LMP8601:Bidirectional Precision Current Sensing Amplifier
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This training module will overview the operation of the LMP8601 current sensing amplifier and its various configurations for different applications
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LMP8601:Bidirectional Precision Current Sensing Amplifier
- 7. Typical Performance V OS vs. V CM , V S =3.3V V OS vs. V CM at V S = 5V RL = ∞, Offset (Pin 7) connected to VS, 10nF between VS and GND for both the condition.
- 8. Second Order Low Pass Filter
- 9. Low Side Current Sensing Application
- 10. High Side Current Sensing Application
- 11. Gain Adjustment Reducing gain is given by: Increasing gain is given by:
- 12. Battery Current Monitor Application
- 13. Advanced Battery Charger Application
Editor's Notes
- Welcome to the training module on LMP8601: Bidirectional Precision Current Sensing Amplifier. This training module will overview the LMP8601 current sensing amplifier and its various configuration for different applications.
- The LMP8601 and LMP8601Q are fixed 20x gain precision amplifiers. The fixed gain is achieved in two separate stages, a preamplifier with a gain of 10x and an output stage buffer amplifier with a gain of 2x. The part will amplify and filter small differential signals in the presence of high common mode voltages. The input common mode voltage range is –22V to +60V when operating from a single 5V supply. With 3.3V supply, the input common mode voltage range is from –4V to +27V. It is available in 8-pin SOIC package. The devices are ideal parts for unidirectional and bidirectional current sensing applications
- Here lists the key features of the amplifiers. The values are under conditions 25 ºC ambient temperature and 5V supply voltage.
- The signal on the input pins is typically a small differential voltage across a current sensing shunt resistor. The input signal may appear at a high common mode voltage. Two input resistors feed the input signals. The proprietary level-shifting chopping circuit pulls or pushes current through the input resistors to bring the common mode voltage at the resistor inputs to within the supply rails. Subsequently, the signal is scaled up by a factor of 10 and brought out on pin A1 through a trimmed 100K resistor. Additional gain adjustment or filtering components can be added between the A1 and A2 pins in your application. The signal present on pin A2 is amplified by a factor of 2 and fed to the OUT pin. The OFFSET pin allows the output signal to be level-shifted to enable bidirectional current sensing.
- The signal on the A1 and OUT pins is ground-referenced when the OFFSET pin is connected to ground. This means that the output signal can only represent positive values of the current through the shunt resistor, so only currents flowing in one direction can be measures. When the OFFSET pin is tied to the positive supply rail, the signal on the A1 and OUT pins is referenced to a mid-rail voltage which allows bidirectional current sensing. When the OFFSET pin is connected to a voltage source, the output signal will be level shifted to that voltage divided by two.
- The figures show typical performance of input offset voltage (V os ) verses common mode voltage (V cm ) for the LMP8601 operating under V cc = 5V and 3.3V at a temperature between -40 to +125 ºC .
- A third order Butterworth filter with a typical bandwidth of 60 kHz is integrated in the preamplifier stage of the LMP8601 and LMP8601Q devices. The bandwidth of the output buffer can be reduced by adding a capacitor to the A1 pin to create a first order low pass filter with a time constant determined by the 100K internal resistor and external filter capacitor. It is also possible to create an additional second order Sallen- Key low pass filter by adding external components R2, C1 and C2, together with internal resistor R1 as illustrated in the figure.
- This figure illustrates a low side current sensing application with a low side driver. The power transistor is pulse width modulated to control the average current flowing through the inductive load which is connected to a relatively high battery voltage. The current through the load is measured across a shunt resistor R SENSE in series with the load. When the power transistor is on, current flows from the battery through the inductive load, the shunt resistor and the power transistor to ground. In this case, the common mode voltage on the shunt is close to ground. When the power transistor is off, current flows through the inductive load, through the shunt resistor and through the freewheeling diode. In this case, the common mode voltage on the shunt is at least one diode voltage drop above the battery voltage.
- This diagram shows about the application of the LMP8601 in a high side sensing application. The application is similar to the low side sensing application, except in the common mode voltage on the shunt drops below ground when the driver is switched off. Because the common mode voltage range of the LMP8601/LMP8601Q extends below the negative rail, the LMP8601/LMP8601Q is also very well suited for this application.
- The gain of the LMP8601/LMP8601Q is 20; however, this gain can be adjusted as the signal path in between the two internal amplifiers is available on the external pins. Here shows the configuration for increasing and reducing gain of the amplifier.
- This diagram shows how the LMP8601 can be used to monitor the current flowing in and out of a battery pack. If the load current of the battery is higher than the charging current, the output voltage of the LMP8601 will be above the “half offset voltage” for a net current flowing out of the battery. When the charging current is higher than the load current the output will be below this “half offset voltage”.
- This diagram shows how the LMP8601 can be used to realize an advanced battery charger that has the capability to monitor the exact net current that flows in and out the battery. The output signal of the LMP8601 is digitized with the A/D converter and used as an input for the charge controller. The Charge controller can be used to regulate the charger circuit to deliver exactly the current that is required by the load, avoiding overcharging a fully loaded battery.
- Thank you for taking the time to view this presentation on “ LMP8601: Bidirectional Precision Current Sensing Amplifier” . If you would like to learn more or go on to purchase some of these devices, you may either click on the part list link, or simple call our sales hotline. For more technical information you may either visit the National Semiconductor site – link shown – or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility.