Datasheet

LTC4365
12
Rev. B
For more information www.analog.com
APPLICATIONS INFORMATION
The resistor values can then be solved:
1.
1. R
1 + R2 =
3mV
10nA
= 300k
2.
R3 = 2•
3mV
10nA
• 5V − 0.5V
( )
= 2.7M
The closest 1% value: R3 = 2.74M:
3.
R1=
300k + 1.82M
2 •18V
= 84.4k
The closest 1% value: R1 = 84.5k:
R2 = 300k – 84.5k = 215.5k
The closest 1% value: R2 = 215k
Therefore: OV = 17.99V, UV = 5.07V.
Reverse V
IN
Protection
The LTC4365’s rugged and hot-swappable V
IN
input helps
protect the more sensitive circuits at the output load. If
the input supply is plugged in backwards, or a negative
supply is inadvertently connected, the LTC4365 prevents
this negative voltage from passing to the output load.
The LTC4365 employs a novel, high speed reverse supply
voltage monitor. When the negative V
IN
voltage is detected,
an internal switch connects the gates of the external back-
to-back N-channel MOSFETs to the negative input supply.
As shown in Figure 7, external back-to-back N-channel
MOSFETs are required for reverse supply protection. When
V
IN
goes negative, the reverse V
IN
comparator closes the
internal switch, which in turn connects the gates of the
external MOSFETs to the negative V
IN
voltage. The body
diode (D1) of M1 turns on, but the body diode (D2) of
M2 remains in reverse blocking mode. This means that
the common source connection of M1 and M2 remains
about a diode drop higher than V
IN
. Since the gate voltage
of M
2 is shorted to V
IN
, M2 will be turned off and no cur-
rent can
flow from V
IN
to the load at V
OUT
. Note that the
voltage rating of M2 must withstand the reverse voltage
excursion at V
IN
.
Figure 8 illustrates the waveforms that result when V
IN
is hot plugged to –20V. V
IN
, GATE and V
OUT
start out at
ground just before the connection is made. Due to the
parasitic inductance of the V
IN
and GATE connections, the
voltage at the V
IN
and GATE pins ring significantly below
–20V. Therefore, a 40V N-channel MOSFET was selected
to survive the overshoot.
The speed of the LTC4365 reverse protection circuits is
evident by how closely the GATE pin follows V
IN
during
the negative transients. The two waveforms are almost
indistinguishable on the scale shown.
The trace at V
OUT
, on the other hand, does not respond
to the negative voltage at V
IN
, demonstrating the desired
reverse supply protection. The waveforms of Figure 8 were
captured using a 40V dual N-channel MOSFET, a 10µF
ceramic output capacitor and no load current on V
OUT
.
Figure 7. Reverse V
IN
Protection Circuits
V
IN
4365 F07
V
OUT
GATE
V
IN
= –40V
REVERSE V
IN
COMPARATOR
CLOSES SWITCH
WHEN V
IN
IS NEGATIVE
GND
LTC4365
M1
D1 D2
M2
+
–
+
TO LOAD
C
OUT
Figure 8. Hot Swapping V
IN
to –20V
–20V
5V/DIV
GND
4365 F07
500ns/DIV
GATE
V
OUT
V
IN
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