Datasheet LTC6104 (Analog Devices) - 8

LTC6104
APPLICATIONS INFORMATION Selection of External Current Sense Resistor
The low offset and corresponding large dynamic range of the LTC6104 make it more fl exible than other solutions The external sense resistor, RSENSE, has a signifi cant effect in this respect. The ±85µV typical offset gives 60dB of on the function of a current sensing system and must be dynamic range for a sense voltage that is limited to ±85mV chosen with care. max, and over 75dB of dynamic range if the rated input First, the power dissipation in the resistor should be maximum of ±500mV is allowed. considered. The system load current will cause both heat and voltage loss in RSENSE. As a result, the sense resis-
Sense Resistor Connection
tor should be as small as possible while still providing Kelvin connection of the –INA/–INB and +INA/+INB in- the input dynamic range required by the measurement. puts to the sense resistor should be used in all but the Note that input dynamic range is the difference between lowest power applications. Solder connections and PC the maximum input signal and the minimum accurately board interconnections that carry high current can cause reproduced signal, and is limited primarily by input DC signifi cant error in measurement due to their relatively offset of the internal amplifi er of the LTC6104. In addition, large resistances. One 10mm × 10mm square trace of RSENSE must be small enough that VSENSE does not exceed one-ounce copper is approximately 0.5mΩ. A 1mV error the maximum input voltage specifi ed by the LTC6104, even can be caused by as little as 2A fl owing through this small under peak load conditions. interconnect. This will cause a 1% error in a 100mV signal. As an example, an application may require that the A 10A load current in the same interconnect will cause maximum sense voltage be ±100mV. If this application a 5% error for the same 100mV signal. By isolating the is expected to draw ±2A at peak load, RSENSE should be sense traces from the high current paths, this error can no more than 50mΩ. be reduced by orders of magnitude. A sense resistor with integrated Kelvin sense terminals will give the best results. V mV 100 R SENSE = = = m SENSE 50 Ω Figure 2 illustrates the recommended method. I 2A PEAK ILOAD VSENSE – + Once the maximum RSENSE value is determined, the mini- TO R + mum sense resistor value will be set by the resolution or SENSE CHARGER/LOAD dynamic range required. The minimum signal that can be R R IN IN accurately represented by this sense amp is limited by the input offset. As an example, the LTC6104 has a typical 8 7 6 5 input offset of ±85µV. If the minimum current is ±20mA, a +INA –INA –INB +INB sense resistor of 4.25mΩ will set VSENSE to ±85µV. This is + – – + A B the same value as the input offset. A larger sense resistor V V S S will reduce the error due to offset by increasing the sense voltage for a given load current. CURRENT Choosing a 50mΩ RSENSE will maximize the dynamic range OUT MIRROR V– LTC6104 and provide a system that has ±100mV across the sense 1 4 6104 F02 + resistor at peak load (±2A), while input offset causes an ROUT error equivalent to only ±1.7mA of load current. Peak dis- VOUT + sipation in the sense resistor is 200mW in this example. VREF – – If instead a 5mΩ sense resistor is employed, then the ef- fective current error is ±17mA, while the peak sense voltage
Figure 2. Kelvin Input Connections Preserve Accuracy
is reduced to ±10mV at ±2A, dissipating only 20mW.
Despite Large Load Currents
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