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LTC4365

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

COMPARATOR

1820k

0.5V

243k

59k

–

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

3.3V

Si5920

1.0V

Si7940

1.5V

12V

≤30V

Si4230

3.0V

20V

30V

≤60V

Si9945

3.0V

20V

60V

VOUT (V)

4365 F02

VIN = 30V

VIN = 12V

VIN = 5V

VIN = 3.3V

VIN = 2.5V

T = 25°C

IGATE = –1μA

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