LT6107 APPLICATIONS INFORMATION Introduction must be small enough that VSENSE does not exceed the The LT6107 high side current sense amplifier (Figure 1) pro- maximum input voltage specified by the LT6107, even vides accurate monitoring of current through a user-selected under peak load conditions. As an example, an application sense resistor. The sense voltage is amplified by a user- may require that the maximum sense voltage be 100mV. selected gain and level shifted from the positive power sup- If this application is expected to draw 2A at peak load, ply to a ground-referred output. The output signal is analog RSENSE should be no more than 50mΩ. and may be used as is, or processed with an output filter. Once the maximum RSENSE value is determined, the min- imum sense resistor value will be set by the resolution or Theory of Operation dynamic range required. The minimum signal that can be An internal sense amplifier loop forces –IN to have the accurately represented by this sense amplifier is limited same potential as +IN. Connecting an external resistor, by the input offset. As an example, the LT6107 has a typi- R cal input offset of 150µV. If the minimum current is 20mA, IN, between –IN and V+ forces a potential across RIN that is the same as the sense voltage across R a sense resistor of 7.5mΩ will set V SENSE. A SENSE to 150µV. This corresponding current, V is the same value as the input offset. A larger sense resis- SENSE/RIN, will flow through RIN. The high impedance inputs of the sense amplifier will not tor will reduce the error due to offset by increasing the conduct this current, so it will flow through an internal sense voltage for a given load current. Choosing a 50mΩ PNP to the output pin as I R OUT. SENSE will maximize the dynamic range and provide a system that has 100mV across the sense resistor at peak The output current can be transformed into a voltage by load (2A), while input offset causes an error equivalent adding a resistor from OUT to V–. The output voltage is to only 3mA of load current. Peak dissipation is 200mW. then VO = V– + IOUT • ROUT. If a 5mΩ sense resistor is employed, then the effective Table 1. Useful Gain Configurations current error is 30mA, while the peak sense voltage is GAIN reduced to 10mV at 2A, dissipating only 20mW. RINROUTVSENSE at VOUT = 5VIOUT at VOUT = 5V 20 499Ω 10k 250mV 500µA The low offset and corresponding large dynamic range 50 200Ω 10k 100mV 500µA of the LT6107 make it more flexible than other solutions 100 100Ω 10k 50mV 500µA in this respect. The 150µV typical offset gives 60dB GAINRINROUT VSENSE at VOUT = 2.5V IOUT at VOUT = 2.5V 20 249Ω 5k 125mV 500µA of dynamic range for a sense voltage that is limited to 50 100Ω 5k 50mV 500µA 150mV maximum, and over 70dB of dynamic range if the 100 50Ω 5k 25mV 500µA rated input maximum of 0.5V is allowed. Selection of External Current Sense ResistorSense Resistor Connection The external sense resistor, RSENSE, has a significant Kelvin connection of the –IN and +IN inputs to the sense effect on the function of a current sensing system and resistor should be used in all but the lowest power appli- must be chosen with care. cations. Solder connections and PC board interconnec- First, the power dissipation in the resistor should be con- tions that carry high current can cause significant error sidered. The system load current will cause both heat and in measurement due to their relatively large resistances. voltage loss in R One 10mm × 10mm square trace of one-ounce copper SENSE. As a result, the sense resistor should be as small as possible while still providing the is approximately 0.5mΩ. A 1mV error can be caused by input dynamic range required by the measurement. Note as little as 2A flowing through this small interconnect. that input dynamic range is the difference between the This will cause a 1% error in a 100mV signal. A 10A load maximum input signal and the minimum accurately mea- current in the same interconnect will cause a 5% error sured signal, and is limited primarily by input DC offset of for the same 100mV signal. By isolating the sense traces the internal amplifier of the LT6107. In addition, R from the high current paths, this error can be reduced SENSE 6107fc For more information www.linear.com/LT6107 7
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