LMP8640,LMP8645
Application Note 1975 LMP8640 / LMP8645 Evaluation Board User Guide
Literature Number: SNOA546B
LMP8640 / LMP8645
Evaluation Board User
Guide
National Semiconductor
Application Note 1975
Domenico Granozio
July 15, 2010
This evaluation board shows a bidirectional high side current
sense made using LMP8640 (LMP8645) and optional differ-
ential amplifier in order to have a single output.
Connectors
POWER SUPPLY
There are two banana plugs labeled GND and VCC to power
the evaluation board. Moreover a banana plug labeled
VCC_Amp is used to power an optional operational amplifier
which makes the difference between the outputs of the two
LMP8640 (LMP8645).
SIGNAL CONNECTORS
There are five connectors for signals.
Input signals
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 LOAD pin to IN pin. The device U2 is able
to sense the current when it flows in the shunt resistor from
IN pin to LOAD pin.
Output signals
OUT+ Output of device U2, the voltage at this pin is
related to the current which flows from IN pin to
LOAD pin.
OUT- Output of device U1, the voltage at this pin is
related to the current which flows from LOAD pin
to IN pin.
OUT Optional Output of the difference amplifier which
makes the difference between OUT+ and OUT-
Hardware Setup
POWER SUPPLY SETUP
1. High side current sense LMP8640 (LMP8645)
Connect a supply voltage in the range between 2.7V and
12V to the VCC and GND turrets.
2. Optional difference amplifier
Connect a supply according to the specs of the amplifier
to the VCC_Amp and GND turrets.
SOURCE AND LOAD SETUP
First case. Current flows from IN to LOAD plugs.
A voltage supply can be connected between the IN and GND
banana plugs, while a load between the LOAD and GND ba-
nana plugs. The voltage applied at IN pin has not to exceed
the max common mode voltage allowed by the LMP8640/HV
(LMP8645/HV). The max allowed common mode voltages
are listed in the following table.
Second case. Current flows from LOAD to IN plugs.
In this case the voltage supply is connected between the
LOAD and GND banana plugs, while the load between the IN
and GND banana plugs. The voltage applied at LOAD pin has
not to exceed the max common mode voltage allowed by the
LMP8640/HV (LMP8645/HV). The max allowed common
mode voltages are listed in the following table.
Max Common Mode Voltage
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 cur-
rents can be sensed with this evaluation board.
COMPONENTS
On the evaluation board there are already the circuit and the
footprint of a standard dual op amp (U3) to implement a dif-
ferential Op amp (U3.B) with reference (U3.A) in order to
provide a single ended output of the bidirectional current.
Optional differential circuit
U3 Standard Dual Op Amp 8 pin.
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+
R1, R2,
R3, R4
resistors which implement the differential circuit
according to the following equation:
R5, R6,
C5
components for voltage reference and its filter.
The voltage reference is calculating according the
following equation:
Using the Evaluation Board
INPUT AND OUTPUT SIGNALS
The evaluation board allow to measure a bidirectional current,
so the IN and LOAD banana connectors can act either as
Source or LOAD pin. If the current flows from the IN pin to the
LOAD pin, a simply way to test the performance of the
LMP8640 (LMP8645) is showed in Figure 1
© 2010 National Semiconductor Corporation 300979 www.national.com
LMP8640 / LMP8645 Evaluation Board User Guide AN-1975
30097901
FIGURE 1. LMP8640 (LMP8645) measurement setup
The DMM1 is configured as a Voltmeter it measures the out-
put of the LMP8640 (LMP8645), while the DMM2 is config-
ured as an Ammeter; it measures the current which flows in
the LOAD. The source is a voltage supply; make sure to set
a voltage in the range of -2Vto +46V for LMP8640 (LMP8645)
or -2V to +76V for LMP8640HV (LMP8645HV).
GAIN SELECTION OF LMP8645
The evaluation board is provided with a shunt resistor Rsns
(10mΩ, @1%, 1W). While two gain resistors Rg1 and Rg2
(10kΩ, @1%) ensure a gain of 2V/V for each current sense.
The Gain is evaluated according to the following formula:
30097903
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).
SINGLE OUTPUT
In the applications where a single measurement of a bidirec-
tional current sense is needed is possible to populate the
evaluation board with two amplifiers and some resistors (see
section COMPONENTS).
For instance in the following configuration:
R1 = R2 = R3 = R4 = 10kΩ,
R5 = R6 =10kΩ
OUT = Vref +(OUT+ -OUT-)
Vref = 0.5*Vcc_Amp.
The Vref level represents the zero level; the voltages greater
than Vref are related to a current which flows from IN pin to
LOAD pin while the voltages less than Vref are related to a
current which flows from LOAD pin to IN pin. To ensure good
results in the measurements the resistors R4 and R3 and the
resistors R1 and R2 need to be well matched.
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AN-1975
Schematic
This schematic shows the evaluation board with a LMP8645 mounted on the PCB. For the version of the PCB stuffed with LMP8640
the only difference will be the gain resistors RG1, RG2. The LMP8640 is a current sense with fixed gain, so it doesn't require any
external gain resistor.
30097906
FIGURE 2. Schematic Diagram
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AN-1975
Layout
30097907
Top Layer
30097908
Bottom Layer
FIGURE 3. PCB Layout
Bill Of Material
Components marked with (*) are not soldered on the board.
Components marked with (**) are soldered only on LMP8645 board.
Designator Component Value Tolerance Package type
C1, C3 Capacitor 0.01μF5% 0603
C2, C6* Capacitor 1μF10% 3216–18
C4 Capacitor 0.1μF10% 0805
C5*, C7 Capacitor 0.1μF10% 0603
C_Filt1*, C_Filt2* Capacitor 0805
CG1*, CG2* Capacitor 0805
R1*, R2*, R3*, R4*, R5*, R6* Resistor min 10kΩ1% 0603
RG1**, RG2** Resistor 10.0kΩ1% 0603
Rsns** Resistor 0.01Ω 1%, 1W 2010
Rsns Resistor 0.01Ω 0.1%, 1W 2512
R_Filt_1, R_Filt2 Resistor 5% 0805
U3* Dual Op Amp MSOP-8
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AN-1975
Notes
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AN-1975
Notes
AN-1975 LMP8640 / LMP8645 Evaluation Board User Guide
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