Texas Instruments AN-1975 LMP8640, AN-1975 LMP8645 User Manual
AN-1975 LMP8640/LMP8645 Evaluation Board
1 Introduction
This evaluation board shows a bidirectional high-side current sense made using LMP8640 (LMP8645) and
optional differential amplifier in order to have a single output.
2 Connectors
2.1 Power Supply
There are two banana plugs labeled GND and VCCto power the evaluation board. Moreover, a banana
plug labeled VCC_Amp is used to power an optional operational amplifier that makes the difference
between the outputs of the two LMP8640 (LMP8645).
2.2 Signal Connectors
There are five connectors for signals.
User's Guide
SNOA546C–November 2009–Revised April 2013
IN It connects the non inverting input of U1 and inverting input of U2.
LOAD It connects the non inverting input of U2 and inverting input of U1.
The device U1 is able to sense the current when it flows in the shunt resistor from the LOAD pin to the IN
pin. The device U2 is able to sense the current when it flows in the shunt resistor from IN pin to LOAD pin.
OUT+ Output of device U2, the voltage at this pin is related to the current that flows from the IN pin to the LOAD pin.
OUT- Output of device U1, the voltage at this pin is related to the current that flows from the LOAD pin to the IN pin.
OUT Optional Output of the difference amplifier that makes the difference between OUT+ and OUT-
3 Hardware Setup
3.1 Power Supply Setup
• High side current sense LMP8640 (LMP8645)
– Connect a supply voltage in the range between 2.7 V and 12 V to the VCCand GND turrets.
• Optional difference amplifier
– Connect a supply according to the specs of the amplifier to the VCC_Amp and GND turrets.
Table 1. Input signals
Table 2. Output signals
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SNOA546C–November 2009–Revised April 2013 AN-1975 LMP8640/LMP8645 Evaluation Board
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x
Vref =
Amp_VCC
5R
6R5R +
OUT = -
¹
·
©
§
R1
R4
* OUT- +
R1
R4
1 +
¹
·
©
§
R2 + R3
R2
* Vref +
R2 + R3
R3
* OUT
+
Hardware Setup
3.2 Source and Load Setup
First case: Current flows from IN to LOAD plugs.
A voltage supply can be connected between the IN and the GND banana plugs, while a load is connected
between the LOAD and the GND banana plugs. The voltage applied at the IN pin should not exceed the
maximum common mode voltage allowed by the LMP8640/HV (LMP8645/HV). The maximum allowed
common mode voltages are listed in Table 3.
Second case: Current flows from LOAD to IN plugs.
In this case, the voltage supply is connected between the LOAD and the GND banana plugs, while the
load is connected between the IN and GND banana plugs. The voltage applied at LOAD pin should not
exceed the maximum common mode voltage allowed by the LMP8640/HV (LMP8645/HV). The maximum
allowed common mode voltages are listed in Table 3.
DEVICE MAX VCM
LMP8640 42V
LMP8645 42V
LMP8640HV 76V
LMP8645HV 76V
According to the shunt resistor (Rsns), to the gain of LMP8640 (LMP8645) and to the supply voltage
different ranges of currents can be sensed with this evaluation board.
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Table 3. Max Common Mode Voltage
3.3 Components
On the evaluation board, there are already the circuit and the footprint of a standard dual op amp (U3) to
implement a differential op amp (U3.B) with reference (U3.A) in order to provide a single ended output of
the bidirectional current.
U3 Standard Dual Op Amp 8 pin.
R1, R2, R3, R4 resistors that implement the differential circuit according to Equation 1:
R5, R6, C5 components for voltage reference and its filter. The voltage reference is calculating according to Equation 2:
U3.1 OUT A
U3.2 -IN A
U3.3 +IN A
U3.4 V
U3.5 +IN B
U3.6 -IN B
U3.7 OUT B
U3.8 V
-
+
Table 4. Optional Differential Circuit
(1)
(2)
2
AN-1975 LMP8640/LMP8645 Evaluation Board SNOA546C–November 2009–Revised April 2013
Copyright © 2009–2013, Texas Instruments Incorporated
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V/V
Rg
Gain =
k5 :
+IN -IN
OUTV-V+
RG
-+
A
-+
SOURCE
V
DMM1
DMM2
Rsns
+IN-IN
OUT
V-
V+
RG
-+
VCC
ohmic
LOAD
Rg1
Rg2
VCC
IN
LOAD
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4 Using the Evaluation Board
4.1 Input and Output Signals
The evaluation board allows you to measure a bidirectional current, so the IN and LOAD banana
connectors can act either as the Source or LOAD pin. If the current flows from the IN pin to the LOAD pin,
a simple way to test the performance of the LMP8640 (LMP8645) is shown in Figure 1.
Using the Evaluation Board
The DMM1 is configured as a voltmeter, which measures the output of the LMP8640 (LMP8645), while the
DMM2 is configured as an Ammeter, which measures the current that flows in the LOAD. The source is a
voltage supply that makes sure to set a voltage in the range of -2 V to +46 V for LMP8640 (LMP8645) or 2 V to +76 V for LMP8640HV (LMP8645HV).
4.2 Gain Selection of LMP8645
The evaluation board is provided with a shunt resistor Rsns (10 mΩ, @ 1%, 1W), while two gain resistors
Rg1 and Rg2 (10 kΩ, @1%) ensure a gain of 2 V/V for each current sense. The Gain is evaluated
according to formula in Equation 3:
The gain resistor must be chosen such that the max output voltage does not exceed the LMP8645 max
output voltage rating for a given common mode voltage (further details in the Datasheet).
4.3 Single Output
In the applications where a single measurement of a bidirectional current sense is needed is possible to
populate the evaluation board with two amplifiers and some resistors (see Section 3.3).
For instance, in the following configuration:
R1 = R2 = R3 = R4 = 10 kΩ,
R5 = R6 =10 kΩ
OUT = V
V
REF
REF
+(OUT+-OUT-)
= 0.5*Vcc_Amp.
Figure 1. LMP8640 (LMP8645) Measurement Setup
(3)
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