Datasheet LTC4365 (Analog Devices) - 10

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applicaTions inForMaTion
Figure 3 highlights the dependence of the gate drive on VIN
Overvoltage and Undervoltage Protection
and VOUT. When system power is first turned on (SHDN The LTC4365 provides two accurate comparators to moni- low to high, VOUT = 0V), gate drive is at a maximum for all tor for overvoltage (OV) and undervoltage (UV) conditions values of VIN. This helps prevent start-up problems into at VIN. If the input supply rises above the user adjustable heavy loads by ensuring that there is enough gate drive OV threshold, the gate of the external MOSFET is quickly to support the load. turned off, thus disconnecting the load from the input. As VOUT ramps up from 0V, the absolute value of the GATE Similarly, if the input supply falls below the user adjust- voltage remains fixed until VOUT is greater than the lower able UV threshold, the gate of the external MOSFET also of (VIN –1V) or 6V. Once VOUT crosses this threshold, is quickly turned off. Figure 4 shows a UV/OV application gate drive begins to increase up to a maximum of 9.8V for an input supply of 12V. (for VIN ≥ 12V). The curves of Figure 3 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. LTC4365 12V VIN Note that when V R3 UV IN is at the lower end of the operating 1820k COMPARATOR range, the external N-channel MOSFET must be selected UV – with a corresponding lower threshold voltage. UVTH = 3.5V 25mV + R2 0.5V DISCHARGE GATE 243k WITH 50mA SINK 12 OV T = 25°C COMPARATOR IGATE = –1µA OV 10 OVTH = 18V + VIN = 30V R1 25mV 8 59k 0.5V – (V) 6 GATE 4365 F04 ∆V V 4 IN = 12V VIN = 5V
Figure 4. UV, OV Comparators Monitor 12V Supply
2 VIN = 3.3V VIN = 2.5V 0 0 3 6 9 12 15 The external resistive divider allows the user to select VOUT (V) 4365 F03 an input supply range that is compatible with the load at VOUT. Furthermore, the UV and OV inputs have very low
Figure 3. Gate Drive (GATE – VOUT) vs VOUT
leakage currents (typically < 1nA at 100°C), allowing for large values in the external resistive divider. In the applica- Table 1 lists some external MOSFETs compatible with tion of Figure 4, the load is connected to the supply only if different VIN supply voltages. 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
Table 1. Dual MOSFETs for Various Supply Ranges
MOSFET is immediately discharged with a 50mA current
VIN MOSFET VTH(MAX) VGS(MAX) VDS(MAX)
sink, thus isolating the load from the supply. 2.5V SiB914 0.8V 5V 8V 3.3V Si5920 1.0V 5V 8V 5V Si7940 1.5V 8V 12V ≤30V Si4214 3.0V 20V 30V ≤60V Si9945 3.0V 20V 60V 4365fa 10 For more information www.linear.com/LTC4365 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Revision History Related Parts