Negative Voltage

APPLICATION NOTE 105: Current Sense Circuit Collection
Negative Voltage
This chapter discusses solutions for negative voltage
current sensing.
To see other chapters in this Application Note, return to
the Introduction.
Telecom Supply Current Monitor
+
LOAD
IL
5V
48V
–
3
7
+
RS
G2
LT1990
2
–
4
G1
5 6
VOUT
8
1
–77V ≤ VCM ≤ 8V
VOUT = VREF – (10 • IL • RS)
REF
VREF = 4V
4
5
IN
OUT
LT6650
1
GND FB
2
1nF
174k
20k
1990 AI01
1µF
mately 4V by the LT6650. The output signal moves
downward from the reference potential in this connection
so that a large output swing can be accommodated.
The LT1990 is a wide common-mode range difference
amplifier used here to amplify the sense resistor drop by
10. To provide the desired input range when using a single 5V supply, the reference potential is set to approxi–48V Hot Swap Controller
GND
RIN
3× 1.8k IN SERIES
1/4W EACH
GND
(SHORT PIN)
CIN
1µF
1
VIN
R1
402k
1%
+
9
R2
32.4k
1%
EN
OV
PWRGD
UV
DRAIN
10
TIMER
3
CT
SS
0.33µF
C1
10nF
CSS
68nF
GATE
VEE
5
LOAD
R3
5.1k
LTC4252-1
8
CL
100µF
SENSE
*
2
7
VOUT
RD 1M
6
Q1
IRF530S
4
RC
10Ω
CC
18nF
RS
0.02Ω
–48V
* M0C207
This load protecting circuit employs low-side current
sensing. The N-MOSFET is controlled to soft-start the
load (current ramping) or to disconnect the load in the
event of supply or load faults. An internal shunt regulator
establishes a local operating voltage.
Negative Voltage-1
APPLICATION NOTE 105: Current Sense Circuit Collection
–48V Low Side Precision Current Sense
The first stage amplifier is basically a complementary
form of the “classic” high-side current sense, designed
to operate with telecom negative supply voltage. The
Zener forms an inexpensive “floating” shunt-regulated
supply for the first op amp. The N-MOSFET drain delivers
a metered current into the virtual ground of the second
stage, configured as a trans-impedance amplifier (TIA).
The second op amp is powered from a positive supply
and furnishes a positive output voltage for increasing
load current. . A dual op amp cannot be used for this implementation due to the different supply voltages for
each stage. This circuit is exceptionally precise due to the
use of Zero Drift op amps. The scaling accuracy is established by the quality of the user-selected resistors. Smallsignal range is limited by VOL in single-supply operation
of the second stage.
Fast Compact –48V Current Sense
VOUT = 3V – 0.1Ω • ISENSE
ISENSE = 0A TO 30A
ACCURACY ≈ 3%
VOUT
Q1
FMMT493
30.1Ω
1%
–
3.3k
0805
×3
+
R1
4.7k
1k
1%
VS = 3V
R1 REDUCES Q1 DISSIPATION
LT1797
0.1µF
SETTLES TO 1% IN 2µs,
1V OUTPUT STEP
BZX84C6V8
VZ = 6.8V
–48V SUPPLY
(–42V TO –56V)
0.003Ω
1% 3W
–
ISENSE
+
This amplifier configuration is essentially the complementary implementation to the classic high-side configuration. The op amp used must support common-mode
operation at its lower rail. A “floating” shunt-regulated
local supply is provided by the Zener diode, and the transistor provides metered current to an output load resis-
Negative Voltage-2
1797 TA01
–48V LOAD
tance (1kΩ in this circuit). In this circuit, the output voltage is referenced to a positive potential and moves
downward when representing increasing –48V loading.
Scaling accuracy is set by the quality of resistors used
and the performance of the NPN transistor.
APPLICATION NOTE 105: Current Sense Circuit Collection
–48V Current Monitor
In this circuit an economical ADC is used to acquire the
sense resistor voltage drop directly. The converter is
powered from a “floating” high-accuracy shunt-regulated
supply and is configured to perform continuous conversions. The ADC digital output drives an opto-isolator,
level-shifting the serial data stream to ground. For wider
supply voltage applications, the 13k biasing resistor may
be replaced with an active 4mA current source such as
shown to the right. For complete dielectric isolation
and/or higher efficiency operation, the ADC may be powered from a small transformer circuit as shown below.
Simple Telecom Power Supply Fuse Monitor
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
The LTC1921 provides an all-in-one telecom fuse and
supply-voltage monitoring function. Three opto-isolated
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
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
status flags are generated that indicate the condition of
the supplies and the fuses.
Negative Voltage-3