DN290 - Monitor Network Compliant -48V Power Supplies

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Monitor Network Compliant –48V Power Supplies
Design Note 290
Brendan Whelan
Introduction
Reliability is a top priority for the designers of
modern telephone and communication equipment.
Designers take extra care to protect circuitry from
failure-causing temperature and voltage changes,
employing redundancy whenever possible, especially
for power supplies. The power supplies are monitored
to obtain early warning of impending failure. Often
complicated circuitry that can include a voltage reference, comparators, voltage regulator and several
precision resistor dividers is used. Designers may also
use discrete components to monitor and indicate the
state of power supply fuses. The resulting circuitry
can be expensive in terms of component cost, board
space and engineering time. The LTC ®1921 replaces
the complicated monitoring circuitry with a simple
integrated precision monitoring system contained
entirely in an 8-lead MSOP package.
Features
The LTC1921 is the only integrated solution that can
monitor two independent –48V power supplies, plus
associated fuses and drive up to three optoisolators
or LEDs to indicate status. The required external
components are three resistors and optocouplers or
LEDs, as shown by the simple circuit in Figure 1. The
LTC1921 can withstand ±100V DC at the supply and
fuse input pins and tolerates ±200V transients.
The LTC1921 monitors supply voltages by dividing
the voltage internally and comparing to an internal
precision reference. Since no critical precision external
components are required, component cost, board
space and design time are minimized while accuracy
is maximized. The LTC1921 comes with telecom industry accepted preset voltage thresholds, including
undervoltage (–38.5V), undervoltage recovery (–43V)
and overvoltage (–70V). The overvoltage threshold
has a 1.3V hysteresis that defines the overvoltage
recovery threshold. These thresholds are trimmed
to meet exacting requirements. This eliminates the
messy worst-case threshold tolerance error calculation
required when using discrete comparators, resistors
and a separate voltage reference.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered
trademarks and Hot Swap is a trademark of Linear Technology Corporation. All
other trademarks are the property of their respective owners.
47k
5V
FUSE
STATUS
–48V
RETURN
R1
100k
MS8
U A L SIZ
R2
100k
MOC207
3
E
A
CT
1
8
RTN
OUT F
VA
5V
SUPPLY A
STATUS
VB
LTC1921
2
47k
4
FUSE B
OUT A
OUT B
SUPPLY A
–48V
SUPPLY B
–48V
F1
D1
F2
D2
5V
SUPPLY B
STATUS
5
6
DN290 F01
MOC207
R3
47k
1/4W
SUPPLY A
STATUS
0
0
1
1
SUPPLY B
STATUS
0
1
0
1
OK: WITHIN SPECIFICATION
OV: OVERVOLTAGE
UV: UNDERVOLTAGE
MOC207
FUSE A
47k
7
VA
VB
OK
OK
OK
UV OR OV
UV OR OV
OK
UV OR OV UV OR OV
–48V OUT
FUSE A
= VA
= VA
≠ VA
≠ VA
FUSE B
= VB
≠ VB
= VB
≠ VB
FUSE STATUS
0
1
1
1*
0: LED/PHOTODIODE ON
1: LED/PHOTODIODE OFF
*IF BOTH FUSES (F1 AND F2) ARE OPEN,
ALL STATUS OUTPUTS WILL BE HIGH
SINCE R3 WILL NOT BE POWERED
= LOGIC COMMON
Figure 1. The LTC1921 Requires Few External Components for Monitoring Two Supplies
07/02/290_conv
The LTC1921 is designed to indicate proper supply
status across a wide variety of conditions. In order
to accomplish this, the internal architecture is symmetrical. The LTC1921 is powered via the supply
monitor input pins, VA and VB. Supply current can
be drawn from either or both pins so the device can
operate properly as long as one supply is within the
operating range. Since power is not drawn from a
combined supply (such as would be available with a
diode OR), the LTC1921 will function properly even
if the fuses or diodes are not functional. In addition,
the LTC1921 has a low voltage lockout. If both supply
voltages are very low, all three outputs of the LTC1921
lock into a fault indication state, thus communicating
to supervisory systems that there is a problem even
though there is not enough power for the LTC1921 to
maintain accuracy.
The LTC1921 can communicate supply and fuse status
by controlling external optocouplers or LEDs. This
allows for intelligent system monitoring despite high
isolation voltage requirements. Control of the LEDs
or optocouplers is accomplished by connecting the
LTC1921 outputs in parallel with the LEDs or photodiodes. During normal supply and fuse conditions, the
LTC1921 outputs are high impedance; current flows
through the external diodes continuously. If a fuse
opens or a supply voltage falls outside of the allowed
window, then the proper LTC1921 output shunts the
current around the diode, thus indicating a fault. The
outputs may be ORed to reduce the number of required
optoisolators.
Application Example
Figure 2 shows an LTC1921 and an LT®4250 Hot Swap™
controller comprising a complete power system
solution. The LTC1921 monitors both –48V supply
inputs from the power bus, as well as the supply
fuses. The status signals may be wired off the board
via optoisolators to an isolated microprocessor or
microcontroller to control system performance and
warning functions. Resistors R9 and R10 pull up the
fuse pins so that damaged fuses can be detected. The
LT4250L controls the combined –48V supply during
hot swapping and low supply conditions, and monitors
the combined supply voltage. The PWRGD pin drives
an optoisolator signalling the status of the LT4250’s
switched output.
The device monitors fuses by comparing the voltage
potentials on each side of each fuse. If a significant
difference (about 2V) is sensed, the LTC1921 signals
that a fuse has opened. The voltage difference across
the damaged fuse may be reduced by diode reverse
leakage, making it difficult to detect a damaged fuse.
Weak pull-up resistors (R1 and R2 in Figure 1) ensure
that the LTC1921 can detect an open fuse circuit. The
value of the pull-up resistors used is a function of the
reverse leakage current of the OR’ing diodes used.
– 48V
RTN
R9
10k
1W
R10
10k
1W
1
8
MOC207
3
RTN
VA
OUT F
7
C8
100nF
100V
MOC207
SUPPLY A
STATUS
R4
549k
1%
FUSE A
OUT A
OUT B
3A
– 48V A
3A
R5
6.49k
1%
5
6
MOC207
R11
47k
1/4W
SUPPLY B
STATUS
PWRGD
3
2
R6
10k
1%
LT4250L
UV
DRAIN
OV
GATE
VEE
*
* DIODES INC. SMAT70A
– 48V B
R7
51k
5%
8
VDD
VB
FUSE B
R8
100Ω
4
LTC1921
2
FUSE
STATUS
7
6
C2
15nF
100V
SENSE
5
4
R1
0.02Ω
5%
LUCENT
JW050A1-E
MOC207
1
C1
470nF
25V
VIN+
R3
1k
5%
R2
10Ω
5%
C3
0.1μF
100V
1N4003
1
VOUT+
LUCENT
FLTR100V10
VIN–
2
VOUT
C4
0.1μF
100V
+
–
C5
100μF
100V
C6
0.1μF
100V
4
CASE
VIN+
VOUT+
SENSE +
TRIM
ON/OFF
SENSE –
VOUT–
VIN–
9
5V
8
7
+
C7
100μF
16V
6
5
CASE
3
DN290 F02
Q1
IRF530
= DIODES INC. B3100
Figure 2. Network Switch Card Monitor with Hot Swap Control
Data Sheet Download
http://www.linear.com
Linear Technology Corporation
For applications help,
call (408) 432-1900
dn290f_conv LT/TP 0702 316.5K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2002
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