LT4256 LINER [Linear Technology], LT4256 Datasheet - Page 11

• Manufacturer Part Number: LT4256
• Description: UV, OV and Reverse Supply Protection Controller Low Operating Current: 125?A
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
The resistor values can then be solved:
The closest 1% value: R3 = 1.82M:
The closest 1% value: R1 = 59k:
The closest 1% value: R2 = 243k
Therefore: OV = 17.93V, UV = 3.51V.
Reverse V
The LTC4365’s rugged and hot-swappable V
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 sup-
ply voltage monitor. When the negative V
V
IN
R2 = 300k – 59k = 241k
3. R1 =
1. R1+ R2 =
2. R3 = 2 •
= –40V
GND
Figure 6. Reverse V
IN
300k +1.82M
V
Protection
IN
REVERSE V
COMPARATOR
10nA
2 • 18V
3mV
10nA
3mV
+
–
M1
• (3.5V – 0.5V) =1.8M
IN
= 300k
D1
CLOSES SWITCH
WHEN V
= 58.9k
IN
GATE
IN
Protection Circuits
IS NEGATIVE
D2
M2
V
LTC4365
OUT
+
4365 F06
IN
IN
C
input helps
OUT
voltage is
TO LOAD
detected, an internal switch connects the gates of the
external back-to-back N-channel MOSFETs to the nega-
tive input supply.
As shown in Figure 6, external back-to-back N-channel
MOSFETs are required for reverse supply protection. When
V
internal switch, which in turn connects the gates of the
external MOSFETs to the negative V
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
M2 is shorted to V
rent can flow from V
voltage rating of M2 must withstand the reverse voltage
excursion at V
Figure 7 illustrates the waveforms that result when V
is hot plugged to –20V. V
ground just before the connection is made. Due to the
parasitic inductance of the V
voltage at the V
–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
the negative transients. The two waveforms are almost
indistinguishable on the scale shown.
IN
goes negative, the reverse V
5V/DIV
–20V
GND
Figure 7. Hot Swapping V
IN
IN
.
and GATE pins ring significantly below
IN
IN
, M2 will be turned off and no cur-
V
OUT
to the load at V
500ns/DIV
IN
IN
, GATE and V
IN
. Since the gate voltage of
and GATE connections, the
GATE
IN
comparator closes the
IN
IN
to –20V
V
IN
voltage. The body
OUT
LTC4365
OUT
. Note that the
4365 F07
start out at
IN
11
during
4365f
IN