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LMP8645MKE Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LMP8645MKE Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 16 page  Application Information GENERAL The LMP8645 and LMP8645HV are single supply high side current sense amplifiers with variable gain selected through an external resistor and a common mode voltage range of -2V to 42V or -2V to 76V depending on the grade. The sense voltage is amplified by a user-selected gain and level shifted from the positive power supply to a ground-re- ferred output. THEORY OF OPERATION As seen from the picture below, the current flowing through R S develops a voltage drop equal to VSENSE across RS. The high impedance inputs of the amplifier doesn’t conduct this current and the high open loop gain of the sense amplifier forces its non-inverting input to the same voltage as the in- verting input. In this way the voltage drop across R IN matches V SENSE. A current proportional to IS according to the following relation: I S′ = VSENSE/RIN = RS*IS/RIN , where RIN = 1/Gm flows entirely in the external gain resistor developing a voltage drop equal to V G = IS′ *RGAIN = (VSENSE/RIN) *RGAIN = ((RS*IS)/RIN)*RGAIN This voltage is buffered and showed at the output with a very low impedance allowing a very easy interface of the LMP8645 with other ICs (ADC, μC…). V OUT = (RS*IS)*G, where G = RGAIN/RIN 30071603 FIGURE 1. Current monitor SELECTION OF THE SHUNT RESISTOR The accuracy of the current measurement strictly depends on the value of the shunt resistor R S. Its value depends on the application and it is a compromise between small-signal ac- curacy and maximum permissible voltage loss in the mea- surement section. High values of R S provide better accuracy at lower currents by minimizing the effects of offset, while low values of R S minimize voltage loss in the supply section. For most applications, best performance is obtained with an R S value that provides a full-scale shunt voltage range of 100 mV to 200 mV. SELECTION OF THE GAIN RESISTOR In the LMP8645 and LMP8645HV the gain is selected through an external resistor connected to the R G pin. Moreover the gain resistor R GAIN determines the voltage of the output buffer which is related to the supply voltage and to the common mode voltage of the input signal. 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. The following equations explain how to select the gain resistor for various range of the input common mode voltage. Case 1 −2V < V CM ≤ 1.8V The max voltage at the RG pin is given by the following in- equality V RG=Vsense*RGAIN *Gm ≤ min(1.3V; Vout_max) where Vout_max is the maximum allowable output voltage according to the Electrical Tables.All the gain resistors (R GAIN) which respect the previous inequality are allowed. The graphical representation in Figure 2 helps in the selection; all the combinations (V SENSE, RGAIN) below the curve are al- lowed. 30071604 FIGURE 2. Allowed Gains for CASE 1 As a consequence once selected the gain (R GAIN) the V SENSE range is fixed too. For example if an application re- quired a Gain of 10, R G will be 50 kΩ and VSENSE will be in the range 10 mV to 100 mV. Case 2 1.8V < V CM ≤ VS In this case the max voltage at the R G pin is related to the common mode voltage and V SENSE. So all the RGAIN resistors which respect the following inequalities are allowed: V R G ≤ min (Vout_max; (V CM - Vsense-250mV)) where V RG = VSENSE*RGAIN*Gm and Vout_max is the maximum allowable output voltage according to the Electrical Tables. The graphical representation in Figure 3 helps in the selec- tion; all the combinations (V SENSE, RGAIN) below the curves for given V CM and supply voltage are allowed. 11 www.national.com |
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