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LT1787 Datasheet(PDF) 10 Page - Linear Technology |
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LT1787 Datasheet(HTML) 10 Page - Linear Technology |
10 / 16 page 10 LT1787/LT1787HV 1787fc The LT1787 high side current sense amplifier (Figure 1) provides accurate bidirectional monitoring of current through a user-selected sense resistor. The sense voltage is amplified by a fixed gain of 8 and level shifted from the positive power supply to the ground referenced outputs. The output signal may be used in a variety of ways to interface with subsequent signal processing circuitry. Input and output filtering are easily implemented to elimi- nate aliasing errors. Theory of Operation Inputs VS+ and VS– apply the sense voltage to matched resistors RG1 and RG2. The opposite ends of resistors RG1 and RG2 are forced to be at equal potentials by the voltage gain of amplifier A1. The currents through RG1 and RG2 are forced to flow through transistors Q1 and Q2 and are summed at node VOUT by the 1:1 current mirror. The net current from RG1 and RG2 flowing through resistor ROUT gives a voltage gain of eight. Positive sense voltages result in VOUT being positive with respect to pin VBIAS. Pins VEE, VBIAS and VOUT may be connected in a variety of ways to interface with subsequent circuitry. Split supply and single supply output configurations are shown in the following sections. Supply current for amplifier A1 is drawn from the VS– pin. The user may choose to include this current in the moni- tored current through RSENSE by careful choice of connec- tion polarity. Selection of External Current Sense Resistor External RSENSE resistor selection is a delicate trade-off between power dissipation in the resistor and current measurement accuracy. The LT1787 makes this decision less difficult than with competitors’ products. The maxi- mum sense voltage may be as large as ±500mV to get maximum resolution, however, high current applications will not want to suffer this much power dissipation in the sense resistor. The LT1787’s input offset voltage of 40 µV gives high resolution for low sense voltages. This wide operating dynamic range gives the user wide latitude in tailoring the range and resolution of his supply monitoring function. APPLICATIONS INFORMATION Kelvin connection of the LT1787’s VS+ and VS– inputs to the sense resistor should be used in all but the lowest power applications. Solder connections and PC board interconnect resistance (approximately 0.5m Ωpersquare) can be a large error in high current systems. A 5-Amp application might choose a 20m Ω sense resistor to give a 100mV full-scale input to the LT1787. Input offset voltage will limit resolution to 2mA. Neglecting contact resistance at solder joints, even one square of PC board copper at each resistor end will cause an error of 5%. This error will grow proportionately higher as monitored current levels rise to tens or hundreds of amperes. Input Noise Filtering The LT1787 provides input signal filtering pins FIL+ and FIL– that are internally connected to the midpoint taps of resistors RG1 and RG2. These pins may be used to filter the input signal entering the LT1787’s internal amplifier, and should be used when fast current ripple or transients may flow through the sense resistor. High frequency signals above the 300kHz bandwidth of the LT1787’s internal amplifier will cause errors. A capacitor connected between FIL+ and FIL– creates a single pole low pass filter with corner frequency: f–3dB = 1/(2πRC) where R = 1.25k. A 0.01 µF capacitor creates a pole at 12.7kHz, a good choice for many applications. Common mode filtering from the FIL+ and FIL– pins should not be attempted, as mismatch in the capacitors from FIL+ and FIL– will create AC common mode errors. Common mode filtering must be done at the power supply output. Output Signal Range The LT1787’s output signal is developed by summing the net currents through RG1 and RG2 into output resistor ROUT. The pins VOUT and VBIAS may be connected in numerous configurations to interface with following cir- cuitry in either single supply or split supply applications. Care must be used in connecting the output pins to preserve signal accuracy. Limitations on the signal swing at VOUT are imposed by the negative supply, VEE, and the input voltage VS+. In the negative direction, internal circuit saturation with loss of accuracy occurs for VOUT < 70mV |
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