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LT1787HVIMS8#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
10
LT1787/LT1787HV
1787fc
The LT1787 high side current sense amplifier (Figure 1)
provides accurate bidirectional monitoring of current
through a user-selected sense resistor. The sense voltage
is amplified by a fixed gain of 8 and level shifted from the
positive power supply to the ground referenced outputs.
The output signal may be used in a variety of ways to
interface with subsequent signal processing circuitry.
Input and output filtering are easily implemented to elimi-
nate aliasing errors.
Theory of Operation
Inputs V
S
+
and V
S
–
apply the sense voltage to matched
resistors R
G1
and R
G2
. The opposite ends of resistors R
G1
and R
G2
are forced to be at equal potentials by the voltage
gain of amplifier A1. The currents through R
G1
and R
G2
are
forced to flow through transistors Q1 and Q2 and are
summed at node V
OUT
by the 1:1 current mirror. The net
current from R
G1
and R
G2
flowing through resistor R
OUT
gives a voltage gain of eight. Positive sense voltages result
in V
OUT
being positive with respect to pin V
BIAS
.
Pins V
EE
, V
BIAS
and V
OUT
may be connected in a variety of
ways to interface with subsequent circuitry. Split supply
and single supply output configurations are shown in the
following sections.
Supply current for amplifier A1 is drawn from the V
S
–
pin.
The user may choose to include this current in the moni-
tored current through R
SENSE
by careful choice of connec-
tion polarity.
Selection of External Current Sense Resistor
External R
SENSE
resistor selection is a delicate trade-off
between power dissipation in the resistor and current
measurement accuracy. The LT1787 makes this decision
less difficult than with competitors’ products. The maxi-
mum sense voltage may be as large as ±500mV to get
maximum resolution, however, high current applications
will not want to suffer this much power dissipation in the
sense resistor. The LT1787’s input offset voltage of 40µV
gives high resolution for low sense voltages. This wide
operating dynamic range gives the user wide latitude in
tailoring the range and resolution of his supply monitoring
function.
APPLICATIONS INFORMATION
WUU
U
Kelvin connection of the LT1787’s V
S
+
and V
S
–
inputs to
the sense resistor should be used in all but the lowest
power applications. Solder connections and PC board
interconnect resistance (approximately 0.5mΩ per square)
can be a large error in high current systems. A 5-Amp
application might choose a 20mΩ sense resistor to give a
100mV full-scale input to the LT1787. Input offset voltage
will limit resolution to 2mA. Neglecting contact resistance
at solder joints, even one square of PC board copper at
each resistor end will cause an error of 5%. This error will
grow proportionately higher as monitored current levels
rise to tens or hundreds of amperes.
Input Noise Filtering
The LT1787 provides input signal filtering pins FIL
+
and
FIL
–
that are internally connected to the midpoint taps of
resistors R
G1
and R
G2
. These pins may be used to filter the
input signal entering the LT1787’s internal amplifier, and
should be used when fast current ripple or transients may
flow through the sense resistor. High frequency signals
above the 300kHz bandwidth of the LT1787’s internal
amplifier will cause errors. A capacitor connected between
FIL
+
and FIL
–
creates a single pole low pass filter with
corner frequency:
f
–3dB
= 1/(2πRC)
where R = 1.25k. A 0.01µF capacitor creates a pole at
12.7kHz, a good choice for many applications.
Common mode filtering from the FIL
+
and FIL
–
pins should
not be attempted, as mismatch in the capacitors from FIL
+
and FIL
–
will create AC common mode errors. Common
mode filtering must be done at the power supply output.
Output Signal Range
The LT1787’s output signal is developed by summing the
net currents through R
G1
and R
G2
into output resistor
R
OUT
. The pins V
OUT
and V
BIAS
may be connected in
numerous configurations to interface with following cir-
cuitry in either single supply or split supply applications.
Care must be used in connecting the output pins to
preserve signal accuracy. Limitations on the signal swing
at V
OUT
are imposed by the negative supply, V
EE
, and the
input voltage V
S
+
. In the negative direction, internal circuit
saturation with loss of accuracy occurs for V
OUT
< 70mV
11
LT1787/LT1787HV
1787fc
APPLICATIONS INFORMATION
WUU
U
with absolute minimum swing at 30mV above V
EE
. V
OUT
may swing positive to within 0.75V of V
S
+
or a maximum
of 35V, a limit set by internal junction breakdown. Within
these contraints, an amplified, level shifted representation
of the R
SENSE
voltage is developed across R
OUT
.
Split Supply Bipolar Output Swing
Figure 2 shows the LT1787 used with split power supplies.
The V
BIAS
pin is connected to ground, and the output
signal appears at the V
OUT
pin. Bidirectional input currents
can be monitored with the output swinging positive for
current flow from V
S
+
and V
S
–
. Input currents in the
opposite direction cause V
OUT
to swing below ground.
Figure 2 shows an optional output capacitor connected
from V
OUT
to ground. This capacitor may be used to filter
the output signal before it is processed by other
circuitry.Figure 3 shows the voltage transfer function of
the LT1787 used in this configuration.
Single Supply with Shifted V
BIAS
Figure 4 shows the LT1787 used in a single supply mode
with the V
BIAS
pin shifted positive using an external
LT1634 voltage reference. The V
OUT
output signal can
swing above and below V
BIAS
to allow monitoring of
positive or negative currents through the sense resistor,
as shown in Figure 5. The choice of reference voltage is not
critical except for the precaution that adequate headroom
must be provided for V
OUT
to swing without saturating the
internal circuitry. The component values shown in Figure 4
allow operation with V
S
supplies as low as 3.1V.
Figure 2. Split Supply Operation
*OPTIONAL
C2
1µF
–5V
1787 F02
OUTPUT
C3*
1000pF
C1
1µF
R
SENSE
15V
TO
CHARGER/
LOAD
1
2
3
4
8
7
6
5
LT1787
FIL
+
FIL
–
V
BIAS
V
OUT
V
S
–
V
S
+
DNC
V
EE
R
OUT
Figure 3. Split Supply Output Voltage
SENSE VOLTAGE (V
S
+
– V
S
–
) (mV)
OUTPUT VOLTAGE – OUTPUT BIAS VOLTAGE (V)
1787 F05
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
–128
–64
0
32
–96 –32
64
96
128
V
S
= 3.3V TO 60V
T
A
= –40°C TO 85°C
Figure 5. Single Supply Output Voltage
with V
BIAS
= 1.25V
Figure 4. Charge/Discharge Current Monitor on
Single Supply with V
BIAS
= 1.25V
C2
1µF
20k
5%
1787 F04
3.3V
LT1634-1.25
*OPTIONAL OUTPUT
C1
1µF
R
SENSE
3.3V
TO
60V
TO
CHARGER/
LOAD
1
2
3
4
8
7
6
5
LT1787HV
FIL
+
FIL
–
V
BIAS
V
OUT
V
S
–
V
S
+
DNC
V
EE
C3*
1000pF
R
OUT
SENSE VOLTAGE (V
S
+
– V
S
–
) (mV)
OUTPUT VOLTAGE (V)
1787 F03
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
–128
–64
0
32
–96 –32
64
96
128
V
S
= 3.3V TO 60V
T
A
= –40°C TO 85°C
12
LT1787/LT1787HV
1787fc
Operation with A/D Converter
Figure 6 shows the LT1787 operating with the LTC1286
A/D converter. This low cost circuit is capable of 12-bit
resolution of unipolar currents. The –IN pin of the A/D
converter is biased at 1V by the resistor divider R1 and R2.
This voltage increases as sense current increases, with the
amplified sense voltage appearing between the A/D con-
verters –IN and +IN terminals. The front page of the data
sheet shows a similar circuit which uses a voltage refer-
ence for improved accuracy and signal range. The LTC1286
converter uses sequential sampling of its –IN and +IN
inputs. Accuracy is degraded if the inputs move between
sampling intervals. A filter capacitor from FIL
+
to FIL
–
as
well as a filter capacitor from V
BIAS
to V
OUT
may be
necessary if the sensed current changes more than 1LSB
within a conversion cycle.
Buffered Output Operation
Figure 7 shows the LT1787’s outputs buffered by an
operational amplifier configured as an I/V converter. This
configuration is ideal for monitoring very low voltage
supplies. The LT1787’s V
OUT
pin is held equal to the
reference voltage appearing at the op amp’s noninverting
input. This allows monitoring V
S
supplies as low as 2.5V.
The op amp’s output may swing from ground to its positive
supply voltage. The low impedance output of the op amp
may drive following circuitry more effectively than the high
output impedance of the LT1787. The I/V converter configu-
ration also works well with split supply voltages.
Single Supply Unidirectional Operation
Figure 8 shows the simplest connection in which the
LT1787 may be used. The V
BIAS
pin is connected to
ground, and the V
OUT
pin swings positive with increasing
sense current. The LT1787’s outputs can swing as low as
30mV as shown in Figure 9. Accuracy is sacrificed at small
output levels, but this is not a limitation in protection
circuit applications or where sensed currents do not vary
greatly. Increased low level accuracy can be obtained by
level shifting V
BIAS
above ground. The level shifting may be
done with resistor dividers, voltage references or a simple
diode. Accuracy is ensured if the output signal is sensed
differentially between V
BIAS
and V
OUT
.
APPLICATIONS INFORMATION
WUU
U
Figure 8. Unidirectional Current Sensing Mode
1787 F08
C
0.1µF
R
SENSE
2.5V TO
60V
V
OUT
TO
LOAD
1
2
3
4
8
7
6
5
LT1787HV
FIL
+
FIL
–
V
BIAS
V
OUT
V
S
–
V
S
+
DNC
V
EE
R
OUT
Figure 7. Single Supply 2.5V Bidirectional Operation
with External Voltage Reference and I/V Converter
2.5V
C1
1µF
R
SENSE
I
SENSE
2.5V + V
SENSE(MAX)
TO
CHARGER/
LOAD
V
OUT A
1M
5%
1787 F07
LT1495
C3
1000pF
LT1389-1.25
2.5V
+
–
A1
1
2
3
4
8
7
6
5
LT1787
FIL
+
FIL
–
V
BIAS
V
OUT
V
S
–
V
S
+
DNC
V
EE
R
OUT
Figure 6. Unidirectional Output into A/D
with Fixed Supply at V
S
+
R2
5k
5%
1787 F06
I
OUT
C1
1µF
5V
V
REF
V
CC
GND
LTC1286
CS
CLK
D
OUT
+IN
–IN
TO µP
R
SENSE
5V
1
2
3
4
8
7
6
5
LT1787
FIL
+
FIL
–
V
BIAS
V
OUT
V
S
–
V
S
+
DNC
V
EE
R1
20k
5%
R
OUT
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

LT1787HVIMS8#PBF

Mfr. #:
Buy LT1787HVIMS8#PBF
Manufacturer:
Analog Devices Inc.
Description:
Current Sense Amplifiers Prec, Hi Side C Sense Amps
Lifecycle:
New from this manufacturer.
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