Datasheet

LTC4365
10
Rev. B
For more information www.analog.com
Overvoltage and Undervoltage Protection
The LTC4365 provides two accurate comparators to moni-
tor for
overvoltage (OV) and undervoltage (UV) conditions
at V
IN
. 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 4 shows a UV/OV application
for an input supply of 12V.
Figure 3. Gate Drive (GATE – V
OUT
) vs V
OUT
APPLICATIONS INFORMATION
Figure 3 highlights the dependence of the gate drive on V
IN
and V
OUT
. When system power is first turned on (SHDN
low to high, V
OUT
= 0V), gate drive is at a maximum for all
values of V
IN
. This helps prevent start-up problems into
heavy loads by ensuring that there is enough gate drive
to support the load.
As V
OUT
ramps up from 0V, the absolute value of the GATE
voltage remains fixed until V
OUT
is greater than the lower
of (V
IN
–1V) or 6V. Once V
OUT
crosses this threshold,
gate drive begins to increase up to a maximum of 9.8V
(for V
IN
≥ 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 V
IN
would be slightly higher.
Note that when V
IN
is at the lower end of the operating
range, the external N-channel MOSFET must be selected
with a corresponding lower threshold voltage.
V
IN
12V
UV
TH
= 5V
OV
TH
= 18V
4365 F04
DISCHARGE GATE
WITH 50mA SINK
LTC4365
OV
COMPARATOR
UV
COMPARATOR
R3
2470k
UV
0.5V
0.5V
OV
R2
215k
R1
84.5k
–
+
25mV
–
+
25mV
Figure 4. 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
V
OUT
. 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 4, the load is connected to the supply only
if V
IN
lies between 5V and 18V. In the event that V
IN
goes
above 18V or below 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 V
IN
supply voltages.
Table 1. Dual MOSFETs for Various Supply Ranges
V
IN
MOSFET V
TH(MAX)
V
GS(MAX)
V
DS(MAX)
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
V
OUT
(V)
0
0
∆V
GATE
(V)
2
4
6
10
8
12
9
15
4365 F03
3 6
12
V
IN
= 30V
V
IN
= 12V
V
IN
= 5V
V
IN
= 3.3V
V
IN
= 2.5V
T = 25°C
I
GATE
= –1µA
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