Datasheet LT1787, LT1787HV (Analog Devices) - 10

LT1787/LT1787HV
U U W U APPLICATIONS INFORMATION
The LT1787 high side current sense amplifier (Figure 1) Kelvin connection of the LT1787’s V + – S and VS inputs to provides accurate bidirectional monitoring of current the sense resistor should be used in all but the lowest through a user-selected sense resistor. The sense voltage power applications. Solder connections and PC board is amplified by a fixed gain of 8 and level shifted from the interconnect resistance (approximately 0.5mΩ per square) positive power supply to the ground referenced outputs. can be a large error in high current systems. A 5-Amp The output signal may be used in a variety of ways to application might choose a 20mΩ sense resistor to give a interface with subsequent signal processing circuitry. 100mV full-scale input to the LT1787. Input offset voltage Input and output filtering are easily implemented to elimi- will limit resolution to 2mA. Neglecting contact resistance nate aliasing errors. at solder joints, even one square of PC board copper at each resistor end will cause an error of 5%. This error will
Theory of Operation
grow proportionately higher as monitored current levels Inputs V + – rise to tens or hundreds of amperes. S and VS apply the sense voltage to matched resistors RG1 and RG2. The opposite ends of resistors RG1
Input Noise Filtering
and RG2 are forced to be at equal potentials by the voltage gain of amplifier A1. The currents through RG1 and RG2 are The LT1787 provides input signal filtering pins FIL+ and forced to flow through transistors Q1 and Q2 and are FIL– that are internally connected to the midpoint taps of summed at node VOUT by the 1:1 current mirror. The net resistors RG1 and RG2. These pins may be used to filter the current from RG1 and RG2 flowing through resistor ROUT input signal entering the LT1787’s internal amplifier, and gives a voltage gain of eight. Positive sense voltages result should be used when fast current ripple or transients may in VOUT being positive with respect to pin VBIAS. flow through the sense resistor. High frequency signals above the 300kHz bandwidth of the LT1787’s internal Pins VEE, VBIAS and VOUT may be connected in a variety of amplifier will cause errors. A capacitor connected between ways to interface with subsequent circuitry. Split supply FIL+ and FIL– creates a single pole low pass filter with and single supply output configurations are shown in the corner frequency: following sections. f Supply current for amplifier A1 is drawn from the V – –3dB = 1/(2πRC) S pin. The user may choose to include this current in the moni- where R = 1.25k. A 0.01µF capacitor creates a pole at tored current through RSENSE by careful choice of connec- 12.7kHz, a good choice for many applications. tion polarity. Common mode filtering from the FIL+ and FIL– pins should not be attempted, as mismatch in the capacitors from FIL+
Selection of External Current Sense Resistor
and FIL– will create AC common mode errors. Common External RSENSE resistor selection is a delicate trade-off mode filtering must be done at the power supply output. between power dissipation in the resistor and current measurement accuracy. The LT1787 makes this decision
Output Signal Range
less difficult than with competitors’ products. The maxi- The LT1787’s output signal is developed by summing the mum sense voltage may be as large as ±500mV to get net currents through R maximum resolution, however, high current applications G1 and RG2 into output resistor R will not want to suffer this much power dissipation in the OUT. The pins VOUT and VBIAS may be connected in numerous configurations to interface with following cir- sense resistor. The LT1787’s input offset voltage of 40µV cuitry in either single supply or split supply applications. gives high resolution for low sense voltages. This wide Care must be used in connecting the output pins to operating dynamic range gives the user wide latitude in preserve signal accuracy. Limitations on the signal swing tailoring the range and resolution of his supply monitoring at V function. OUT are imposed by the negative supply, VEE, and the input voltage V + S . In the negative direction, internal circuit saturation with loss of accuracy occurs for VOUT < 70mV 1787fc 10