICATIONS INFORMATION Figure 2 highlights the dependence of the gate drive on VIN and VOUT. When system power is first turned on (SHDN low to high, VOUT = 0V), gate drive is at a maximum for all values of VIN. This helps prevent start-up problems into heavy loads by ensuring that there is enough gate drive to support the load. As VOUT ramps up from 0V, the absolute value of the GATE voltage remains fixed until VOUT is greater than the lower of (VIN –1V) or 6V. Once VOUT crosses this threshold, gate drive begins to increase up to a maximum of 9.8V (for VIN ≥ 12V). The curves of Figure 2 were taken with a GATE load of –1μA. If there were no load on GATE, the gate drive for each VIN would be slightly higher. Note that when VIN is at the lower end of the operating range, the external N-channel MOSFET must be selected with a corresponding lower threshold voltage. VIN 12V UVTH = 3.5V OVTH = 18V 4365 F03 DISCHARGE GATE WITH 50mA SINK LTC4365 OV COMPARATOR UV COMPARATOR R3 1820k UV 0.5V 0.5V OV R2 243k R1 59k – + 25mV 25mV Figure 3. UV, OV Comparators Monitor 12V Supply The external resistive divider allows the user to select an input supply range that is compatible with the load at VOUT. Furthermore, the UV and OV inputs have very low leakage currents (typically < 1nA at 100°C), allowing for large values in the external resistive divider. In the applica- tion of Figure 3, the load is connected to the supply only if VIN lies between 3.5V and 18V. In the event that VIN goes above 18V or below 3.5V, the gate of the external N-channel MOSFET is immediately discharged with a 50mA current sink, thus isolating the load from the supply. Table 1 lists some external MOSFETs compatible with different VIN supply voltages. Table 1. Dual MOSFETs for Various Supply Ranges VIN MOSFET VTH(MAX) VGS(MAX) VDS(MAX) 2.5V SiB914 0.8V 5V 8V 3.3V Si5920 1.0V 5V 8V 5V Si7940 1.5V 8V 12V ≤30V Si4230 3.0V 20V 30V ≤60V Si9945 3.0V 20V 60V VOUT (V) 0 0 2 4 6 10 8 12 9 15 4365 F02 36 12 VIN = 30V VIN = 12V VIN = 5V VIN = 3.3V VIN = 2.5V T = 25°C IGATE = –1μA |
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Similar Description - LTC1735 |
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