LTC4252-2 Linear Technology, LTC4252-2 Datasheet - Page 33

• Manufacturer Part Number: LTC4252-2
• Description: Negative Voltage Hot Swap Controllers
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
The internal fault latches of the LTC4252 are disabled as
the TIMER pin is always held low by the SS voltage when
Q2 and Q3 are in analog current limit.
If both power supplies from channel A and B are exactly
equal, then equal load current will flow through Q2 and Q3
to the load module via the Hot Swap section.
If the channel A supply is greater than the channel B by
more than 100mV, the sense voltage will rise above the
fast comparator trip threshold of 200mV, the GATE will be
pulled low and Q2 is turned off. The GATE ramps up and
regulates Q2 when the channel A supply is equal to the
channel B supply. Likewise, if the channel B supply is
greater than channel A by more than 100mV, it trips the
fast comparator and GATE is pulled low and Q3 is turned
off. The GATE ramps up and regulates Q3 when the
channel B supply is equal to the channel A supply.
Resistors R4, R7 and external FETs Q4 and Q7 limit the
current flow into Q5 and Q8 during their respective supply
source short. When the channel A supply is shorted to the
– 48V RTN (or GND), large current flows into Q4 momen-
tarily and creates a voltage drop across R4, which in turn
reduces the gate-to-source voltage of Q4, limiting the
(SHORT PIN)
(LONG PIN)
–48V RTN
–48V RTN
–48V
30.1k
390k
1%
1%
R1
R2
C1
10nF
C
0.68µF
U
T
10
8
9
3
OV
TIMER
UV
SS
C
68nF
U
SS
LTC4252A-1
V
V
EE
IN
1
PWRGD
5
R
3× 1.8k
1/4W
DRAIN
SENSE
IN
GATE
1µF
C
IN
W
2
7
6
4
Figure 23. Typical Application Using a Filter Module
R
1M
D
R
10Ω
C
C
10nF
C
R3
5.1k
U
Q1
IRF530S
R
0.02Ω
*MOC207
S
*
1N4003
current flow. The sense voltage is lifted up and causes the
fast comparator of LTC4252 to trip and pull the GATE low
instantly. The channel A supply short will not cause Q3 of
channel B diode-OR circuit to turn off.
Similarly, when the channel B supply is shorted to the
– 48V RTN (or GND), large current flows into Q7 momen-
tarily and creates a voltage drop across R7, which in turn
reduces the gate-to-source voltage of Q7, thus limiting the
current flow. The increase in sense voltage will trip the fast
comparator of LTC4252 and pull the GATE low instantly.
The channel B supply short will not cause Q2 of channel A
diode-OR circuit to turn off. The load short at the output of
Q1 is protected by the Hot Swap section.
Using an EMI Filter Module
Many applications place an EMI filter module in the power
path to prevent switching noise of the module from being
injected back onto the power supply. A typical application
using the Lucent FLTR100V10 filter module is shown in
Figure 23. When using a filter, an optoisolator is required
to prevent common mode transients from destroying the
PWRGD and ON/OFF pins.
C2
0.1µF
100V
LTC4252A-1/LTC4252A-2
V
V
FLTR100V10
IN
IN
LUCENT
+
–
CASE
V
V
OUT
OUT
LTC4252-1/LTC4252-2
+
–
C3
0.1µF
100V
+
C4
100µF
100V
C5
0.1µF
100V
1
2
4
V
ON/OFF
V
JW050A1-E
IN
IN
LUCENT
+
–
CASE
SENSE
SENSE
V
V
TRIM
3
OUT
OUT
4252-1/2 F20
+
+
–
–
9
8
7
6
5
33
+
425212fb
C6
100µF
16V
5V