Datasheet LTC6103 (Analog Devices) - 7

LTC6103
THEORY OF OPERATION
An internal sense amplifi er loop forces –IN to have the an internal MOSFET to the OUT pin. In most application same potential as +IN. Connecting an external resistor, RIN, cases, IS << ILOAD, so IOUT ≈ ILOAD • RSENSE/RIN. between –IN and V+ forces a potential across RIN that is the The output current can be transformed into a voltage by same as the sense voltage across RSENSE. A corresponding adding a resistor from OUT to V–. The output voltage is current, (ILOAD + IS) • RSENSE/RIN, will fl ow through RIN. then VOUT = (V–) + (IOUT • ROUT). The high impedance inputs of the sense amplifi er will not conduct this input current, so the current will fl ow through
APPLICATIONS INFORMATION
In this dual current sense device, amplifi ers A and B are Once the maximum RSENSE value is determined, the mini- independent except for sharing the same V– pin. So supply mum sense resistor value will be set by the resolution or voltage and component values can be chosen independently dynamic range required. The minimum signal that can be for each amplifi er. accurately represented by this sense amp is limited by the input offset. As an example, the LTC6103 has a typical
Selection of External Current Sense Resistor
input offset of 85µV. If the minimum current is 20mA, a The external sense resistor, R sense resistor of 4.25mΩ will set V SENSE, has a signifi cant effect SENSE to 85µV. This is on the function of a current sensing system and must be the same value as the input offset. A larger sense resistor chosen with care. will reduce the error due to offset by increasing the sense voltage for a given load current. First, the power dissipation in the resistor should be considered. The system load current will cause both heat Choosing a 50mΩ RSENSE will maximize the dynamic range and voltage loss in R and provide a system that has 100mV across the sense SENSE. As a result, the sense resis- tor should be as small as possible while still providing resistor at peak load (2A), while input offset causes an the input dynamic range required by the measurement. error equivalent to only 1.7mA of load current. Note that input dynamic range is the difference between Peak dissipation is 200mW. If instead a 5mΩ sense resis- the maximum input signal and the minimum accurately tor is employed, then the effective current error is 17mA, reproduced signal, and is limited primarily by input DC while the peak sense voltage is reduced to 10mV at 2A, offset of the internal amplifi er of the LTC6103. dissipating only 20mW. As an example, an application may require that the The low offset and corresponding large dynamic range of maximum sense voltage be 100mV. If this application is the LTC6103 make it more fl exible than other solutions expected to draw 2A at peak load, RSENSE should be no in this respect. The 85µV typical offset gives 60dB of larger than 50mΩ. dynamic range for a sense voltage that is limited to 85mV V mV 100 max, and over 75dB of dynamic range for a maximum R SENSE = = = m SENSE 50 Ω I 2A input of 500mV. PEAK 6103f 7