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LTC6104 Datasheet(PDF) 7 Page - Linear Technology |
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LTC6104 Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page LT6106 7 6106fa APPLICATIONS INFORMATION Introduction The LT6106 high side current sense amplifier (Figure 1) pro- videsaccuratemonitoringofcurrentthroughauser-selected sense resistor. The sense voltage is amplified by a user- selected gain and level shifted from the positive power sup- ply to a ground-referred output. The output signal is analog and may be used as is, or processed with an output filter. Theory of Operation An internal sense amplifier loop forces –IN to have the same potential as +IN. Connecting an external resistor, RIN, between –IN and V+ forces a potential across RIN that is the same as the sense voltage across RSENSE. A corresponding current, VSENSE/RIN, will flow through RIN. The high impedance inputs of the sense amplifier will not conduct this current, so it will flow through an internal PNP to the output pin as IOUT. The output current can be transformed into a voltage by adding a resistor from OUT to V–. The output voltage is then VO = V– + IOUT • ROUT. Table 1. Useful Gain Configurations GAIN RIN ROUT VSENSE at VOUT = 5V IOUT at VOUT = 5V 20 499Ω 10k 250mV 500μA 50 200Ω 10k 100mV 500μA 100 100Ω 10k 50mV 500μA GAIN RIN ROUT VSENSE at VOUT = 2.5V IOUT at VOUT = 2.5V 20 249Ω 5k 125mV 500μA 50 100Ω 5k 50mV 500μA 100 50Ω 5k 25mV 500μA Selection of External Current Sense Resistor The external sense resistor, RSENSE, has a significant ef- fect on the function of a current sensing system and must be chosen with care. First, the power dissipation in the resistor should be con- sidered. The system load current will cause both heat and voltage loss in RSENSE. As a result, the sense resistor 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 mea- sured signal, and is limited primarily by input DC offset of the internal amplifier of the LT6106. In addition, RSENSE must be small enough that VSENSE does not exceed the maximum input voltage specified by the LT6106, even un- der 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 amplifier is limited by the input offset. As an example, the LT6106 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 resis- tor will reduce the error due to offset by increasing the sense voltage for a given load current. Choosing a 50mΩ RSENSE will maximize the dynamic range 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, dissipating only 20mW. The low offset and corresponding large dynamic range of the LT6106 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 maximum, and over 70dB of dynamic range if the rated input maximum of 0.5V 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 appli- cations. 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 × 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 cur- rent 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 |
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