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OPA2388IDR Datasheet(PDF) 19 Page - Texas Instruments |
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OPA2388IDR Datasheet(HTML) 19 Page - Texas Instruments |
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19 / 34 page  Gain = Diff_Amp V V = 15.5 V V R (I I ) OUT_Max OUT_Min SHUNT MAX MIN - - ´ 3.2 V 100 mV 100 m - W ´ - - [1 A ( 1A)] = R = SHUNT(Max) = = 100 mW V I SHUNT(Max) LOAD(Max) 100 mV 1 A V = V REF CC ´ R R R 6 5 6 + Gain = Diff_Amp R R 4 3 V = I R SHUNT LOAD ´ SHUNT V = V Gain + V OUT SHUNT Diff_Amp REF ´ 19 OPA388, OPA2388, OPA4388 www.ti.com SBOS777 – DECEMBER 2016 Product Folder Links: OPA388 OPA2388 OPA4388 Submit Documentation Feedback Copyright © 2016, Texas Instruments Incorporated Typical Application (continued) 8.2.1 Design Requirements This solution has the following requirements: • Supply voltage: 3.3 V • Input: –1 A to 1 A • Output: 1.65 V ±1.54 V (110 mV to 3.19 V) 8.2.2 Detailed Design Procedure The load current, ILOAD, flows through the shunt resistor (RSHUNT) to develop the shunt voltage, VSHUNT. The shunt voltage is then amplified by the difference amplifier consisting of U1A and R1 through R4. The gain of the difference amplifier is set by the ratio of R4 to R3. To minimize errors, set R2 = R4 and R1 = R3. The reference voltage, VREF, is supplied by buffering a resistor divider using U1B. The transfer function is given by Equation 1. where • • • (1) There are two types of errors in this design: offset and gain. Gain errors are introduced by the tolerance of the shunt resistor and the ratios of R4 to R3 and, similarly, R2 to R1. Offset errors are introduced by the voltage divider (R5 and R6) and how closely the ratio of R4 / R3 matches R2 / R1. The latter value affects the CMRR of the difference amplifier, ultimately translating to an offset error. The value of VSHUNT is the ground potential for the system load because VSHUNT is a low-side measurement. Therefore, a maximum value must be placed on VSHUNT. In this design, the maximum value for VSHUNT is set to 100 mV. Equation 2 calculates the maximum value of the shunt resistor given a maximum shunt voltage of 100 mV and maximum load current of 1 A. (2) The tolerance of RSHUNT is directly proportional to cost. For this design, a shunt resistor with a tolerance of 0.5% was selected. If greater accuracy is required, select a 0.1% resistor or better. The load current is bidirectional; therefore, the shunt voltage range is –100 mV to 100 mV. This voltage is divided down by R1 and R2 before reaching the operational amplifier, U1A. Take care to ensure that the voltage present at the noninverting node of U1A is within the common-mode range of the device. Therefore, use an operational amplifier, such as the OPA388, that has a common-mode range that extends below the negative supply voltage. Finally, to minimize offset error, note that the OPA388 has a typical offset voltage of merely ±0.25 µV (±5 µV maximum). Given a symmetric load current of –1 A to 1 A, the voltage divider resistors (R5 and R6) must be equal. To be consistent with the shunt resistor, a tolerance of 0.5% was selected. To minimize power consumption, 10-kΩ resistors were used. To set the gain of the difference amplifier, the common-mode range and output swing of the OPA388 must be considered. Equation 3 and Equation 4 depict the typical common-mode range and maximum output swing, respectively, of the OPA388 given a 3.3-V supply. –100 mV < VCM < 3.4 V (3) 100 mV < VOUT < 3.2 V (4) The gain of the difference amplifier can now be calculated as shown in Equation 5. (5) |
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