Linear Technology LT1787HV, LT1787 Datasheet

FEATURES

■

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

U

APPLICATIO S

■

Battery Monitoring

■

Power Monitoring

■

Portable Phones

■

Cellular Phones

■

Portable Test/Measurement Systems

■

Battery-Operated Systems

LT1787/LT1787HV

Precision, High Side

Current Sense Amplifiers

U

DESCRIPTIO

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.

, LTC and LT are registered trademarks of Linear Technology Corporation.

U

TYPICAL APPLICATIO

12-Bit Dynamic Resolution Unidirectional Output into LTC®1286 ADC

R

SENSE

FIL
V

S

DNC

V

EE

LOAD

–

LT1787HV

–

• R

0.0016Ω

SENSE

V

R
20k

V

)

FIL

V

BIAS

OUT

OUT

S

+

+

0.1µF

2.5V TO 60V

R1
15k

V

REFVCC

+IN

–IN

C2

LT1634-1.25

LTC1286

GND

1µF

D

C1

CS

CLK

OUT

LOAD

V

OUT

TO

= V

I = 100A

18

27

36

45

+ (8 • I

BIAS

TO µP

1787 TA01

Input Offset Voltage vs Supply Voltage

50
40
30
20

5V

10

0
–10
–20
–30

INPUT OFFSET VOLTAGE (µV)

–40
–50

10

0

20

TOTAL SUPPLY VOLTAGE (V)

30 40

50

1787 TA01b

60

1

LT1787/LT1787HV

1
2
3
4

8
7
6
5

TOP VIEW

MS8 PACKAGE

8-LEAD PLASTIC MSOP

FIL

+

V

S

+

V

BIAS

V

OUT

FIL

–

V

S

–

DNC*

V

EE

WW

W

ABSOLUTE MAXIMUM RATINGS

U

(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)

U

W

PACKAGE/ORDER INFORMATION

ORDER PART NUMBER

LT1787CMS8
LT1787IMS8
LT1787HVCMS8
LT1787HVIMS8

* DO NOT CONNECT

T

= 150°C, θJA = 250°C/W

JMAX

MS8 PART MARKING

LTGM
LTGN
LTKJ
LTKK

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

U

FIL

V

DNC*

V

–

1

–

2

S

3

4

EE

T

JMAX

8-LEAD PLASTIC SO

* DO NOT CONNECT

TOP VIEW

8

7

6

5

S8 PACKAGE

= 150°C, θJA = 190°C/W

FIL
V
V
V

S

BIAS

OUT

ORDER PART NUMBER

+

+

LT1787CS8
LT1787IS8
LT1787HVCS8
LT1787HVIS8

S8 PART MARKING

1787
1787I
1787HV
787HVI

Consult factory for Military grade parts.

ELECTRICAL CHARACTERISTICS

The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
Total supply = (V

–

– VEE) = 2.5V to 36V (LT1787), 2.5V to 60V (LT1787HV) unless otherwise specified.

S

(Note 4)

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

–

+

V

, V

S

V

SENSE

V

OS

Sense Amplifier Supply Voltage Single Supply Operation (LT1787) ● 2.5 36 V

S

Input Sense Voltage Full Scale V
Input Offset Voltage (S8) I

Input Offset Voltage (MS8) I

Single Supply Operation (LT1787HV)

+

= V

SENSE

= 0, VS Supply = 5V –75 ±40 75 µV

OUT

≤ 70°C ● –135 135 µV

0°C ≤ T

A

–40°C ≤ T
I

= 0 (LT1787) –100 100 µV

OUT

≤ 70°C ● –160 160 µV

0°C ≤ T

A

–40°C ≤ T
I

= 0 (LT1787HV) –100 100 µV

OUT

≤ 70°C ● –160 160 µV

0°C ≤ T

A

–40°C ≤ T

= 0, VS Supply = 5V –125 ±40 125 µV

OUT

≤ 70°C ● –230 230 µV

0°C ≤ T

A

–40°C ≤ T
I

= 0 (LT1787) –150 150 µV

OUT

0°C ≤ T

≤ 70°C ● –250 250 µV

A

–40°C ≤ T
I

= 0 (LT1787HV) –150 150 µV

OUT

≤ 70°C ● –250 250 µV

0°C ≤ T

A

–40°C ≤ T

–

– V

, VS = 10V, V

S

S

≤ 85°C ● –200 200 µV

A

≤ 85°C ● –225 225 µV

A

≤ 85°C ● –225 225 µV

A

≤ 85°C ● –250 250 µV

A

≤ 85°C ● –280 280 µV

A

≤ 85°C ● –280 280 µV

A

= 5V, AV = 8 ±10% ● 500 mV

BIAS

● 2.5 60 V

2

LT1787/LT1787HV

ELECTRICAL CHARACTERISTICS

The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
Total supply = (V

–

– VEE) = 2.5V to 36V (LT1787), 2.5V to 60V (LT1787HV) unless otherwise specified.

S

(Note 4)

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

VOS TC Temperature Coefficient of VOSVS Supply = 5V (Note 5) 0.5 2 µV/°C
I

OUT(O)

V

OUT(O)

g

m

A

V

No-Load Output Current Error V
No-Load Output Voltage Error V

SENSE

SENSE

(S8) 0°C ≤ T

–40°C ≤ T

No-Load Output Voltage Error V

SENSE

(MS8) 0°C ≤ T

–40°C ≤ T

Tranconductance, I

Gain, V

OUT/VSENSE

OUT/VSENSE±VSENSE

V

Supply = Total Supply + |V

S

±V

SENSE

= 0V 4 nA
= 0V, VS Supply = 5V –600 600 µV

≤ 70°C ● –1080 1080 µV

A

≤ 85°C ● –1600 1600 µV

A

= 0V, VS Supply = 5V –1000 1000 µV

≤ 70°C ● –1840 1840 µV

A

≤ 85°C ● –2000 2000 µV

A

= 10mV, 50mV, 100mV, 150mV, 250mV, 400 µA/V

|

SENSE

= 100mV, VS Supply = 5V ● 7.6 8 8.4 V/V

Output Voltage Gain Error ● –5 2 5 %

VS PSRR VS Supply Rejection Ratio V

VEE PSRR Negative Supply Rejection Ratio V

∆V
∆V

I

S+(O)

I

S–(O)

I

EE(O)

I

OUT

V

OUT

OS

BIAS

Change in Input Offset Voltage V
with Change in V

Voltage V

BIAS

Positive Input Sense Current V
Negative Input Sense Current V
Negative Supply Current V
Output Current V
Output Voltage V
Ripple Rejection V

V

OMIN

Minimum Output Voltage V

Unipolar Output V
Saturation Voltage V

V

OMAX

R

G1A, RG2A

R

OUT

Maximum Output Voltage V
Input Gain-Setting Resistor Pin 1 to Pin 2, Pin 7 to Pin 8 1.25 kΩ
Output Resistor Pin 5 to Pin 6 20 kΩ

= 0V, VS Supply = 2.5V to 36V (LT1787) ● 120 135 dB

SENSE

= 0V, VS Supply = 2.5V to 60V (LT1787HV) ● 120 135 dB

V

SENSE

= 0V, VS Supply = 15V, V

SENSE

= –1V to –15V (LT1787)

V

EE

V

= 0V, VS Supply = 40V, V

SENSE

= –1V to –15V (LT1787HV)

V

EE

= 0V, VS Supply = 36V, V

SENSE

= 0V, VS Supply = 60V, V

SENSE

= 0V ● 10 20 µA

SENSE

= 0V ● 50 100 µA

SENSE

= 0V ● 60 120 µA

SENSE

= ±128mV ±50 µA

SENSE

SENSE

S

SENSE

V

SENSE

SENSE
SENSE

V

SENSE

V

SENSE

+

= V

= ±128mV, V

–

= 20V, ∆VS Supply = 1V, f = 1kHz ● 80 88 dB

S

= 0V, V

+

= V

– V

S

= 2mV, V
= 4mV, V
= 5mV, V
= 6mV, V

+

≥ 3.3V V

S

= 0V ● 30 45 mV

BIAS

–

= –128mV, V

S

= 0V ● 32 50 mV

BIAS

= 0V ● 38 55 mV

BIAS

= 0V ● 43 60 mV

BIAS

= 0V ● 49 65 mV

BIAS

= 0V, ● 100 130 dB

BIAS

= 0V, ● 100 130 dB

BIAS

= 0.5V to 25V (LT1787) ● 100 130 dB

BIAS

= 0.5V to 25V (LT1787HV) ● 100 130 dB

BIAS

±1.024 V

BIAS

= 0V 10 mV

BIAS

+

– 0.75 V

S

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

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, VEE = 0V unless otherwise

BIAS

specified.
Note 5: This parameter is not 100% tested.

3

LT1787/LT1787HV

SENSE VOLTAGE (V

S

+

– V

S

–

) (mV)

–250

OUTPUT VOLTAGE (V)

2.5

2.0

1.5

1.0

0.5

V

BIAS

–0.5
–1.0
–1.5
–2.0
–2.5

150

1787 G06

–150

–50

50

250

VS = 5.5V TO 60V
V

BIAS

= 2.5V

V

EE

= 0V

W

U

TYPICAL PERFORMANCE CHARACTERISTICS

Input Offset Voltage vs
Supply Voltage

50

+

–

V

= V

S

V

BIAS

V

EE

S

= 0V

= –1.25V

40
30
20
10

0
–10
–20
–30

INPUT OFFSET VOLTAGE (µV)

–40
–50

0

10

20

TOTAL SUPPLY VOLTAGE (V)

Input Offset Voltage vs
Negative Supply Voltage

30

+

–

V

= V

= 2.5V

S

S

= 1V

V

BIAS

20

10

0

–10

INPUT OFFSET VOLTAGE (µV)

–20

–30

0

–5 –10 –15 –20 –25

NEGATIVE SUPPLY VOLTAGE (V)

T

T

T

A

T

= 25°C

A

T

= –40°C

A

A

= 85°C

A

= 25°C

= –40°C

30 40

T

A

= 85°C

No Load Output Voltage
vs Supply Voltage

400

+

–

V

= V

S

S

V

BIAS

V

EE

= 0V

= –1.25V

T

= 85°C

A

300

200

100

0

T

= 25°C

A

–100

OUTPUT VOLTAGE (µV)

–200

–300

50

60

–400

010

= –40°C

T

A

20 4030

50

60

TOTAL SUPPLY VOLTAGE (V)

1787 G01

1787 G02

Input Offset Voltage vs
Temperature

50

+

–

V

= V

S

V

BIAS

V

EE

S

= 0V

= –1.25V

40
30

No Load Output Current vs
Supply Voltage

10

T

= –40°C

A

8
6

T

= 25°C

A

4
2

0
–2
–4

OUTPUT CURRENT (nA)

–6

V

= 1V

BIAS

= 0V

V

EE

–8

+

= V

V

S

–10

S

0

10

= 85°C

T

A

–

30 40

20

TOTAL SUPPLY VOLTAGE (V)

Output Voltage vs Sense Voltage
(Bidirectional Mode)

50

60

1787 G03

20
10

0
–10
–20

INPUT OFFSET VOLTAGE (µV)

–30
–40

–30

1787 G04

–50

–20 0 20 40 60

–40

TEMPERATURE (°C)

80

1787 G05

85

1.4

1.2

1.0

0.8

0.6

OUPUT VOLTAGE (V)

0.4

0.2

4

Output Voltage vs Sense Voltage
(Unidirectional Mode)

VS = 2.5V TO 60V
T

= –40°C TO 85°C

A

= V

V

BIAS

EE

0

30 60 90 150

0

SENSE VOLTAGE (V

+

– V

S

S

120

–

) (mV)

1787 G07

8.195

8.185

8.175

8.165

GAIN (V/V)

8.155

8.145

8.135

V

= (2.5V + |V

S

–40

–20 0 20 40 60

TEMPERATURE (°C)

Gain vs FrequencyGain vs Temperature

|)TO 60V

SENSE

+

–

V

> V

S

S

+

–

V

< V

S

S

80

85

1787 G08

30

20

10

0

–10

GAIN (dB)

–20

–30

–40

–50

0.1k 1k

10k 1M100k

FREQUENCY (Hz)

V

SENSE

= 10mV

10M

1787 G09

100M

W

SENSE VOLTAGE (V

S

+

– V

S

–

) (mV)

–128

POSITIVE INPUT SENSE CURRENT (µA)

60

50

40

30

20

10

0

–64

0

32

1787 G17

–96 –32

64

96

128

VS = (2.5V + |V

SENSE

|) TO 60V

TA = 85°C

TA = –40°C

TA = 25°C

U

TYPICAL PERFORMANCE CHARACTERISTICS

LT1787/LT1787HV

Supply Current vs Supply Voltage

75

70

65

60

55

50

SUPPLY CURRENT (µA)

45

+

V

= V

S

40

0

10

TA = 85°C

TA = 25°C

T

= –40°C

A

–

S

20 30 40 50 60

TOTAL SUPPLY VOLTAGE (V)

Step Response at
V

= 0V to 10mV

SENSE

10mV

0V

80mV

0V

1787 G10

Negative Input Sense Current vs
Sense Voltage

120

110

100

90

80

70

60

50

40

NEGATIVE INPUT SENSE CURRENT (µA)

30

–128

–96 –32

VS = (2.5V + |V

–64

SENSE VOLTAGE (V

0

+

S

Step Response at
V

= 0V to 128mV

SENSE

100mV

0V

1V

500mV

0V

SENSE

32

– V

|) TO 60V

TA = 85°C

TA = 25°C

TA = –40°C

64

–

) (mV)

S

96

1787 G11

128

Positive Input Sense Current vs
Sense Voltage

Step Response at
V

= 0V to 128mV

SENSE

100mV

0V

1V

500mV

0V

= 0pF 1787 G12

C

OUT

Step Response at
V

SENSE

0V

–100mV

0V

–500mV

–1V

C

= 0 1787 G19

OUT

= 0V to –128mV

C

OUT

Step Response at
V

SENSE

0V

–100mV

0V

–500mV

–1V

C

OUT

= 0pF 1787 G18

= 0V to –128mV

= 1000pF 1787 G14

C

OUT

Step Response at
V

SENSE

100mV

0V

–100mV

1V

0V

–1V

C

OUT

= 1000pF 1787 G13

= –128mV to 128mV

= 0 1787 G20

5

LT1787/LT1787HV

W

U

TYPICAL PERFORMANCE CHARACTERISTICS

Step Response at
V

SENSE

100mV

–100mV

1V
0V

–1V

C

OUT

= 128mV to –128mV

= 2200pF 1787 G15

V

Error vs Supply Ripple

OUT

Voltage (V

1000

900
800
700
600
500
400
300
200

SUPPLY RIPPLE VOLTAGE (mV)

LESS THAN 0.1%

100

0

100 10k 100k 1M

V

OUT

ERROR

1k

SENSE

FREQUENCY (Hz)

UUU

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

= 1/(2πRC), R = 1.25kΩ.

–3dB

–

V

(Pin 2): Negative Input Sense Terminal. Negative

S

sense voltage will result in an output sinking current
proportional to the sense current. V
internal gain-setting resistor R
rent to the internal amplifier.

DNC (Pin 3): Do Not Connect. Connected internally. Do not
connect external circuitry to this pin.

V

(Pin 4): Negative Supply or Ground for Single Supply

EE

Operation.

–

is connected to an

S

and supplies bias cur-

G1A

= ±128mV)

0.5%

V

5%

1%

2%

1787 G16

(Pin 6): Output Bias Pin. For single supply, bidirec-

BIAS

tional current sensing operation, V
external bias voltage, so that at V
V

OUT(O)

+ V

. For dual supply, bidirectional current

BIAS

sensing operation, V
V

= V

OUT

+

V

(Pin 7): Positive Input Sense Terminal. Positive sense

S

OUT(O)

at V

Output Voltage vs Sense Voltage

16

–

V

= 18V

14

S

= 0V

V

BIAS

12

V

= –18V

EE

10

8
6
4
2
0

OUTPUT VOLTAGE (V)

–2
–4
–6
–8

–0.8 –0.4

SENSE VOLTAGE (V

is connected to ground. Thus,

BIAS

= 0V.

SENSE

0.4

0 0.8 2.0

BIAS

SENSE

+

– V

S

is connected to an

= 0V, V

1.2 1.6

–

) (V)

S

OUT

1787 G21

voltage will result in an output sourcing current propor­tional to the sense current. V
gain-setting resistor R
a load to V

–

will allow the LT1787 to measure its own

S

G2A

+

is connected to an internal

S

. Connecting a supply to V

+

and

S

supply current.

=

V

(Pin 5): Voltage Output or Current Output propor-

OUT

tional to the magnitude of the sense current flowing
through R
tion, V

OUT

. For bidirectional current sensing opera-

SENSE

= AV • V

SENSE

+ V

OUT(O)

+ V

BIAS

,

where:

+

V
V

V

OUT(O)

OUT
OUT

> V
< V

BIAS
BIAS

for V
for V

S
S

is the no load output voltage at V

> V

+

< V

–

S

–

S

= 0V.

SENSE

6

BLOCK DIAGRAM

W

LT1787/LT1787HV

R

–

V

S

SENSE

I

SENSE

+

V

S

R

G1A

1.25k

R

G1B

1.25k

–

Q1

CURRENT MIRROR

A1

U

–

FIL

V

EE

WUU

Figure 1. LT1787 Functional Diagram

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 V

+

and V

S

resistors RG1 and RG2. The opposite ends of resistors R
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 V
current from RG1 and RG2 flowing through resistor R
gives a voltage gain of eight. Positive sense voltages result
in V

Pins VEE, V

being positive with respect to pin V

OUT

BIAS

ways to interface with subsequent circuitry. Split supply

–

apply the sense voltage to matched

S

by the 1:1 current mirror. The net

OUT

BIAS

and V

may be connected in a variety of

OUT

G1

OUT

.

R

G2A

1.25k

R

G2B

1.25k

+

I

OUT

R

OUT

Q2

20k

1787 F 01

+

FIL

V

BIAS

V

OUT

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 moni­tored current through R

by careful choice of connec-

SENSE

tion polarity.

Selection of External Current Sense Resistor

External R

resistor selection is a delicate trade-off

SENSE

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.

7

LT1787/LT1787HV

U

WUU

APPLICATIONS INFORMATION

Kelvin connection of the LT1787’s V

+

and V

S

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

= 1/(2πRC)

–3dB

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
R

. The pins V

OUT

OUT

and V

may be connected in

BIAS

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

–

inputs to

S

+

at V
input voltage V
saturation with loss of accuracy occurs for V
with absolute minimum swing at 30mV above VEE. V
may swing positive to within 0.75V of V

are imposed by the negative supply, VEE, and the

OUT

+

. In the negative direction, internal circuit

S

< 70mV

OUT

+

or a maximum

S

OUT

of 35V, a limit set by internal junction breakdown. Within
these contraints, an amplified, level shifted representation
of the R

voltage is developed across R

SENSE

OUT

.

Split Supply Bipolar Output Swing

Figure 2 shows the LT1787 used with split power supplies.
The V
signal appears at the V

pin is connected to ground, and the output

BIAS

pin. Bidirectional input currents

OUT

can be monitored with the output swinging positive for
current flow from V

+

and V

S

opposite direction cause V

–

. Input currents in the

S

to swing below ground.

OUT

Figure 2 shows an optional output capacitor connected
from V

to ground. This capacitor may be used to filter

OUT

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
LT1634 voltage reference. The V
swing above and below V

pin shifted positive using an external

BIAS

output signal can

OUT

to allow monitoring of

BIAS

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

to swing without saturating the

OUT

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

8

LT1787/LT1787HV

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

U

WUU

APPLICATIONS INFORMATION

CHARGER/

–5V

*OPTIONAL

LOAD

TO

1

2

3

4

C2
1µF

Figure 2. Split Supply Operation

1.5
VS = 3.3V TO 60V

T

1.0

0.5

0

R

SENSE

–

FIL

LT1787

–

V

S

DNC

V

EE

= –40°C TO 85°C

A

C1

8

+

FIL

+

V

S

7

V

BIAS

6

R

OUT

5

V

OUT

C3*
1000pF

1µF

OUTPUT

1787 F02

15V

CHARGER/

LOAD

TO

*OPTIONAL OUTPUT

R

SENSE

C1

C3*

1000pF

1µF

1µF

C2

1

–

FIL

LT1787HV

–

V

S

2

3

DNC

4

V

EE

8

+

FIL

+

V

S

7

V

6

BIAS

R

OUT

5

V

OUT

Figure 4. Charge/Discharge Current Monitor on
Single Supply with V

1.5
VS = 3.3V TO 60V

= –40°C TO 85°C

T

A

1.0

0.5

0

BIAS

= 1.25V

3.3V
TO
60V

3.3V

20k
5%

LT1634-1.25

1787 F04

–0.5

OUTPUT VOLTAGE (V)

–1.0

–1.5

–128

–96 –32

–64

SENSE VOLTAGE (V

0

32

+

– V

S

–

S

64

) (mV)

96

128

1787 F03

Figure 3. Split Supply Output Voltage

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 V

BIAS

to V

may be

OUT

necessary if the sensed current changes more than 1LSB
within a conversion cycle.

–0.5

–1.0

OUTPUT VOLTAGE – OUTPUT BIAS VOLTAGE (V)

–1.5

–128

–96 –32

SENSE VOLTAGE (V

–64

0

32

+

– V

S

–

S

64

) (mV)

96

128

1787 F05

Figure 5. Single Supply Output Voltage
with V

Figure 6. Unidirectional Output into A/D
with Fixed Supply at V

BIAS

= 1.25V

+

S

9

LT1787/LT1787HV

U

WUU

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 V
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.

Single Supply Unidirectional Operation

Figure 8 shows the simplest connection in which the
LT1787 may be used. The V
ground, and the V

pin swings positive with increasing

OUT

sense current. The LT1787’s outputs can swing as low as
30mV as shown in Figure 9. Accuracy is sacrificed at small

pin is held equal to the

OUT

pin is connected to

BIAS

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

above ground. The level shifting may

BIAS

be done with resistor dividers, voltage references or a
simple diode. Accuracy is ensured if the output signal is
sensed differentially between V

R

FIL
V

DNC

V

S

EE

0.1µF

–

LT1787HV

–

SENSE

C

TO

LOAD

1

2

3

4

Figure 8. Unidirectional Current Sensing Mode

V

R

V

BIAS

FIL

V

S

BIAS

OUT

OUT

1787 F08

+

+

and V

8

7

6

5

V

OUT

OUT

2.5V TO
60V

.

I

TO

CHARGER/

LOAD

R

SENSE

1

–

FIL

LT1787

–

V

S

2

3

DNC

4

V

EE

2.5V

SENSE

C1

8

+

FIL

+

V

S

7

V

BIAS

6

R

OUT

5

V

OUT

1M

5%

1µF

C3
1000pF

LT1389-1.25

Figure 7. Single Supply 2.5V Bidirectional Operation
with External Voltage Reference and I/V Converter

+

2.5V

–

A1

2.5V + V

LT1495

1787 F07

SENSE(MAX)

V

OUT A

0.30

0.25

0.20

0.15

0.10

OUTPUT VOLTAGE (V)

0.05

0

IDEAL

0

0.005

0.010

0.015 0.020

+

V

– V

S

–

S

0.025 0.030

(V)

1787 F09

Figure 9. Expanded Scale of Unidirectional Output

10

LT1787/LT1787HV

(

U

WUU

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 R
resistor. When an external resistor is used, leave the V
pin floating or connected to the V
the internal R

from the circuit.

OUT

The voltage gain will be gm • R

pin. This will remove

OUT

where gm is the

OUT

OUT

BIAS

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 R

AV = gm • R

= 400µA/V • 100k = 40

OUT

OUT

:

U

PACKAGE DESCRIPTION

0.007
(0.18)

0.021

± 0.006

(0.53 ± 0.015)

* 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"

° – 6° TYP

0

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)

SEATING

PLANE

0.012
(0.30)

0.0256

REF

(0.65)

BSC

0.152mm) PER SIDE

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 V

0.034 ± 0.004
(0.86 ± 0.102)

0.006 ± 0.004

(0.15 ± 0.102)

specification of VS + –0.75V.

OMAX

0.118 ± 0.004*
(3.00 ± 0.102)

0.193 ± 0.006

(4.90 ± 0.15)

8

7

12

6

3

5

0.118 ± 0.004**
(3.00 ± 0.102)

MSOP (MS8) 1098

4

0.010 – 0.020

(0.254 – 0.508)

0.008 – 0.010

(0.203 – 0.254)

*

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

× 45°

0.016 – 0.050

(0.406 – 1.270)

0.053 – 0.069

(1.346 – 1.752)

0°– 8° TYP

0.014 – 0.019

(0.355 – 0.483)

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.

S8 Package

8-Lead Plastic Small Outline (Narrow 0.150)

(LTC DWG # 05-08-1610)

0.004 – 0.010

(0.101 – 0.254)

0.228 – 0.244

(5.791 – 6.197)

TYP

0.050

(1.270)

BSC

0.189 – 0.197*
(4.801 – 5.004)

7

8

1

2

5

6

0.150 – 0.157**
(3.810 – 3.988)

3

4

SO8 1298

11

LT1787/LT1787HV

TYPICAL APPLICATION

Split or Single Supply Operation, Bidirectional Output into A/D

IS = ±125mA

SRCE

V

–5V

1

–

FIL

–

V

S

2

3

DNC

4

EE

V

EE

V

≈4.75V

U

1Ω
1%

LT1787

V

20k

V

FIL

V

BIAS

OUT

8

+

+

S

7

6

5

(±1V)

V

OUT

OPTIONAL SINGLE

SUPPLY OPERATION:

DISCONNECT V

AND CONNECT IT TO V

FROM GROUND

REPLACE –5V SUPPLY

WITH GROUND.
OUTPUT CODE FOR ZERO
CURRENT WILL BE ~2430

BIAS

REF

V

CC

5V

10µF
16V

1

7

LTC1404

4

V

EE

–5V

8

CONV

CLK

DOUT

6
5

10µF
16V

CLOCKING

CIRCUITRY

D

OUT

1787 TA02

2

A

IN

3

V

REF

GND

10µF
16V

.

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-Top

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.

TM

Inputs

12

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone]

●

www.linear-tech.com

1787f LT/TP 0100 4K • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 1999