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LTC2051 Datasheet(PDF) 10 Page - Linear Technology |
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LTC2051 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 22 page  LTC6101/LTC6101HV 10 6101fh APPLICATIONS INFORMATION Figure 2. Kelvin Input Connection Preserves Accuracy Despite Large Load Current LTC6101 ROUT VOUT 6101 F02 RIN V+ LOAD RSENSE V+ V– OUT –IN +IN Useful Gain Configurations Gain RIN ROUT VSENSE at VOUT = 5V IOUT at VOUT = 5V 20 499 10k 250mV 500μA 50 200 10k 100mV 500μA 100 100 10k 50mV 500μA Selection of External Current Sense Resistor The external sense resistor, RSENSE, has a significant effect on the function of a current sensing system and must be chosen with care. First, the power dissipation in the resistor should be considered. The system load current will cause both heat and voltage loss in RSENSE. As a result, the sense resis- tor should be as small as possible while still providing the input dynamic range required by the measurement. Note that input dynamic range is the difference between the maximum input signal and the minimum accurately reproduced signal, and is limited primarily by input DC offset of the internal amplifier of the LTC6101. In addition, RSENSE must be small enough that VSENSE does not exceed the maximum input voltage specified by the LTC6101, even under peak load conditions. As an example, an application may require that the maximum sense voltage be 100mV. If this application is expected to draw 2A at peak load, RSENSE should be no more than 50mΩ. Once the maximum RSENSE value is determined, the mini- mum sense resistor value will be set by the resolution or dynamic range required. The minimum signal that can be accurately represented by this sense amp is limited by the input offset. As an example, the LTC6101B has a typical input offset of 150μV. If the minimum current is 20mA, a sense resistor of 7.5mΩ will set VSENSE to 150μV. This is the same value as the input offset. A larger sense resistor will reduce the error due to offset by increasing the sense voltage for a given load current. Choosing a 50mΩ RSENSE willmaximizethedynamicrange and provide a system that has 100mV across the sense resistor at peak load (2A), while input offset causes an error equivalent to only 3mA of load current. Peak dissipation is 200mW. If a 5mΩ sense resistor is employed, then the effective current error is 30mA, while the peak sense voltage is reduced to 10mV at 2A, dis- sipating only 20mW. The low offset and corresponding large dynamic range of the LTC6101 make it more flexible than other solutions in this respect. The 150μV typical offset gives 60dB of dy- namic range for a sense voltage that is limited to 150mV max, and over 70dB of dynamic range if the rated input maximum of 500mV is allowed. Sense Resistor Connection Kelvin connection of the IN– and IN+ inputs to the sense resistor should be used in all but the lowest power ap- plications. Solder connections and PC board interconnec- tions that carry high current can cause significant error in measurement due to their relatively large resistances. One 10mm x 10mm square trace of one-ounce copper is approximately 0.5mΩ. A 1mV error can be caused by as little as 2A flowing through this small interconnect. This will cause a 1% error in a 100mV signal. A 10A load current in the same interconnect will cause a 5% error for the same 100mV signal. By isolating the sense traces from the high-current paths, this error can be reduced by orders of magnitude. A sense resistor with integrated Kelvin sense terminals will give the best results. Figure 2 illustrates the recommended method. |
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