LINER LT1787IMS8 Precision, high side current sense amplifier Datasheet

LT1787/LT1787HV
Precision, High Side
Current Sense Amplifiers
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DESCRIPTIO
FEATURES
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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
The LT ®1787 is a complete micropower precision high
side current sense amplifier. The LT1787 monitors bidirectional 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 fullscale input voltage. A fixed gain of 8 is set by onboard
precision resistors. Input signal filtering is easily implemented 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.
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APPLICATIO S
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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
I = 100A
1
FIL–
–
2 VS
3
4
DNC
FIL+
LT1787HV
40
8
VS+ 7
VBIAS 6
ROUT
20k
VEE
50
RSENSE
0.0016Ω
5
VOUT
VOUT = VBIAS + (8 • ILOAD • RSENSE)
C2
0.1µF
2.5V TO 60V
R1
15k
C1
1µF
VREF VCC
CS
+IN
LTC1286 CLK
–IN
D
GND OUT
LT1634-1.25
5V
TO µP
1787 TA01
INPUT OFFSET VOLTAGE (µV)
TO
LOAD
Input Offset Voltage vs Supply Voltage
30
20
10
0
–10
–20
–30
–40
–50
0
10
30
40
50
20
TOTAL SUPPLY VOLTAGE (V)
60
1787 TA01b
1
LT1787/LT1787HV
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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
MS8 PACKAGE
8-LEAD PLASTIC MSOP
ORDER PART NUMBER
LT1787CMS8
LT1787IMS8
LT1787HVCMS8
LT1787HVIMS8
* DO NOT CONNECT
MS8 PART MARKING
TJMAX = 150°C, θJA = 250°C/ W
LTGM
LTGN
LTKJ
LTKK
TOP VIEW
FIL–
VS–
DNC*
VEE
1
2
3
4
8
7
6
5
FIL+
VS+
VBIAS
VOUT
ORDER PART NUMBER
LT1787CS8
LT1787IS8
LT1787HVCS8
LT1787HVIS8
TOP VIEW
–
FIL
+
1
8
FIL
VS– 2
7
VS+
DNC* 3
6
VBIAS
VEE 4
5
VOUT
S8 PART MARKING
S8 PACKAGE
8-LEAD PLASTIC SO
1787
1787I
1787HV
787HVI
* DO NOT CONNECT
TJMAX = 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
VS –, VS +
Sense Amplifier Supply Voltage Single Supply Operation (LT1787)
Single Supply Operation (LT1787HV)
VSENSE
Input Sense Voltage Full Scale
VSENSE = VS+ – VS–, VS = 10V, VBIAS = 5V, AV = 8 ±10%
●
500
VOS
Input Offset Voltage (S8)
IOUT = 0, VS Supply = 5V
0°C ≤ TA ≤ 70°C
– 40°C ≤ TA ≤ 85°C
●
●
– 75
– 135
– 200
IOUT = 0 (LT1787)
0°C ≤ TA ≤ 70°C
– 40°C ≤ TA ≤ 85°C
●
●
IOUT = 0 (LT1787HV)
0°C ≤ TA ≤ 70°C
– 40°C ≤ TA ≤ 85°C
Input Offset Voltage (MS8)
2
CONDITIONS
MIN
●
●
TYP
2.5
2.5
MAX
36
60
UNITS
V
V
mV
±40
75
135
200
µV
µV
µV
– 100
– 160
– 225
100
160
225
µV
µV
µV
●
●
– 100
– 160
– 225
100
160
225
µV
µV
µV
IOUT = 0, VS Supply = 5V
0°C ≤ TA ≤ 70°C
– 40°C ≤ TA ≤ 85°C
●
●
– 125
– 230
– 250
125
230
250
µV
µV
µV
IOUT = 0 (LT1787)
0°C ≤ TA ≤ 70°C
– 40°C ≤ TA ≤ 85°C
●
●
– 150
– 250
– 280
150
250
280
µV
µV
µV
IOUT = 0 (LT1787HV)
0°C ≤ TA ≤ 70°C
– 40°C ≤ TA ≤ 85°C
●
●
– 150
– 250
– 280
150
250
280
µV
µV
µV
±40
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
CONDITIONS
VOS TC
Temperature Coefficient of VOS
VS Supply = 5V (Note 5)
MIN
TYP
MAX
UNITS
0.5
2
µV/°C
IOUT(O)
No-Load Output Current Error
VSENSE = 0V
VOUT(O)
No-Load Output Voltage Error
(S8)
VSENSE = 0V, VS Supply = 5V
0°C ≤ TA ≤ 70°C
– 40°C ≤ TA ≤ 85°C
●
●
–600
– 1080
– 1600
600
1080
1600
µV
µV
µV
No-Load Output Voltage Error
(MS8)
VSENSE = 0V, VS Supply = 5V
0°C ≤ TA ≤ 70°C
– 40°C ≤ TA ≤ 85°C
●
●
– 1000
– 1840
– 2000
1000
1840
2000
µV
µV
µV
gm
Tranconductance, IOUT/ VSENSE
±VSENSE = 10mV, 50mV, 100mV, 150mV, 250mV,
VS Supply = Total Supply + |VSENSE|
AV
Gain, VOUT/ VSENSE
±VSENSE = 100mV, VS Supply = 5V
4
nA
µA/V
400
●
7.6
8
8.4
V/V
●
–5
2
5
%
●
●
120
120
135
135
dB
dB
VEE PSRR Negative Supply Rejection Ratio VSENSE = 0V, VS Supply = 15V, VBIAS = 0V,
VEE = – 1V to – 15V (LT1787)
●
100
130
dB
VSENSE = 0V, VS Supply = 40V, VBIAS = 0V,
VEE = – 1V to – 15V (LT1787HV)
●
100
130
dB
100
100
130
130
dB
dB
Output Voltage Gain Error
VS PSRR
VS Supply Rejection Ratio
VSENSE = 0V, VS Supply = 2.5V to 36V (LT1787)
VSENSE = 0V, VS Supply = 2.5V to 60V (LT1787HV)
∆VOS
∆VBIAS
Change in Input Offset Voltage
with Change in VBIAS Voltage
VSENSE = 0V, VS Supply = 36V, VBIAS = 0.5V to 25V (LT1787)
●
VSENSE = 0V, VS Supply = 60V, VBIAS = 0.5V to 25V (LT1787HV) ●
IS+(O)
Positive Input Sense Current
VSENSE = 0V
●
10
20
µA
IS–(O)
Negative Input Sense Current
VSENSE = 0V
●
50
100
µA
IEE(O)
Negative Supply Current
VSENSE = 0V
●
60
120
µA
IOUT
Output Current
VSENSE = ±128mV
VOUT
VOMIN
VOMAX
+ ≥ 3.3V
±50
µA
VBIAS ±1.024
V
Output Voltage
VSENSE = ±128mV, VS
Ripple Rejection
VS+
Minimum Output Voltage
VSENSE = 0V, VBIAS = 0V
VSENSE = VS+ – VS– = –128mV, VBIAS = 0V
●
30
10
45
mV
mV
Unipolar Output
Saturation Voltage
VSENSE = 2mV, VBIAS = 0V
VSENSE = 4mV, VBIAS = 0V
VSENSE = 5mV, VBIAS = 0V
VSENSE = 6mV, VBIAS = 0V
●
●
●
●
32
38
43
49
50
55
60
65
mV
mV
mV
mV
= VS = 20V, ∆VS Supply = 1V, f = 1kHz
–
80
88
dB
VS + – 0.75
Maximum Output Voltage
RG1A, RG2A Input Gain-Setting Resistor
Pin 1 to Pin 2, Pin 7 to Pin 8
ROUT
Pin 5 to Pin 6
Output Resistor
●
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.
Note 3: The LT1787C/LT1787HVC are guaranteed to meet specified
performance from 0°C to 70° and are designed, characterized and
V
1.25
kΩ
20
kΩ
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 VBIAS = 1.25V, VEE = 0V unless otherwise
specified.
Note 5: This parameter is not 100% tested.
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LT1787/LT1787HV
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TYPICAL PERFORMANCE CHARACTERISTICS
No Load Output Voltage
vs Supply Voltage
50
VS+ = VS–
VBIAS = 0V
VEE = –1.25V
300
TA = 85°C
20
OUTPUT VOLTAGE (µV)
INPUT OFFSET VOLTAGE (µV)
30
10
400
VS+ = VS–
VBIAS = 0V
VEE = –1.25V
40
10
0
TA = 25°C
–10
–20
–30
TA = –40°C
No Load Output Current vs
Supply Voltage
200
6
TA = 85°C
100
0
TA = 25°C
–100
–200
0
10
30
40
50
20
TOTAL SUPPLY VOLTAGE (V)
–400
60
–10
0
50
20
30
40
10
TOTAL SUPPLY VOLTAGE (V)
0
60
10
30
40
50
20
TOTAL SUPPLY VOLTAGE (V)
60
1787 G03
Output Voltage vs Sense Voltage
(Bidirectional Mode)
50
20
10
0
TA = 25°C
–10
TA = –40°C
–20
2.5
VS+ = VS–
VBIAS = 0V
VEE = –1.25V
40
TA = 85°C
30
2.0
1.5
OUTPUT VOLTAGE (V)
VS+ = VS– = 2.5V
VBIAS = 1V
INPUT OFFSET VOLTAGE (µV)
INPUT OFFSET VOLTAGE (µV)
VBIAS = 1V
VEE = 0V
VS+ = VS–
–8
Input Offset Voltage vs
Temperature
30
20
10
0
–10
–20
0
–5
–10
–15
–20
–25
NEGATIVE SUPPLY VOLTAGE (V)
1.0
0.5
VBIAS
–0.5
–1.0
–2.0
–50
–40
–30
–2.5
–20
0
20
40
TEMPERATURE (°C)
60
1787 G04
80
85
–250
–150
50
150
–50
SENSE VOLTAGE (VS+ – VS–) (mV)
1787 G05
Output Voltage vs Sense Voltage
(Unidirectional Mode)
Gain vs Temperature
Gain vs Frequency
30
VS = (2.5V + |VSENSE|)TO 60V
VS = 2.5V TO 60V
TA = –40°C TO 85°C
VBIAS = VEE
VSENSE = 10mV
20
8.185
VS+ > VS–
1.0
250
1787 G06
8.195
1.4
1.2
VS = 5.5V TO 60V
VBIAS = 2.5V
VEE = 0V
–1.5
–30
–40
10
0.8
0.6
GAIN (dB)
8.175
GAIN (V/V)
OUPUT VOLTAGE (V)
TA = 85°C
–4
1787 G02
Input Offset Voltage vs
Negative Supply Voltage
8.165
8.155
0.4
0
–10
–20
VS+ < VS–
–30
8.145
0.2
–40
0
0
30
60
90
120
SENSE VOLTAGE (VS+ – VS–) (mV)
150
1787 G07
4
0
–2
–6
1787 G01
–30
2
TA = –40°C
–50
TA = 25°C
4
–300
–40
TA = –40°C
8
OUTPUT CURRENT (nA)
Input Offset Voltage vs
Supply Voltage
8.135
–40
–20
0
20
40
TEMPERATURE (°C)
60
80
85
1787 G08
–50
0.1k
1k
10k 100k
1M
FREQUENCY (Hz)
10M
100M
1787 G09
LT1787/LT1787HV
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TYPICAL PERFORMANCE CHARACTERISTICS
Negative Input Sense Current vs
Sense Voltage
Supply Current vs Supply Voltage
NEGATIVE INPUT SENSE CURRENT (µA)
TA = 85°C
70
65
60
TA = 25°C
55
TA = –40°C
50
45
VS+ = VS–
40
0
20
30
40
50
10
TOTAL SUPPLY VOLTAGE (V)
60
60
VS = (2.5V + |VSENSE |) TO 60V
110
100
90
80
70
TA = 85°C
60
TA = 25°C
50
TA = –40°C
40
30
–128 –96 –64 –32 0
32 64 96
SENSE VOLTAGE (VS+ – VS–) (mV)
128
POSITIVE INPUT SENSE CURRENT (µA)
120
75
SUPPLY CURRENT (µA)
Positive Input Sense Current vs
Sense Voltage
VS = (2.5V + |VSENSE |) TO 60V
50
40
TA = 85°C
30
20
TA = 25°C
TA = –40°C
10
0
–128 –96 –64 –32 0
32 64 96
SENSE VOLTAGE (VS+ – VS–) (mV)
1787 G11
1787 G10
Step Response at
VSENSE = 0V to 10mV
128
1787 G17
Step Response at
VSENSE = 0V to 128mV
Step Response at
VSENSE = 0V to 128mV
10mV
100mV
100mV
0V
0V
0V
80mV
1V
1V
500mV
500mV
0V
0V
0V
COUT = 0pF
COUT = 0pF
1787 G12
Step Response at
VSENSE = 0V to –128mV
COUT = 1000pF
1787 G18
Step Response at
VSENSE = 0V to –128mV
Step Response at
VSENSE = – 128mV to 128mV
0V
0V
– 100mV
–100mV
100mV
0V
– 100mV
0V
0V
1V
– 500mV
–500mV
0V
– 1V
–1V
–1V
COUT = 0
1787 G19
COUT = 1000pF
1787 G13
1787 G14
COUT = 0
1787 G20
5
LT1787/LT1787HV
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TYPICAL PERFORMANCE CHARACTERISTICS
Step Response at
VSENSE = 128mV to –128mV
VOUT Error vs Supply Ripple
Voltage (VSENSE = ±128mV)
16
900
14
0V
–1V
COUT = 2200pF
1787 G15
12
800
OUTPUT VOLTAGE (V)
1V
SUPPLY RIPPLE VOLTAGE (mV)
100mV
–100mV
Output Voltage vs Sense Voltage
1000
700
600
500
0.5%
400
5%
1%
300
200
2%
VOUT ERROR
LESS THAN 0.1%
8
6
4
2
0
–2
–4
100
0
100
10
VS– = 18V
VBIAS = 0V
VEE = –18V
–6
–8
1k
10k
100k
FREQUENCY (Hz)
1M
1787 G16
0.4 0.8 1.2 1.6
0
–0.8 –0.4
SENSE VOLTAGE (VS+ – VS–) (V)
2.0
1787 G21
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PIN FUNCTIONS
FIL–, FIL+ (Pins 1, 8): Negative and Positive Filter Terminals. Differential mode noise can be filtered by connecting
a capacitor across FIL– and FIL+ . Pole frequency
f– 3dB = 1/(2πRC), R = 1.25kΩ.
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 current to the internal amplifier.
DNC (Pin 3): Do Not Connect. Connected internally. Do not
connect external circuitry to this pin.
VEE (Pin 4): Negative Supply or Ground for Single Supply
Operation.
VOUT (Pin 5): Voltage Output or Current Output proportional to the magnitude of the sense current flowing
through RSENSE. For bidirectional current sensing operation, 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.
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VBIAS (Pin 6): Output Bias Pin. For single supply, bidirectional 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 7): Positive Input Sense Terminal. Positive sense
voltage will result in an output sourcing current proportional to the sense current. VS + is connected to an internal
gain-setting resistor RG2A. Connecting a supply to VS+ and
a load to VS– will allow the LT1787 to measure its own
supply current.
LT1787/LT1787HV
W
BLOCK DIAGRAM
RSENSE
VS–
ISENSE
RG1A
1.25k
RG2A
1.25k
RG1B
1.25k
RG2B
1.25k
FIL–
VS+
FIL+
–
+
A1
IOUT
VBIAS
Q1
Q2
ROUT
20k
VOUT
VEE
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 eliminate aliasing errors.
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
gain of amplifier A1. The currents through RG1 and RG2 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
gives a voltage gain of eight. Positive sense voltages result
in VOUT being positive with respect to pin VBIAS.
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 monitored current through RSENSE by careful choice of connection 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 maximum 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.
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
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 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
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 RG1 and RG2 into output resistor
ROUT. The pins VOUT and VBIAS may be connected in
numerous configurations to interface with following circuitry 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
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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
these contraints, an amplified, level shifted representation
of the RSENSE voltage is developed across ROUT.
Split Supply Bipolar Output Swing
Figure 2 shows the LT1787 used with split power supplies.
The VBIAS pin is connected to ground, and the output
signal appears at the VOUT pin. Bidirectional input currents
can be monitored with the output swinging positive for
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,
as shown in Figure 5. The choice of reference voltage is not
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.
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
LT1787/LT1787HV
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APPLICATIONS INFORMATION
RSENSE
TO
CHARGER/
LOAD
1
FIL–
–
2 VS
3
DNC
C1
1µF
8
FIL+
LT1787
15V
RSENSE
TO
CHARGER/
LOAD
1
VS + 7
FIL–
2 VS
VBIAS 6
3
FIL+
LT1787HV
–
C2
1µF
–5V
VEE
5
DNC
VOUT
1787 F02
20k
5%
C2
1µF
5
LT1634-1.25
C3*
1000pF
OUTPUT
1787 F04
Figure 4. Charge/Discharge Current Monitor on
Single Supply with VBIAS = 1.25V
OUTPUT VOLTAGE – OUTPUT BIAS VOLTAGE (V)
1.5
VS = 3.3V TO 60V
TA = – 40°C TO 85°C
0.5
0
–0.5
–1.0
–1.5
–128 –96 –64 –32 0
32 64 96
SENSE VOLTAGE (VS+ – VS–) (mV)
3.3V
7
VOUT
*OPTIONAL
Figure 2. Split Supply Operation
OUTPUT VOLTAGE (V)
VEE
C3*
1000pF
*OPTIONAL
1.0
ROUT
4
OUTPUT
VS
8
VBIAS 6
ROUT
4
+
3.3V
TO
60V
C1
1µF
128
1.5
1.0
VS = 3.3V TO 60V
TA = – 40°C TO 85°C
0.5
0
–0.5
–1.0
–1.5
–128 –96 –64 –32 0
32 64 96
SENSE VOLTAGE (VS+ – VS–) (mV)
128
1787 F05
1787 F03
Figure 5. Single Supply Output Voltage
with VBIAS = 1.25V
Figure 3. Split Supply Output Voltage
RSENSE
amplified sense voltage appearing between the A/D converters –IN and +IN terminals. The front page of the data
sheet shows a similar circuit which uses a voltage reference 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 VBIAS to VOUT may be
necessary if the sensed current changes more than 1LSB
within a conversion cycle.
5V
1
–
2 VS
3
4
DNC
LT1787
VS + 7
VBIAS 6 IOUT
ROUT
VEE
C1
1µF
8
FIL+
FIL–
5V
R1
20k
5%
5
VCC
CS
LTC1286 CLK
–IN
D
VREF GND OUT
+IN
VOUT
R2
5k
5%
TO µP
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
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 converter configuration also works well with split supply
voltages.
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
sensed differentially between VBIAS and VOUT.
RSENSE
TO
LOAD
8
FIL+
LT1787HV
–
VS+ 7
2 VS
1
Single Supply Unidirectional Operation
3
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
30mV as shown in Figure 9. Accuracy is sacrificed at small
4
FIL–
DNC
VBIAS 6
ROUT
VEE
5
VOUT
VOUT
1787 F08
Figure 8. Unidirectional Current Sensing Mode
ISENSE
RSENSE
TO
CHARGER/
LOAD
2.5V TO
60V
C
0.1µF
0.30
FIL–
–
2 VS
3
4
DNC
FIL+
LT1787
8
2.5V + VSENSE(MAX)
VS+ 7
2.5V
VBIAS 6
ROUT
VEE
5
VOUT
C3
1000pF
–
VOUT A
A1
+
2.5V
1M
5%
0.25
OUTPUT VOLTAGE (V)
1
C1
1µF
0.20
0.15
0.10
0.05
LT1495
0
LT1389-1.25
1787 F07
IDEAL
0
0.005 0.010 0.015 0.020
VS+ – VS– (V)
0.025 0.030
1787 F09
Figure 7. Single Supply 2.5V Bidirectional Operation
with External Voltage Reference and I/V Converter
10
Figure 9. Expanded Scale of Unidirectional Output
LT1787/LT1787HV
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APPLICATIONS INFORMATION
Adjusting Gain Setting
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.
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:
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 accuracy. After trimming, the temperature stability of the gm
and therefore gain will be –200ppm/°C.
The only limitations placed upon the resistor choice is care
must be taken not to saturate the internal circuitry by
violating the VOMAX specification of VS + –0.75V.
AV = gm • ROUT = 400µA/V • 100k = 40
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PACKAGE DESCRIPTION
Dimensions in inches (millimeters) unless otherwise noted.
MS8 Package
8-Lead Plastic MSOP
(LTC DWG # 05-08-1660)
0.040 ± 0.006
(1.02 ± 0.15)
0.007
(0.18)
0.118 ± 0.004*
(3.00 ± 0.102)
0.034 ± 0.004
(0.86 ± 0.102)
8
7 6
5
0° – 6° TYP
SEATING
PLANE 0.012
(0.30)
0.0256
REF
(0.65)
BSC
0.021 ± 0.006
(0.53 ± 0.015)
0.006 ± 0.004
(0.15 ± 0.102)
0.118 ± 0.004**
(3.00 ± 0.102)
0.193 ± 0.006
(4.90 ± 0.15)
MSOP (MS8) 1098
1
* 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
4
2 3
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
× 45°
(0.254 – 0.508)
0.008 – 0.010
(0.203 – 0.254)
0.053 – 0.069
(1.346 – 1.752)
0°– 8° TYP
0.016 – 0.050
(0.406 – 1.270)
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
8
7
6
5
0.004 – 0.010
(0.101 – 0.254)
0.050
(1.270)
BSC
0.150 – 0.157**
(3.810 – 3.988)
0.228 – 0.244
(5.791 – 6.197)
1
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
2
3
4
SO8 1298
11
LT1787/LT1787HV
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TYPICAL APPLICATION
Split or Single Supply Operation, Bidirectional Output into A/D
1Ω
1%
IS = ±125mA
VSRCE
≈4.75V
1
–
2 VS
3
VCC
5V
8
FIL+
FIL–
LT1787
DNC
VEE 4
VEE
–5V
VS+ 7
10µF
16V
VBIAS 6
20k
VOUT
1
VOUT (±1V)
5
OPTIONAL SINGLE
SUPPLY OPERATION:
DISCONNECT VBIAS
FROM GROUND
AND CONNECT IT TO VREF.
REPLACE –5V SUPPLY
WITH GROUND.
OUTPUT CODE FOR ZERO
CURRENT WILL BE ~2430
CONV
7
2
6
AIN LTC1404 CLK
VREF
5
DOUT
GND
10µF
16V
4
8
CLOCKING
CIRCUITRY
3
10µF
16V
VEE
–5V
DOUT
1787 TA02
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTC1043
Dual Precision Instrumentation Switched Capacitor Building Block
120dB CMRR, 3V to 18V Operation
LT1490/LT1491
Dual and Quad Micropower Rail-to-Rail Input and Output Op Amps
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.
12
Linear Technology Corporation
1787f LT/TP 0100 4K • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com
 LINEAR TECHNOLOGY CORPORATION 1999
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