Aug 2002 Simplify Telecom Power Supply Monitoring with the LTC1921 Integrated Dual -48V Supply and Fuse Monitor

DESIGN FEATURES
Simplify Telecom Power Supply
Monitoring with the LTC1921
Integrated Dual –48V Supply and
Fuse Monitor
by Brendan Whelan
Introduction
How it Works
The LTC1921 is the only fully integrated dual –48V supply and fuse
monitor that meets common telecom
specifications for supply range warning and that can withstand the high
transient voltages required by telecom systems. This device improves
system reliability by monitoring both
supply inputs at the card edge and
indicating the status of both supply
fuses. The input pins are designed to
withstand the large DC and transient
voltages that may occur on the backplane supply. The outputs are
designed to drive up to three LEDs or
optoisolators, allowing warnings to
be transmitted across an isolation
barrier. The LTC1921 achieves high
accuracy, high reliability and ease of
use by combining an accurate internal reference, precision comparators
and trimmed resistor networks in one
package. Few external components
The LTC1921 monitors supply voltages by dividing the voltage internally
and comparing the result to an internal precision reference. Since no
precision external components are
required, component cost, board
space and engineering requirements
LTC1921 Features
❏ Independently monitors two
–48V supplies for undervoltage
(–38.5V ±1VMAX) and
overvoltage (–70V ±1.5VMAX)
faults
❏ Accurately detects
undervoltage fault recovery:
–43V±0.5VMAX
❏ Monitors two external fuses
❏ Operates from –10V to –80V
❏ Tolerates DC faults to –100V
❏ Tolerates accidental supply
reversal to 100V
❏ Withstands transient voltages
up to 200V/–200V
❏ Small footprint: 8-lead MSOP
and SO packages
…the LTC1921…can
accurately provide
warnings even if there is no
power at all.
are required, as shown in Figure 1,
and none affect the threshold
accuracy.
are minimized, while accuracy is
maximized. The LTC1921 comes with
telecom industry accepted preset voltage thresholds, as illustrated by
Figure 2, including undervoltage
(–38.5V), undervoltage recovery (–43V)
and overvoltage (–70V). The overvoltage threshold has a 1.3V hysteresis
47k
5V
FUSE
STATUS
–48V
RETURN
R1
100k
R2
100k
MOC207
3
RTN
1
8
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
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
VB
VA
OK
OK
OK
UV OR OV
UV OR OV
OK
UV OR OV UV OR OV
–48V OUT
VFUSE A
= VA
= VA
≠ VA
≠ VA
VFUSE 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
16
Linear Technology Magazine • August 2002
DESIGN FEATURES
TIME
0
NOMINAL
VOLTAGE
UNDERVOLTAGE
FAULT
–38.5
–43
–48
SUPPLY VOLTAGE (V)
that defines the overvoltage recovery
threshold. These thresholds are
trimmed to meet exacting requirements that are based on commonly
used power supply specifications. This
eliminates the need, as in the case of
discretes, to calculate the aggregate
error of a separate reference, multiple
comparator offsets, and resistors.
Internal resistors eliminate error due
to board leakage, allowing the use of
large internal resistor values that reduce power dissipation.
The LTC1921 is designed to indicate proper supply status over a wide
UNDERVOLTAGE
RECOVERY
–68.7
–70
OVERVOLTAGE
RECOVERY
OVERVOLTAGE
FAULT
Figure 2: Voltage monitor thresholds
range of conditions. In order to
accomplish this, the internal archi-
tecture is symmetrical. The LTC1921
is powered via the supply monitor
input pins, VA and VB, as shown in
Figure 1. Supply current can be drawn
from either or both pins, so the device
can operate properly as long as at
least 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.
A useful feature of the LTC1921
architecture is that it can accurately
47k
5V
STATUS
–48V
RETURN
100k
100k
MOC207
3
RTN
1
VA
8
OUT F
4
LOGIC
COMMON
VB
LTC1921
2
VFUSE A VFUSE B VA VB STATUS
= VA
= VB
OK OK
0
FUSE A
7
FUSE B
OUT A
OUT B
ALL OTHER CONDITIONS
OK: WITHIN SPECIFICATION
0: LED/PHOTODIODE ON
1: LED/PHOTODIODE OFF
5
1
6
47k
1/4W
SUPPLY A
–48V
–48V OUT
SUPPLY B
–48V
47k
5V
FUSE STATUS
–48V
RETURN
R1
100k
R2
100k
3
MOCD207
RTN
1
8
OUT F
VA
4
47k
5V
SUPPLY
STATUS
7
FUSE A
FUSE B
OUT A
OUT B
SUPPLY A
–48V
SUPPLY B
–48V
SUPPLY STATUS
0
1
1
1
OK: WITHIN SPECIFICATION
OV: OVERVOLTAGE
UV: UNDERVOLTAGE
VB
LTC1921
2
VA
VB
OK
OK
OK
UV OR OV
UV OR OV
OK
UV OR OV UV OR OV
VFUSE A
= VA
= VA
≠ VA
≠ VA
5
6
R3
47k
1/4W
F1
–48V OUT
VFUSE 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
F2
= LOGIC COMMON
Figure 3: Output OR allows for fewer components
Linear Technology Magazine • August 2002
17
DESIGN FEATURES
provide warnings even if there is no
power at all. This is accomplished
with a low voltage lockout circuit. 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 power supply
problem, even though the LTC1921
does not have enough power to maintain accuracy. As an example, if both
supplies are active and fall below a
magnitude of 13V, all outputs shunt
until the supplies either recover or
fall so low that the LTC1921 cannot
keep its outputs shorted. At this point,
the supply voltages are so low that the
output diodes do not receive enough
current through R3 (Figure 1) to turn
on, so they continue to indicate a
warning. The low supply lockout ensures that the LTC1921 provides
proper warning if there is insufficient
supply voltage to power its internal
circuitry, and it occurs well below the
undervoltage threshold of –38.5V, so
supply warning accuracy is not compromised.
Finally, the LTC1921 is designed
to monitor the supply voltages at the
edge-connector, upstream of the series-connected supply diodes and
fuses, which allows the LTC1921 to
provide the most accurate assessment of supply condition possible.
LTC1921 monitors supply fuses
F1 and F2, in Figure 1, by comparing
the voltage potentials on each side of
each fuse. This is accomplished by
optoisolators 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 have been
designed to accommodate series connection of the output status diodes.
This allows the use of one resistor
(R3, Figure 1) instead of three, and
cuts the total output current by the
same factor. The outputs may be ORed
to reduce the number of required
optoisolators as shown in Figure 3.
The supply outputs may be combined,
or all outputs may be combined. The
required warnings will be provided in
all cases. The only difference in function is that the exact source of the
warnings cannot be distinguished
when the outputs are combined.
comparing the voltage at VA (pin 1) to
the voltage at Fuse A (pin 2) and the
voltage at VB (pin 8) to the voltage at
Fuse B (pin 7). If a significant difference (about 2V) arises, 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, Figure 1) en-
The LTC1921 replaces
complicated monitoring
circuitry with a simple
integrated precision
monitoring system
contained entirely in an
MSOP-8 or SO-8 package.
sure that the voltage across a damaged fuse is sufficient for the LTC1921
to detect an open-circuit fuse. The
size of these resistors is determined
by the reverse leakage of the ORing
diodes used in the application. The
higher reverse leakage current exhibited by Schottky diodes may require
lower-valued resistors to be used (as
with R9 and R10, Figure 4).
The LTC1921 can communicate
supply and fuse status by controlling
external optoisolators or LEDs. This
allows for intelligent system monitoring despite high isolation voltage
requirements. Control of the LEDs or
Application Example
Figure 4 shows an LTC1921 and an
LT4250 Hot Swap controller comprising a complete telecommunications
power system solution. The LTC1921
monitors both –48V supply inputs
from the power bus, as well as the
supply fuses. Because the LTC1921
measures both supplies at the card
edge, it can provide warnings for conditions that other solutions cannot
– 48V
RTN
R9
10k
1W
R10
10k
1W
MOC207
3
RTN
1
8
VA
OUT F
7
– 48V A
3A
– 48V B
MOC207
SUPPLY A
STATUS
R4
549k
1%
FUSE A
FUSE B
OUT A
R8
100Ω
R7
51k
5%
8
VDD
VB
OUT B
3A
C8
100nF
100V
4
LTC1921
2
FUSE
STATUS
R5
6.49k
1%
5
6
MOC207
R11
47k
1/4W
SUPPLY B
STATUS
R6
10k
1%
PWRGD
3
2
LT4250L
DRAIN
UV
OV
GATE
VEE
*
7
6
C2
15nF
100V
SENSE
5
4
R1
0.02Ω
5%
* DIODES INC. SMAT70A (805) 446-4800
LUCENT
JW050A1-E
MOC207
1
C1
470nF
25V
VIN+
R3
1k
5%
R2
10Ω
5%
C3
0.1µF
100V
1N4003
1
2
VOUT+
C4
0.1µF
100V
LUCENT
FLTR100V10
VIN–
VOUT–
CASE
+
C5
100µF
100V
C6
0.1µF
100V
4
VIN+
VOUT+
SENSE +
TRIM
ON/OFF
SENSE –
VOUT–
VIN–
9
5V
8
7
+
C7
100µF
16V
6
5
CASE
3
Q1
IRF530
= DIODES INC. B3100
Figure 4: Network switch card application with Hot Swap control
18
Linear Technology Magazine • August 2002
DESIGN FEATURES
measure, such as one supply failing
or one fuse damaged. The supply
measurement is also more accurate,
since the voltage drop across the fuses
or diodes does not affect it. Resistors
R9 and R10 pull up the fuse pins so
that damaged fuses can be detected.
The status signals may be wired off
the card, with optoisolators, to an
isolated microprocessor or microcontroller that controls system
performance and warning functions.
This allows an automated system
supervisor to issue a warning or record
the event, despite operating from an
isolated supply. The L T4250L
switches the –48V supply via Q1 during hot swapping and low supply
conditions, and monitors the supply
voltage provided to the load. The
PWRGD output of the LT4250 drives
an optoisolator, providing a supply
status signal to the DC/DC converter.
This signal may also be used to monitor the condition of the ORing diodes
by comparing it to the supply status
signals from the LTC1921.
Conclusion
Reliability is 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. They monitor supplies for early warnings of impending
failure, often using complicated circuitry that can include a voltage
reference, comparators, an LDO and
several precision resistor dividers.
Designers may also use discrete components to indicate the state of power
supply fuses. The resulting circuits
can be expensive in terms of component cost, board space and
engineering time. The LTC1921 replaces this complicated monitoring
circuitry with a simple integrated precision monitoring system contained
entirely in an MSOP-8 or SO-8
package.
LT3430, continued from page 8
VIN
8V TO 40V
C3
4.7µF
CER
50V
OFF ON
R1
3.3k
C2
0.022µF
D2
MMSD914T1
VIN BOOST
SYNC
SW
LT3430EFE
BIAS
SHDN
VC
FB
C1
220pF
VOUT
5V AT 2A
C4
L1
0.68µF 22µH
IL1
1A/DIV
R2
15.4k
VOUT
GND
D1
30BQ060
R3
4.99k
C5
100µF
CER
OUTPUT
RIPPLE
VOLTAGE
20mV/DIV
DN302 F03
C3: TDK C5750X7R1H475K
C5: TDK C4532X5R0J107M
L1: SUMIDA CEI-122 220
(408) 392-1400
VIN = 24V
VOUT = 5V
IOUT = 2A
(847) 956-0667
Figure 4. Low profile (max height of 3.0mm) FireWire
peripheral supply with low output ripple voltage
Figure 4 shows a 5V/2A solution for
FireWire peripherals which takes advantage of the LT3430 current mode
architecture by using a low ESR ceramic capacitor at the output. The
circuit provides a low profile (all components less than 3.0mm height), low
output ripple voltage solution. Output ripple voltage is only 26mVP–P, as
shown in Figure 5, using a 22µH
inductor, with VIN = 24V and VOUT =
5V at 2A.
2µs/DIV
Figure 5. Output ripple voltage for
the circuit shown in Figure 4
Conclusion
The LT3430 features a 3A peak switch
current limit, 100mΩ internal power
switch and a 5.5V to 60V operating
range, making it well suited to automotive, industrial and FireWire
peripheral applications. It is highly
efficient over the entire operating
range, and it includes important features to save space and reduce output
ripple—including a 200kHz fixed operating frequency, a current mode
architecture and availability in a small
thermally enhanced 16-pin TSSOP
package.
Notes
1 The ‘no connect’ pins 3 and 5 of the LT1766 and
LT1956 must be connected for the LT3430 to
handle the increased current in the SW output
(pins 2 and 5) and the VIN input (pins 3 and 4).
For more information on parts featured in this issue, see
http://www.linear.com/go/ltmag
Linear Technology Magazine • August 2002
19
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