High Current

APPLICATION NOTE 105: Current Sense Circuit Collection
High Current (100mA to Amps)
Sensing high currents accurately requires excellent control of the sensing resistance, which is typically a very
small value to minimize losses, and the dynamic range of
the measurement circuitry
Shunt Diode Limits Maximum Input Voltage
to Allow Better Low Input Resolution
Without Over-Ranging the LTC6101
V+
To see other chapters in this Application Note, return to
the Introduction.
RSENSE
Kelvin Input Connection Preserves
Accuracy Despite Large Load Currents
LOAD
If low sense currents must be resolved accurately in a
system that has very wide dynamic range, more gain can
be taken in the sense amplifier by using a smaller value
for resistor RIN. This can result in an operating current
greater than the max current spec allowed unless the
max current is limited in another way, such as with a
Schottky diode across RSENSE. This will reduce the high
current measurement accuracy by limiting the result,
while increasing the low current measurement resolution.
This approach can be helpful in cases where an occasional large burst of current may be ignored.
V+
RSENSE
RIN
4
3
+
–
LOAD
2
DSENSE
6101 F03a
5
1
VOUT
LTC6101
ROUT
Kelvin Sensing
6101 F02
DIRECTION OF CHARGING CURRENT
Kelvin connection of the IN– and IN+ inputs to the sense
resistor should be used in all but the lowest power applications. Solder connections and PC board interconnections that carry high current can cause significant error in
measurement due to their relatively large resistances. By
isolating the sense traces from the high current paths,
this error can be reduced by orders of magnitude. A
sense resistor with integrated Kelvin sense terminals will
give the best results.
RSENSE
4008 F12
CSP
BAT
In any high current, >1Amp, application, Kelvin contacts
to the sense resistor are important to maintain accuracy.
This simple illustration from a battery charger application
shows two voltage-sensing traces added to the pads of
the current sense resistor. If the voltage is sensed with
high impedance amplifier inputs, no IxR voltage drop
errors are developed.
High Current (100mA to Amps)-1
APPLICATION NOTE 105: Current Sense Circuit Collection
0A to 33A High Side Current Monitor with Filtering
4.4V TO 48V
SUPPLY
Single Supply RMS Current Measurement
3V
2
LT6100
VCC
7
6
A4
A2
+
8 VS
VOUT 5
RSENSE
3mΩ
–
1 VS
LOAD
VEE
FIL
4
3
VOUT = 2.5V
ISENSE = 33A
6100 TA01a
220pF
CONFIGURED FOR GAIN = 25V/V
The LT1966 is a true RMS-to-DC converter that takes a
single-ended or differential input signal with rail-to-rail
range. The output of a pcb mounted current sense transformer can be connected directly to the converter. Up to
75A of AC current is measurable without breaking the
signal path from a power source to a load. The accurate
operating range of the circuit is determined by the selection of the transformer termination resistor. All of the
math is built in to the LTC1966 to provide a dc output
voltage that is proportional to the true rms value of the
current. This is valuable in determining the power/energy
consumption of ac powered appliances.
High current sensing on a high voltage supply rail is easily accomplished with the LT6100. The sense amplifier is
biased from a low 3V supply and pin-strapped to a gain
of 25V/V to output a 2.5V full scale reading of the current
flow. A capacitor at the FIL pin to ground will filter out
noise of the system (220pF produces a 12KHz low pass
corner frequency).
Dual LTC6101’s Allow High-Low Current Ranging
VLOGIC
(3.3V TO 5V)
CMPZ4697
7
10k
3
M1
Si4465
VIN
RSENSE HI
10m
ILOAD
VOUT
301
RSENSE LO
100m
4
+
–
2
+ –
LTC6101
Q1
CMPT5551
40.2k 6
301
301
301
4.7k
1.74M
4
8
5
3
4
5
2
VIN
1
3
+ –
2
5
619k
1
LTC6101
LTC1540
1
HIGH
RANGE
INDICATOR
(ILOAD > 1.2A)
HIGH CURRENT RANGE OUT
250mV/A
7.5k
VLOGIC
BAT54C
R5
7.5k
(VLOGIC +5V) ≤ VIN ≤ 60V
0 ≤ ILOAD ≤ 10A
High Current (100mA to Amps)-2
LOW CURRENT RANGE OUT
2.5V/A
6101 F03b
APPLICATION NOTE 105: Current Sense Circuit Collection
Using two current sense amplifiers with two values of
sense resistors is an easy method of sensing current
over a wide range. In this circuit the sensitivity and resolution of measurement is 10 times greater with low cur-
rents, less than 1.2 Amps, than with higher currents. A
comparator detects higher current flow, up to 10 Amps,
and switches sensing over to the high current circuitry.
LDO Load Balancing
VIN
1.8V TO 20V
IN
+
10µF
BALLAST RESISTANCE:
IDENTICAL LENGTH
THERMALLY MATED
WIRE OR PCB TRACE
OUT
LT1763
0.01µF
10µF
SHDN BYP
FB
R2
2k
IN
R1
2k
⎛ R1⎞
VOUT = 1.22V ⎜1 + ⎟
⎝ R2⎠
OUT
LT1763
0.01µF
10µF
SHDN BYP
100Ω
ILOAD
FB
LOAD
2k
2k
1k
0.1µF
–
A
10k
IN
+
OUT
LT1763
0.01µF
10µF
SHDN BYP
100Ω
FB
2k
0 ≤ ILOAD ≤ 1.5A
1.22V ≤ VOUT ≤ VDD
LDO LOADS MATCH TO WITHIN
1mA WITH 10mΩ OF BALLAST
RESISTANCE (2 INCHES OF AWG
28 GAUGE STRANDED WIRE)
A, B: LTC6078
2k
0.1µF
1k
VDD
–
B
10k
As system design enhancements are made there is often
the need to supply more current to a load than originally
expected. A simple way to modify power amplifiers or
voltage regulators, as shown here, is to parallel devices.
When paralleling devices it is desired that each device
shares the total load current equally. In this circuit two
adjustable “slave” regulator output voltages are sensed
+
60789 TA09
and servo’ed to match the master regulator output voltage. The precise low offset voltage of the LTC6078 dual
op amp (10uV) balances the load current provided by
each regulator to within 1mA. This is achieved using a
very small 10mΩ current sense resistor in series with
each output. This sense resistor can be implemented
with pcb copper traces or thin gauge wire.
High Current (100mA to Amps)-3
APPLICATION NOTE 105: Current Sense Circuit Collection
Sensing Output Current
VCC
0V TO 1V
12V
VCSRC
VCSNK
+IN
–IN
EN
VCC
V+
ISRC
ISNK
RS
0.2Ω
TSD
OUT
LT1970
SENSE+
–
SENSE
FILTER
–
V
VEE
RLOAD
COMMON
R4
255k
LT1787
RG
RF
VS–
–12V
VS+
20k
VEE
–12V
BIAS
–12V
R1
60.4k
R2
10k
12V
–
+
VOUT
2.5V
±5mV/mA
LT1880
R3
20k
1kHz FULL CURRENT
BANDWIDTH
–12V
0V TO 5V
A/D
1970 F10
OPTIONAL DIGITAL FEEDBACK
The LT1970 is a 500mA power amplifier with voltage
programmable output current limit. Separate DC voltage
inputs and an output current sensing resistor control the
maximum sourcing and sinking current values. These
control voltages could be provided by a D-to-A Converter
High Current (100mA to Amps)-4
in a microprocessor controlled system. For closed loop
control of the current to a load an LT1787 can monitor
the output current. The LT1880 op amp provides scaling
and level shifting of the voltage applied to an A-to-D
Converter for a 5mV/mA feedback signal.