APPLICATION NOTE 105: Current Sense Circuit Collection Digitizing To see other chapters in this Application Note, return to the Introduction. In many systems the analog voltage quantity indicating current flow must be input to a system controller. In this chapter several examples of the direct interface of a current sense amplifier to an A to D converter are shown. Sensing Output Current VCC 0V TO 1V 12V VCSRC VCSNK +IN –IN EN VCC V+ ISRC ISNK RS 0.2Ω TSD OUT LT1970 SENSE+ SENSE– FILTER V– VEE RLOAD COMMON R4 255k LT1787 RG RF VS– –12V VS+ 20k VEE –12V BIAS –12V R1 60.4k R2 10k 12V – + VOUT 2.5V ±5mV/mA LT1880 R3 20k 1kHz FULL CURRENT BANDWIDTH –12V 0V TO 5V A/D 1970 F10 OPTIONAL DIGITAL FEEDBACK The LT1970 is a 500mA power amplifier with voltage programmable output current limit. Separate DC voltage inputs and an output current sensing resistor control the maximum sourcing and sinking current values. These control voltages could be provided by a D-to-A Converter in a microprocessor controlled system. For closed loop control of the current to a load an LT1787 can monitor the output current. The LT1880 op amp provides scaling and level shifting of the voltage applied to an A-to-D Converter for a 5mV/mA feedback signal. Digitizing-1 APPLICATION NOTE 105: Current Sense Circuit Collection Split or Single Supply Operation, Bidirectional Output into A/D 1Ω 1% IS = ±125mA VSRCE ≈4.75V 1 FIL– – 2 VS 3 DNC VEE 4 VEE –5V VCC 5V 8 FIL+ LT1787 VS+ 7 10µF 16V VBIAS 6 20k VOUT 5 1 CONV VOUT (±1V) 7 2 6 AIN LTC1404 CLK VREF 5 DOUT GND 10µF 16V 4 8 3 OPTIONAL SINGLE SUPPLY OPERATION: DISCONNECT VBIAS FROM GROUND AND CONNECT IT TO VREF. REPLACE –5V SUPPLY WITH GROUND. OUTPUT CODE FOR ZERO CURRENT WILL BE ~2430 CLOCKING CIRCUITRY 10µF 16V DOUT VEE –5V 1787 TA02 In this circuit, split supply operation is used on both the LT1787 and LT1404 to provide a symmetric bidirectional measurement. In the single-supply case, where the LT1787 pin 6 is driven by VREF, the bidirectional measurement range is slightly asymmetric due to VREF being somewhat greater than mid-span of the ADC input range. 16-Bit Resolution Unidirectional Output into LTC2433 ADC 12-Bit Resolution Unidirectional Output into LTC1286 ADC TO LOAD I = 100A 1 FIL– 2 VS 3 4 RSENSE 0.0016Ω – DNC FIL+ LT1787HV ROUT 20k VEE Digitizing-2 8 2.5V TO 60V 7 R1 15k VBIAS 6 5 VOUT VOUT = VBIAS + (8 • ILOAD • RSENSE) The LTC2433-1 can accurately digitize signal with source impedances up to 5kΩ. This LTC6101 current sense circuit uses a 4.99kΩ output resistance to meet this requirement, thus no additional buffering is necessary. VS + C2 0.1µF C1 1µF VREF VCC CS +IN LTC1286 CLK –IN D GND OUT LT1634-1.25 5V TO µP 1787 TA01 While the LT1787 is able to provide a bidirectional output, in this application the economical LTC1286 is used to digitize a unidirectional measurement. The LT1787 has a nominal gain of eight, providing a 1.25V full-scale output at approximately 100A of load current.