Final Electrical Specifications
LTC1921
Dual –48V Supply
and Fuse Monitor
FEATURES
s
May 2002
DESCRIPTIO
s
s
s
s
s
s
s
s
s
Independently Monitors Two –48V Supplies for
– Undervoltage Faults: –38.5V
±1V
MAX
– Overvoltage Faults: –70V
±1.5V
MAX
Accurately Detects Undervoltage Fault Recovery:
–43V
±0.5V
MAX
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 Package
Requires No Precision External Components
Specified from – 40°C to 85°C
The LTC
®
1921 monitors two independent – 48V supplies,
including their fuses, and drives up to three optoisolators
to indicate status, in accordance with standard backplane
specifications. Requiring only three noncritical resistors
and optoisolators, the LTC1921 replaces multiple voltage
comparators, a voltage reference and several precision
resistors.
The monitor features dual supply overvoltage and under-
voltage detection circuits. The preset trip thresholds in-
clude overvoltage, undervoltage and undervoltage recovery
that are guaranteed over temperature and meet or exceed
common backplane specifications. Additional built-in cir-
cuitry detects the condition of supply fuses. Overvoltage
and undervoltage detectors ignore fast supply transients,
eliminating false detection. The LTC1921 operates from
–10V to –80V with a typical power dissipation of less than
10mW.
The LTC1921 is available in an 8-pin MSOP package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
s
s
s
Telecom Backplanes or Switch Cards
Networking Backplanes or Switch Cards
High Voltage Fuse Monitoring
TYPICAL APPLICATIO
– 48V
RETURN
R1
100k
R2
100k
3
RTN
1
8
2
7
V
A
V
B
LTC1921
FUSE A
FUSE B
OUT A
5
MOC207
OUT F
MOC207
4
47k
47k
47k
FUSE GOOD
SUPPLY A GOOD SUPERVISOR
µP
SUPPLY B GOOD
OUT B
F1
F2
D1 MURS320
D2 MURS320
6
MOC207
R3
47k
1/4W
1921 TA01
SUPPLY A
–48V
SUPPLY B
–48V
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
U
LOGIC
SUPPLY
LOGIC
COMMON
U
U
–48V LOAD
1921i
1
LTC1921
ABSOLUTE
AXI U
RATI GS
(Note 1) All voltages referred to RTN
PACKAGE/ORDER I FOR ATIO
TOP VIEW
V
A
FUSE A
RTN
OUT F
1
2
3
4
8
7
6
5
V
B
FUSE B
OUT B
OUT A
Supply Voltage
(V
A
, V
B
, FUSE A, FUSE B) ....................... 100V to –100V
Transient Voltage (V
A
, V
B
, FUSE A, FUSE B)
(Note 2) ........................................................ 0V to 200V
Transient Voltage (V
A
, V
B
, FUSE A, FUSE B)
(Note 2) ...................................................... 0V to –200V
OUT A, OUT B, OUT F Pins ......................... 0.3V to – 8V
Maximum Junction Temperature ......................... 150°C
Operating Temperature Range
LTC1921CMS8 ........................................ 0°C to 70°C
LTC1921IMS8 .................................... – 40°C to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LTC1921CMS8
LTC1921IMS8
MS8 PART MARKING
LTZV
LTZU
MS8 PACKAGE
8-LEAD PLASTIC MSOP
T
JMAX
= 150°C,
θ
JA
= 250°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
PARAMETER
Power Supply
Supply Voltage Range (RTN – V
A
, RTN – V
B
)
Supply Current (I
A
+ I
B
)
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. (Note 3) RTN = 0V, V
A
= –48V, V
B
= –48V, FUSE A = –48V,
FUSE B = –48V, unless otherwise noted.
CONDITIONS
q
q
MIN
10
TYP
MAX
80
UNITS
V
µA
µA
V
V
V
V
MΩ
–160
–250
–300
–37.5
–42.5
–68.5
1.6
Supply Monitor
Undervoltage Threshold
Undervoltage Recovery Threshold
Overvoltage Threshold
Overvoltage Threshold Hysteresis
Fuse Monitor
Input Resistance, FUSE A, FUSE B
Fuse Comparison Threshold |V
FUSEA
– V
A
|, |V
FUSEB
– V
B
|
LTC1921C
LTC1921I
Output
Propagation Delay
Output Switch Resistance, OUT F, OUT A, OUT B
Output Switch Off Leakage
Output Switch Resistance in Undervoltage Lockout,
OUT F, OUT A, OUT B
V
A
= V
B
= –10V, I
OUT
= 10mA
V
A
= –10V, V
B
= 0V, I
OUT
= 10mA
V
A
= 0V, V
B
= –10V, I
OUT
= 10mA
q
q
q
q
q
q
q
q
q
–39.5
–43.5
–71.5
1
11
2.5
0.9
0.5
220
25
4.5
4.5
C
OUT
= 100pF, Overdrive = 1V
V
A
= V
B
= –35V, V
FUSEA
= V
FUSEB
= 0V
I
OUT
= 10mA
q
50
500
60
80
80
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
Transient voltage for less than 10µs. This parameter is not 100%
tested. Voltage should not exceed 200V between any two pins.
Note 3:
All currents into device pins are positive; all currents out of device
pins are negative. All voltages are referenced to RTN unless otherwise
specified.
1921i
2
U
V
V
V
µs
Ω
Ω
pA
Ω
Ω
Ω
W
U
U
W W
W
LTC1921
TYPICAL PERFOR A CE CHARACTERISTICS
Supply Current vs Supply Voltage
400
350
V
A
= V
B
= V
FUSEA
= V
FUSEB
180
I
A
+ I
B
160
UNDERVOLTAGE THRESHOLD (V)
SUPPLY CURRENT (µA)
SUPPLY CURRENT (µA)
300
250
200
150
100
50
0
0
10 20 30 40 50 60 70 80 90 100
SUPPLY VOLTAGE (V)
1921 G01
Undervoltage Recovery Threshold
vs Temperature
UNDERVOLTAGE RECOVERY THRESHOLD (V)
–42.5
–42.6
–42.7
–42.8
–42.9
–43.0
–43.1
–43.2
–43.3
–43.4
–43.5
–50
–30
–10 10
30
50
TEMPERATURE (°C)
70
90
–68.0
–68.5
–69.0
–69.5
–70.0
–70.5
–71.0
–71.5
OVERVOLTAGE HYSTERESIS (V)
OVERVOLTAGE THRESHOLD (V)
Fuse Window Positive Threshold
vs Temperature
2.7
FUSE POSITIVE THRESHOLD (V)
2.5
2.3
2.1
1.9
1.7
1.5
–50
FUSE NEGATIVE THRESHOLD (V)
–2.0
–2.2
–2.4
–2.6
–2.8
–3.0
OUTPUT R
DS(ON)
(Ω)
–30
–10 10
30
50
TEMPERATURE (°C)
U W
70
Supply Current vs Temperature
–37.5
–37.7
–37.9
–38.1
–38.3
–38.5
–38.7
–38.9
–39.1
–39.3
70
90
Undervoltage Threshold
vs Temperature
140
120
100
80
60
40 V
A
= –48V
V
B
= –48V
20 V
FUSEA
= –48V
V
FUSEB
= –48V
0
30
50
–50 –30 –10 10
TEMPERATURE (°C)
I
A
, I
B
–39.5
–50
–30
–10 10
30
50
TEMPERATURE (°C)
70
90
1921 G02
1921 G04
Overvoltage Threshold
vs Temperature
1.50
1.45
1.40
1.35
1.30
1.25
1.20
1.15
1.10
1.05
–30
–10 10
30
50
TEMPERATURE (°C)
70
90
Overvoltage Hysteresis
vs Temperature
–72.0
–50
1.00
–50
–30
–10 10
30
50
TEMPERATURE (°C)
70
90
1921 G05
1921 G06
1921 G07
Fuse Window Negative Threshold
vs Temperature
30
25
Output R
DS(ON)
vs Temperature
OUT A (OUT F = 0V)
20
OUT F, OUT B (OUT A = 0V)
15
10
5
V
A
= –35V
V
B
= –35V
I = –10mA
30
50
–10 10
TEMPERATURE (°C)
70
90
90
–3.2
–50
–30
–10 10
30
50
TEMPERATURE (°C)
70
90
0
–50 –30
1921 G08
1921 G09
1921 G03
1921i
3
LTC1921
TYPICAL PERFOR A CE CHARACTERISTICS
Undervoltage Response Time
10000
T
A
= 25°C
10000
RESPONSE TIME (µs)
1000
100
4.5
RESPONSE TIME (µs)
4.6 4.7 4.8 4.9 5.0
5.1
SUPPLY STEP FROM UNDERVOLTAGE
RECOVERY THRESHOLD (V)
PI FU CTIO S
V
A
(Pin 1):
Supply to be Monitored. The voltage at this pin
is compared to the valid supply voltage window and the
result is output at OUT A (Pin 5). Supply current is drawn
from this pin as well as from V
B
(Pin 8).
FUSE A (Pin 2):
This pin monitors the state of a fuse by
comparing the voltage at this pin to the voltage at V
A
(Pin 1). The result is output at OUT F (Pin 4).
RTN (Pin 3):
Supply Return Reference. This pin must be
at an equal or higher potential than the other pins and
should be wired to the – 48V return.
OUT F (Pin 4):
This pin indicates the state of the external
fuses by ORing the comparisons made to the FUSE A and
FUSE B pins. If V
FUSEA
≅
V
A
(V
FUSEA
is within the specified
window around V
A
) and V
FUSEB
≅
V
B
, then OUT F will
exhibit a high internal impedance to the RTN pin. If
V
FUSEA
≠
V
A
or V
FUSEB
≠
V
B
, then OUT F is shorted
internally to the RTN pin and can shunt enough current to
turn off an optocoupler or LED wired between these pins.
OUT F should be clamped externally so that it cannot be
driven more than 8V below RTN. This is done automati-
cally by the optocoupler or LED diodes shown in the
application circuits.
OUT A (Pin 5):
Indicates the State of V
A
. If V
A
is within the
specified voltage window (neither undervoltage nor over-
voltage), OUT A will exhibit a high internal impedance to the
OUT F pin. If V
A
is outside the specified overvoltage or
undervoltage limits, then OUT A is shorted internally to the
OUT F pin and can shunt enough current to turn off an
optocoupler or LED wired between these pins. OUT A should
be clamped externally so that it cannot be driven more than
8V below RTN. This is done automatically by the
optocoupler or LED diodes shown in the application circuits.
OUT B (Pin 6):
Indicates the State of V
B
. If V
B
is within the
specified voltage window (neither undervoltage nor over-
voltage), OUT B will exhibit a high internal impedance to the
OUT A pin. If V
B
is outside the specified overvoltage or
undervoltage limits, then OUT B is shorted internally to the
OUT A pin and can shunt enough current to turn off an
optocoupler or LED wired between these pins. OUT B should
be clamped externally so that it cannot be driven more than
8V below RTN. This is done automatically by the
optocoupler or LED diodes shown in the application circuits.
FUSE B (Pin 7):
This pin monitors the state of a fuse by
comparing the voltage at this pin to the voltage at V
B
(Pin 8). The result is output at OUT F (Pin 4).
V
B
(Pin 8):
Supply to be Monitored. The voltage at this pin
is compared to the valid supply voltage window and the
result is output at OUT B (Pin 6). Supply current is drawn
from this pin as well as from V
A
(Pin 1).
1921i
4
U W
Overvoltage Response Time
T
A
= 25°C
1000
5.2
100
1.3
1921 G10
1.7
1.5
2.1
2.3
1.9
SUPPLY STEP FROM OVERVOLTAGE
RECOVERY THRESHOLD (V)
2.5
1921 G11
U
U
U
LTC1921
BLOCK DIAGRA
REGULATOR
1
V
A
W
RTN
3
REF
+
–
+
–
+
–
+
–
+
–
+
–
5
OUT A
4
OUT F
+
–
+
–
8
V
B
2
7
1921 BD
6
OUT B
FUSE A FUSE B
1921i
5
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