LT1787IS8#TRPBF [Linear]
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Linear 
LT1787/LT1787HV
Precision, High Side
Current Sense Amplifiers
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DESCRIPTIO
FEATURES
The LT®1787 is a complete micropower precision high
side current sense amplifier. The LT1787 monitors bidi-
rectional currents via the voltage across an external sense
resistor. A current or voltage output depicts the direction
and magnitude of the sense current. The LT1787 delivers
greater than a 12-bit dynamic range with ultralow 40µV
input offset voltage compared to a typical 250mV full-
scale input voltage. A fixed gain of 8 is set by onboard
precision resistors. Input signal filtering is easily imple-
mented with a capacitor between the FIL– and FIL+ pins.
The LT1787HV operates from 2.5V to 60V total supply
voltage and the LT1787 operates from 2.5V to 36V total
supply voltage. Both versions have a PSRR in excess of
120dB. The LT1787/LT1787HV draw only 60µA and are
available in 8-lead SO and MSOP packages.
■
Input Offset Voltage: 75µV (Max)
■
60V Supply Operation (LT1787HV)
12-Bit Dynamic Range
■
■
■
■
■
■
■
Operating Current: 60µA
User-Selectable External Sense Resistor
Bidirectional High Side Current Sensing
Unidirectional or Bidirectional Output
Input Noise Filtering
Available in 8-Lead SO and MSOP Packages
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APPLICATIO S
■
Battery Monitoring
■
Power Monitoring
■
Portable Phones
Cellular Phones
Portable Test/Measurement Systems
Battery-Operated Systems
■
■
, LTC and LT are registered trademarks of Linear Technology Corporation.
■
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TYPICAL APPLICATIO
12-Bit Dynamic Resolution Unidirectional Output into LTC®1286 ADC
Input Offset Voltage vs Supply Voltage
50
40
30
20
R
SENSE
0.0016Ω
I = 100A
TO
LOAD
2.5V TO 60V
1
8
–
+
+
FIL
FIL
LT1787HV
–
V
V
S
2
3
S
7
6
R1
15k
10
C1
5V
1µF
V
R
BIAS
0
–10
–20
DNC
OUT
20k
V
V
CC
REF
4
5
CS
CLK
V
+IN
EE
LTC1286
TO µP
V
OUT
–30
–40
–50
–IN
D
OUT
GND
C2
0.1µF
1787 TA01
LT1634-1.25
V
= V
+ (8 • I
• R
)
SENSE
OUT
BIAS
LOAD
0
10
30
40
50
60
20
TOTAL SUPPLY VOLTAGE (V)
1787 TA01b
1
LT1787/LT1787HV
W W
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ABSOLUTE MAXIMUM RATINGS
(Notes 1, 2)
Differential Sense Voltage...................................... ±10V
Total Supply Voltage (LT1787) ................................ 40V
Total Supply Voltage (LT1787HV) ........................... 65V
Output Voltage..................... (VEE – 0.3V) to (VEE + 35V)
Output Bias Voltage ............. (VEE – 0.3V) to (VEE + 35V)
Operating Temperature Range ................ –40°C to 85°C
Specified Temperature Range (Note 3)... –40°C to 85°C
Storage Temperature Range ..................–65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
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PACKAGE/ORDER INFORMATION
TOP VIEW
TOP VIEW
ORDER PART NUMBER
ORDER PART NUMBER
–
–
+
+
FIL
1
2
3
4
8
7
6
5
FIL
–
–
+
FIL
1
2
3
4
8
7
6
5
FIL
LT1787CMS8
LT1787IMS8
LT1787HVCMS8
LT1787HVIMS8
LT1787CS8
LT1787IS8
LT1787HVCS8
LT1787HVIS8
V
V
V
V
S
S
+
DNC*
EE
BIAS
OUT
V
S
V
V
V
S
V
DNC*
BIAS
OUT
MS8 PACKAGE
8-LEAD PLASTIC MSOP
V
EE
* DO NOT CONNECT
S8 PACKAGE
MS8 PART MARKING
S8 PART MARKING
8-LEAD PLASTIC SO
T
JMAX = 150°C, θJA = 250°C/ W
LTGM
LTGN
LTKJ
LTKK
1787
1787I
1787HV
787HVI
* DO NOT CONNECT
T
JMAX = 150°C, θJA = 190°C/ W
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
(Note 4)
The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
Total supply = (VS– – VEE) = 2.5V to 36V (LT1787), 2.5V to 60V (LT1787HV) unless otherwise specified.
SYMBOL PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
–
+
V
, V
Sense Amplifier Supply Voltage Single Supply Operation (LT1787)
Single Supply Operation (LT1787HV)
●
●
2.5
2.5
36
60
V
V
S
S
+
–
V
V
Input Sense Voltage Full Scale
Input Offset Voltage (S8)
V
= V – V , V = 10V, V
= 5V, A = 8 ±10%
●
500
mV
SENSE
OS
SENSE
S
S
S
BIAS
V
I
= 0, V Supply = 5V
0°C ≤ T ≤ 70°C
–40°C ≤ T ≤ 85°C
–75
–135
–200
±40
75
135
200
µV
µV
µV
OUT
S
●
●
A
A
I
= 0 (LT1787)
A
–100
–160
–225
100
160
225
µV
µV
µV
OUT
0°C ≤ T ≤ 70°C
–40°C ≤ T ≤ 85°C
●
●
A
I
= 0 (LT1787HV)
A
–100
–160
–225
100
160
225
µV
µV
µV
OUT
0°C ≤ T ≤ 70°C
–40°C ≤ T ≤ 85°C
●
●
A
Input Offset Voltage (MS8)
I
= 0, V Supply = 5V
A
–125
–230
–250
±40
125
230
250
µV
µV
µV
OUT
S
0°C ≤ T ≤ 70°C
–40°C ≤ T ≤ 85°C
●
●
A
I
= 0 (LT1787)
A
–150
–250
–280
150
250
280
µV
µV
µV
OUT
0°C ≤ T ≤ 70°C
–40°C ≤ T ≤ 85°C
●
●
A
I
= 0 (LT1787HV)
A
–150
–250
–280
150
250
280
µV
µV
µV
OUT
0°C ≤ T ≤ 70°C
–40°C ≤ T ≤ 85°C
●
●
A
2
LT1787/LT1787HV
ELECTRICAL CHARACTERISTICS (Note 4)
The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
Total supply = (VS– – VEE) = 2.5V to 36V (LT1787), 2.5V to 60V (LT1787HV) unless otherwise specified.
SYMBOL PARAMETER
TC Temperature Coefficient of V
CONDITIONS
V Supply = 5V (Note 5)
MIN
TYP
0.5
4
MAX
UNITS
µV/°C
nA
V
2
OS
OS
S
I
No-Load Output Current Error
V
= 0V
OUT(O)
SENSE
V
No-Load Output Voltage Error
(S8)
V
= 0V, V Supply = 5V
A
–600
–1080
–1600
600
1080
1600
µV
µV
µV
OUT(O)
SENSE
S
0°C ≤ T ≤ 70°C
–40°C ≤ T ≤ 85°C
●
●
A
No-Load Output Voltage Error
(MS8)
V
= 0V, V Supply = 5V
A
–1000
–1840
–2000
1000
1840
2000
µV
µV
µV
SENSE
S
0°C ≤ T ≤ 70°C
–40°C ≤ T ≤ 85°C
●
●
A
g
Tranconductance, I /V
±V = 10mV, 50mV, 100mV, 150mV, 250mV,
SENSE
400
µA/V
m
OUT SENSE
V Supply = Total Supply + |V
|
S
SENSE
A
Gain, V / V
±V
= 100mV, V Supply = 5V
●
●
7.6
–5
8
2
8.4
5
V/V
%
V
OUT SENSE
SENSE
S
Output Voltage Gain Error
V PSRR V Supply Rejection Ratio
V
V
= 0V, V Supply = 2.5V to 36V (LT1787)
●
●
120
120
135
135
dB
dB
S
S
SENSE
SENSE
S
= 0V, V Supply = 2.5V to 60V (LT1787HV)
S
V
PSRR Negative Supply Rejection Ratio V
V
= 0V, V Supply = 15V, V
= –1V to –15V (LT1787)
= 0V,
= 0V,
●
100
130
dB
EE
SENSE
S
BIAS
BIAS
EE
V
V
= 0V, V Supply = 40V, V
S
= –1V to –15V (LT1787HV)
●
100
130
dB
SENSE
EE
∆V
Change in Input Offset Voltage
with Change in V Voltage
V
V
= 0V, V Supply = 36V, V
= 0.5V to 25V (LT1787)
= 0.5V to 25V (LT1787HV)
●
●
100
100
130
130
dB
dB
OS
SENSE
SENSE
S
BIAS
BIAS
∆V
BIAS
= 0V, V Supply = 60V, V
BIAS
S
+
I
I
I
I
Positive Input Sense Current
Negative Input Sense Current
Negative Supply Current
Output Current
V
V
V
V
V
V
= 0V
= 0V
= 0V
●
●
●
10
50
20
µA
µA
µA
µA
V
S (O)
SENSE
SENSE
SENSE
SENSE
–
100
120
S (O)
60
EE(O)
OUT
= ±128mV
±50
+
V
Output Voltage
= ±128mV, V ≥ 3.3V
V
±1.024
BIAS
OUT
SENSE
+
S
–
Ripple Rejection
= V = 20V, ∆V Supply = 1V, f = 1kHz
●
●
80
88
dB
S
S
S
V
Minimum Output Voltage
V
V
= 0V, V
= 0V
30
10
45
mV
mV
OMIN
SENSE
SENSE
BIAS
+
–
= V – V = –128mV, V
= 0V
S
S
BIAS
Unipolar Output
Saturation Voltage
V
V
V
V
= 2mV, V
= 4mV, V
= 5mV, V
= 6mV, V
= 0V
= 0V
= 0V
= 0V
●
●
●
●
32
38
43
49
50
55
60
65
mV
mV
mV
mV
SENSE
SENSE
SENSE
SENSE
BIAS
BIAS
BIAS
BIAS
+
V
Maximum Output Voltage
Input Gain-Setting Resistor
Output Resistor
V
– 0.75
S
V
kΩ
kΩ
OMAX
R
R
R
Pin 1 to Pin 2, Pin 7 to Pin 8
Pin 5 to Pin 6
1.25
20
G1A, G2A
OUT
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: ESD (Electrostatic Discharge) sensitive devices. Extensive use of
ESD protection devices are used internal to the LT1787/LT1787HV,
however, high electrostatic discharge can damage or degrade the device.
Use proper ESD handling precautions.
expected to meet these extended temperature limits, but are not tested at
–40°C and 85°C. The LT1787I/LT1787HVI are guaranteed to meet the
extended temperature limits.
Note 4: Testing done at V
= 1.25V, V = 0V unless otherwise
EE
BIAS
specified.
Note 5: This parameter is not 100% tested.
Note 3: The LT1787C/LT1787HVC are guaranteed to meet specified
performance from 0°C to 70° and are designed, characterized and
3
LT1787/LT1787HV
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TYPICAL PERFORMANCE CHARACTERISTICS
No Load Output Voltage
Input Offset Voltage vs
No Load Output Current vs
Supply Voltage
vs Supply Voltage
Supply Voltage
10
8
50
40
400
+
–
+
–
V
V
V
= V
V
V
V
= V
T
= –40°C
S
S
S S
A
= 0V
= –1.25V
= 0V
300
BIAS
EE
BIAS
= –1.25V
EE
6
30
T
= 85°C
T
A
= 85°C
A
200
T
= 25°C
= 85°C
A
4
20
100
2
10
0
0
0
T
A
= 25°C
T
A
= 25°C
–2
–4
–6
–8
–10
–10
–20
–30
–40
–50
T
A
–100
–200
–300
–400
V
V
V
= 1V
–
T
A
= –40°C
BIAS
T
= –40°C
A
= 0V
EE
+
= V
S
S
0
10
30
40
50
60
20
0
10
30
40
50
60
50
20
0
10
20
30
40
60
TOTAL SUPPLY VOLTAGE (V)
TOTAL SUPPLY VOLTAGE (V)
TOTAL SUPPLY VOLTAGE (V)
1787 G03
1787 G01
1787 G02
Input Offset Voltage vs
Temperature
Input Offset Voltage vs
Negative Supply Voltage
Output Voltage vs Sense Voltage
(Bidirectional Mode)
30
20
50
40
+
–
+
–
2.5
2.0
1.5
1.0
0.5
V
V
V
= V
S
V
V
= V = 2.5V
S
V
V
V
= 5.5V TO 60V
= 2.5V
BIAS
EE
S
S
S
= 0V
= 1V
T
= 85°C
BIAS
= –1.25V
BIAS
A
= 0V
30
EE
20
10
10
0
0
V
BIAS
T
= 25°C
A
–10
–20
–30
–40
–50
–0.5
–1.0
–1.5
–2.0
–2.5
–10
–20
–30
T
= –40°C
A
0
–5
–10
–15
–20
–25
–30
–40 –20
0
20
40
60
80
85
–250
–150
–50
50
+
150
250
–
NEGATIVE SUPPLY VOLTAGE (V)
TEMPERATURE (°C)
SENSE VOLTAGE (V – V ) (mV)
S
S
1787 G04
1787 G05
1787 G06
Output Voltage vs Sense Voltage
(Unidirectional Mode)
Gain vs Temperature
Gain vs Frequency
8.195
8.185
8.175
8.165
8.155
8.145
8.135
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
30
20
V
= (2.5V + |V
|)TO 60V
V
= 10mV
V
T
BIAS
= 2.5V TO 60V
S
SENSE
–
SENSE
S
A
V
= –40°C TO 85°C
= V
EE
+
V
V
> V
S
S
S
10
0
–10
–20
–30
–40
–50
+
–
< V
40
S
–40 –20
0
20
60
80
85
30
60
90
+
150
0
120
10M
0.1k
1k
10k 100k
1M
100M
–
TEMPERATURE (°C)
FREQUENCY (Hz)
SENSE VOLTAGE (V – V ) (mV)
S
S
1787 G08
1787 G07
1787 G09
4
LT1787/LT1787HV
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TYPICAL PERFORMANCE CHARACTERISTICS
Negative Input Sense Current vs
Sense Voltage
Positive Input Sense Current vs
Sense Voltage
Supply Current vs Supply Voltage
120
110
100
90
60
50
40
30
20
10
0
75
70
65
60
55
50
45
40
V
S
= (2.5V + |V
|) TO 60V
SENSE
V
= (2.5V + |V
|) TO 60V
SENSE
S
T
= 85°C
A
T
= 25°C
A
80
T = 85°C
A
T
= –40°C
70
A
T
T
= 85°C
= 25°C
A
60
A
T
= 25°C
T
= –40°C
A
A
50
T
A
= –40°C
40
+
–
V
= V
10
S
S
30
–128
0
64 96
–128
0
64 96
–96 –64 –32
SENSE VOLTAGE (V – V ) (mV)
32
128
–96 –64 –32
SENSE VOLTAGE (V – V ) (mV)
32
128
0
20
30
40
50
60
+
–
+
–
TOTAL SUPPLY VOLTAGE (V)
S
S
S
S
1787 G11
1787 G17
1787 G10
Step Response at
VSENSE = 0V to 10mV
Step Response at
VSENSE = 0V to 128mV
Step Response at
VSENSE = 0V to 128mV
10mV
0V
100mV
0V
100mV
0V
80mV
1V
500mV
0V
1V
500mV
0V
0V
COUT = 1000pF
1787 G13
COUT = 0pF
1787 G12
COUT = 0pF
1787 G18
Step Response at
VSENSE = 0V to –128mV
Step Response at
VSENSE = 0V to –128mV
Step Response at
VSENSE = –128mV to 128mV
0V
100mV
0V
–100mV
0V
–100mV
–100mV
0V
–500mV
–1V
1V
0V
0V
–500mV
–1V
–1V
COUT = 1000pF
1787 G14
COUT = 0
1787 G20
C
OUT = 0
1787 G19
5
LT1787/LT1787HV
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TYPICAL PERFORMANCE CHARACTERISTICS
Step Response at
SENSE = 128mV to –128mV
VOUT Error vs Supply Ripple
Voltage (VSENSE = ±128mV)
Output Voltage vs Sense Voltage
V
1000
900
800
700
600
500
400
300
200
100
0
16
14
12
10
8
–
V
V
V
= 18V
S
100mV
= 0V
BIAS
EE
–100mV
= –18V
1V
0V
6
4
0.5%
5%
1%
2
–1V
0
2%
–2
–4
–6
–8
V
ERROR
LESS THAN 0.1%
OUT
COUT = 2200pF
1787 G15
0.4
SENSE VOLTAGE (V – V ) (V)
–0.8 –0.4
0
0.8 1.2 1.6 2.0
100
1k
10k
100k
1M
+
–
FREQUENCY (Hz)
S
S
1787 G16
1787 G21
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PIN FUNCTIONS
FIL–, FIL+ (Pins 1, 8): Negative and Positive Filter Termi-
nals. Differential mode noise can be filtered by connecting
a capacitor across FIL– and FIL+. Pole frequency
f–3dB = 1/(2πRC), R = 1.25kΩ.
VBIAS (Pin 6): Output Bias Pin. For single supply, bidirec-
tional current sensing operation, VBIAS is connected to an
external bias voltage, so that at VSENSE = 0V, VOUT
=
VOUT(O) + VBIAS. For dual supply, bidirectional current
sensing operation, VBIAS is connected to ground. Thus,
VOUT = VOUT(O) at VSENSE = 0V.
–
VS (Pin 2): Negative Input Sense Terminal. Negative
sense voltage will result in an output sinking current
proportional to the sense current. VS– is connected to an
internal gain-setting resistor RG1A and supplies bias cur-
rent to the internal amplifier.
VS+ (Pin 7): Positive Input Sense Terminal. Positive sense
voltage will result in an output sourcing current propor-
tional to the sense current. VS+ is connected to an internal
+
gain-settingresistorRG2A. ConnectingasupplytoVS and
DNC(Pin3):DoNotConnect.Connectedinternally.Donot
connect external circuitry to this pin.
–
a load to VS will allow the LT1787 to measure its own
supply current.
VEE (Pin 4): Negative Supply or Ground for Single Supply
Operation.
VOUT (Pin 5): Voltage Output or Current Output propor-
tional to the magnitude of the sense current flowing
through RSENSE. For bidirectional current sensing opera-
tion, VOUT = AV • VSENSE + VOUT(O) + VBIAS
,
where:
–
VOUT > VBIAS for VS+ > VS–
VOUT < VBIAS for VS+ < VS
VOUT(O) is the no load output voltage at VSENSE = 0V.
6
LT1787/LT1787HV
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BLOCK DIAGRAM
R
SENSE
I
SENSE
–
–
+
V
S
V
S
R
R
G1A
G2A
1.25k
1.25k
+
FIL
FIL
R
R
G2B
1.25k
G1B
1.25k
–
+
A1
I
OUT
R
V
BIAS
OUT
OUT
20k
Q1
Q2
V
V
EE
CURRENT MIRROR
1787 F 01
Figure 1. LT1787 Functional Diagram
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APPLICATIONS INFORMATION
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.
and single supply output configurations are shown in the
following sections.
Supply current for amplifier A1 is drawn from the VS– pin.
The user may choose to include this current in the moni-
tored current through RSENSE by careful choice of connec-
tion polarity.
Selection of External Current Sense Resistor
External RSENSE 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
tailoringtherangeandresolutionofhissupplymonitoring
function.
Theory of Operation
+
–
Inputs VS and VS apply the sense voltage to matched
resistors RG1 and RG2. The opposite ends of resistors RG1
and RG2 are forced to be at equal potentials by the voltage
gainofamplifierA1. ThecurrentsthroughRG1 andRG2 are
forced to flow through transistors Q1 and Q2 and are
summed at node VOUT by the 1:1 current mirror. The net
current from RG1 and RG2 flowing through resistor ROUT
givesavoltagegainofeight. Positivesensevoltagesresult
in VOUT being positive with respect to pin VBIAS
.
Pins VEE, VBIAS and VOUT may be connected in a variety of
ways to interface with subsequent circuitry. Split supply
7
LT1787/LT1787HV
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APPLICATIONS INFORMATION
Kelvin connection of the LT1787’s VS+ and VS– inputs to
the sense resistor should be used in all but the lowest
power applications. Solder connections and PC board
interconnectresistance(approximately0.5mΩpersquare)
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.
at VOUT are imposed by the negative supply, VEE, and the
+
input voltage VS . In the negative direction, internal circuit
saturation with loss of accuracy occurs for VOUT < 70mV
with absolute minimum swing at 30mV above VEE. VOUT
may swing positive to within 0.75V of VS+ or a maximum
of 35V, a limit set by internal junction breakdown. Within
thesecontraints, anamplified, levelshiftedrepresentation
of the RSENSE voltage is developed across ROUT
.
Split Supply Bipolar Output Swing
Figure2showstheLT1787usedwithsplitpowersupplies.
The VBIAS pin is connected to ground, and the output
signalappearsattheVOUT pin. Bidirectionalinputcurrents
can be monitored with the output swinging positive for
Input Noise Filtering
+
–
The LT1787 provides input signal filtering pins FIL+ and
FIL– that are internally connected to the midpoint taps of
resistors RG1 and RG2. 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
amplifierwillcauseerrors.Acapacitorconnectedbetween
FIL+ and FIL– creates a single pole low pass filter with
corner frequency:
current flow from VS and VS . Input currents in the
opposite direction cause VOUT to swing below ground.
Figure 2 shows an optional output capacitor connected
from VOUT 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 VBIAS
Figure 4 shows the LT1787 used in a single supply mode
with the VBIAS pin shifted positive using an external
LT1634 voltage reference. The VOUT output signal can
swing above and below VBIAS to allow monitoring of
positive or negative currents through the sense resistor,
asshowninFigure5.Thechoiceofreferencevoltageisnot
critical except for the precaution that adequate headroom
must be provided for VOUT to swing without saturating the
internal circuitry. The component values shown in Figure 4
allow operation with VS supplies as low as 3.1V.
f–3dB = 1/(2πRC)
where R = 1.25k. A 0.01µF capacitor creates a pole at
12.7kHz, a good choice for many applications.
CommonmodefilteringfromtheFIL+ andFIL– pinsshould
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
Operation with A/D Converter
The LT1787’s output signal is developed by summing the
net currents through RG1 and RG2 into output resistor
ROUT. The pins VOUT and VBIAS 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
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
converterisbiasedat1VbytheresistordividerR1andR2.
Thisvoltageincreasesassensecurrentincreases,withthe
8
LT1787/LT1787HV
U
W U U
APPLICATIONS INFORMATION
R
TO
CHARGER/
LOAD
SENSE
R
TO
CHARGER/
LOAD
SENSE
3.3V
C1
1µF
C1
TO
15V
1µF
1
8
60V
–
+
+
1
8
–
+
FIL
FIL
FIL
FIL
3.3V
LT1787
LT1787HV
–
–
+
V
V
S
2
3
S
7
6
V
V
S
2
3
S
7
6
20k
5%
V
R
BIAS
V
R
BIAS
DNC
DNC
OUT
C2
1µF
OUT
LT1634-1.25
4
5
4
5
V
OUTPUT
EE
V
EE
V
OUT
V
OUT
C2
1µF
C3*
1000pF
C3*
1000pF
–5V
1787 F02
1787 F04
*OPTIONAL
OUTPUT
*OPTIONAL
Figure 4. Charge/Discharge Current Monitor on
Single Supply with VBIAS = 1.25V
Figure 2. Split Supply Operation
1.5
1.5
V
T
= 3.3V TO 60V
= –40°C TO 85°C
V
T
= 3.3V TO 60V
= –40°C TO 85°C
S
A
S
A
1.0
0.5
1.0
0.5
0
0
–0.5
–1.0
–1.5
–0.5
–1.0
–1.5
–128
0
64 96
–96 –64 –32
32
128
–128
0
64 96
–96 –64 –32
32
128
+
–
+
–
SENSE VOLTAGE (V – V ) (mV)
SENSE VOLTAGE (V – V ) (mV)
S
S
S
S
1787 F05
1787 F03
Figure 5. Single Supply Output Voltage
with VBIAS = 1.25V
Figure 3. Split Supply Output Voltage
R
SENSE
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-
enceforimprovedaccuracyandsignalrange.TheLTC1286
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 VBIAS to VOUT may be
necessary if the sensed current changes more than 1LSB
within a conversion cycle.
5V
C1
5V
1µF
1
8
–
+
+
FIL
FIL
LT1787
–
R1
20k
5%
V
V
S
2
3
S
7
6
I
V
OUT
BIAS
DNC
R
OUT
V
4
5
CC
CS
V
+IN
–IN
EE
V
OUT
LTC1286 CLK
TO µP
D
OUT
V
R2
5k
5%
GND
REF
1787 F06
Figure 6. Unidirectional Output into A/D
+
with Fixed Supply at VS
9
LT1787/LT1787HV
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APPLICATIONS INFORMATION
Buffered Output Operation
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 VBIAS 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
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 VOUT pin is held equal to the
reference voltage appearing at the op amp’s noninverting
input. This allows monitoring VS 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 con-
verter configuration also works well with split supply
voltages.
sensed differentially between VBIAS and VOUT
.
R
SENSE
C
TO
LOAD
2.5V TO
60V
0.1µF
1
8
–
+
+
FIL
FIL
LT1787HV
–
V
V
S
2
3
S
7
6
Single Supply Unidirectional Operation
V
R
BIAS
DNC
Figure 8 shows the simplest connection in which the
LT1787 may be used. The VBIAS pin is connected to
ground, and the VOUT pin swings positive with increasing
sense current. The LT1787’s outputs can swing as low as
30mVasshowninFigure9. Accuracyissacrificedatsmall
OUT
4
5
V
V
EE
OUT
V
OUT
1787 F08
Figure 8. Unidirectional Current Sensing Mode
I
SENSE
R
TO
CHARGER/
LOAD
SENSE
0.30
0.25
0.20
0.15
0.10
0.05
C1
2.5V + V
SENSE(MAX)
1µF
1
8
–
+
+
FIL
FIL
LT1787
–
V
V
S
2
3
S
7
6
2.5V
V
BIAS
DNC
C3
1000pF
R
OUT
4
5
V
EE
V
OUT
–
A1
V
OUT A
2.5V
+
LT1495
1M
5%
IDEAL
0
LT1389-1.25
0
0.010 0.015 0.020 0.025 0.030
0.005
1787 F07
+
–
V
– V (V)
S
S
1787 F09
Figure 7. Single Supply 2.5V Bidirectional Operation
with External Voltage Reference and I/V Converter
Figure 9. Expanded Scale of Unidirectional Output
10
LT1787/LT1787HV
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APPLICATIONS INFORMATION
Adjusting Gain Setting
The transconductance gm is set by on-chip resistors on
the LT1787. These resistors match well but have loose
absolute tolerance. This will normally require that the
external gain setting resistor be trimmed for initial accu-
racy. After trimming, the temperature stability of the gm
and therefore gain will be –200ppm/°C.
The LT1787 may be used in all operating modes with an
external resistor used in place of the internal 20k ROUT
resistor. When an external resistor is used, leave the VBIAS
pin floating or connected to the VOUT pin. This will remove
the internal ROUT from the circuit.
Theonlylimitationsplacedupontheresistorchoiceiscare
must be taken not to saturate the internal circuitry by
violating the VOMAX specification of VS + –0.75V.
The voltage gain will be gm • ROUT where gm is the
LT1787’s transconductance, 400µA/V typical. A nominal
gain of 40 may be obtained with an external 100k resistor
used in place of the internal 20k ROUT
:
AV = gm • ROUT = 400µA/V • 100k = 40
U
PACKAGE DESCRIPTION Dimensions in inches (millimeters) unless otherwise noted.
MS8 Package
8-Lead Plastic MSOP
(LTC DWG # 05-08-1660)
0.118 ± 0.004*
(3.00 ± 0.102)
8
7
6
5
0.040 ± 0.006
(1.02 ± 0.15)
0.034 ± 0.004
(0.86 ± 0.102)
0.007
(0.18)
0° – 6° TYP
0.118 ± 0.004**
(3.00 ± 0.102)
SEATING
PLANE
0.193 ± 0.006
(4.90 ± 0.15)
0.012
(0.30)
REF
0.021 ± 0.006
(0.53 ± 0.015)
0.006 ± 0.004
(0.15 ± 0.102)
0.0256
(0.65)
BSC
MSOP (MS8) 1098
1
2
3
4
* DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH,
PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
S8 Package
8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.189 – 0.197*
(4.801 – 5.004)
0.010 – 0.020
(0.254 – 0.508)
7
5
8
6
× 45°
0.053 – 0.069
(1.346 – 1.752)
0.004 – 0.010
(0.101 – 0.254)
0.008 – 0.010
(0.203 – 0.254)
0°– 8° TYP
0.150 – 0.157**
(3.810 – 3.988)
0.228 – 0.244
(5.791 – 6.197)
0.016 – 0.050
(0.406 – 1.270)
0.050
(1.270)
BSC
0.014 – 0.019
(0.355 – 0.483)
TYP
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
1
3
4
2
SO8 1298
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-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
11
LT1787/LT1787HV
TYPICAL APPLICATION
U
Split or Single Supply Operation, Bidirectional Output into A/D
1Ω
1%
I
= ±125mA
S
V
CC
5V
1
8
V
–
+
+
SRCE
FIL
FIL
≈4.75V
LT1787
–
10µF
16V
V
V
S
2
3
S
7
6
V
BIAS
1
DNC
7
6
5
CONV
CLK
20k
V
(±1V)
4
5
OUT
2
V
CLOCKING
CIRCUITRY
EE
V
A
V
LTC1404
EE
IN
–5V
3
V
OUT
OPTIONAL SINGLE
SUPPLY OPERATION:
DISCONNECT V
REF
DOUT
GND
10µF
16V
BIAS
FROM GROUND
AND CONNECT IT TO V
4
8
10µF
16V
.
D
REF
OUT
REPLACE –5V SUPPLY
WITH GROUND.
OUTPUT CODE FOR ZERO
CURRENT WILL BE ~2430
V
EE
1787 TA02
–5V
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTC1043
Dual Precision Instrumentation Switched Capacitor Building Block
Dual and Quad Micropower Rail-to-Rail Input and Output Op Amps
120dB CMRR, 3V to 18V Operation
LT1490/LT1491
50µA Amplifier, 2.7V to 40V Operation,
Over-The-TopTM Inputs
LT1620/LT1621
Rail-to-Rail Current Sense Amplifiers
Accurate Output Current Programming, Battery
Charging to 32V
Over-The-Top is a trademark of Linear Technology Corporation.
1787f LT/TP 0100 4K • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
12
●
●
(408)432-1900 FAX:(408)434-0507 www.linear-tech.com
LINEAR TECHNOLOGY CORPORATION 1999
相关型号：
LT1789CS8-1#PBF
LT1789 - Micropower, Single Supply Rail-to-Rail Output Instrumentation Amplifier; Package: SO; Pins: 8; Temperature Range: 0°C to 70°C
Linear
LT1789CS8-1#TRPBF
LT1789 - Micropower, Single Supply Rail-to-Rail Output Instrumentation Amplifier; Package: SO; Pins: 8; Temperature Range: 0°C to 70°C
Linear
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