Datasheet LTC6103 (Analog Devices) - 8

LTC6103
APPLICATIONS INFORMATION Sense Resistor Connection
For example, if RIN = 100Ω, then: Kelvin connections should be used between the inputs (+IN VSENSE = and –IN) and the sense resistor in all but the lowest power IOUT 100Ω applications. Solder connections and PC board intercon- nections that carry high current can cause signifi cant error or IOUT = 1mA for VSENSE = 100mV. in measurement due to their relatively large resistances. RIN should be chosen to allow the required resolution One 10mm × 10mm square trace of one-ounce copper while limiting the output current. At low supply voltage, is approximately 0.5mΩ. A 1mV error can be caused by IOUT may be as much as 1mA. By setting RIN such that as little as 2A fl owing through this small interconnect. the largest expected sense voltage gives IOUT = 1mA, then This will cause a 1% error in a 100mV signal. A 10A load the maximum output dynamic range is available. Output current in the same interconnect will cause a 5% error dynamic range is limited by both the maximum allowed for the same 100mV signal. By isolating the sense traces output current and the maximum allowed output voltage, from the high current paths, this error can be reduced as well as the minimum practical output signal. If less by orders of magnitude. A sense resistor with integrated dynamic range is required, then RIN can be increased Kelvin sense terminals will give the best results. Figure 2 accordingly, reducing the maximum output current and illustrates the recommended method. power dissipation. If low sense currents must be resolved accurately in a system that has very wide dynamic range,
Selection of External Input Resistor, RIN
a smaller RIN than the maximum current specifi cation The external input resistor, RIN, controls the trans- allows may be used if the maximum current is limited in conductance of the current sense circuit. Since: another way, such as with a Schottky diode across RSENSE V (Figure 3a). This will reduce the high current measurement 1 I SENSE OUT = , transconductance g = m accuracy by limiting the result, while increasing the low RIN RIN current measurement resolution. This approach can be helpful in cases where occasional V+ ILOAD RSENSE large burst currents may be ignored. It can also be used in a multi-range confi guration where a low current circuit LOAD RIN is added to a high current circuit (Figure 3b). Note that a comparator (LTC1540) is used to select the range, and +IN –IN transistor M1 limits the voltage across RSENSE(LO). + – VS V+ 1/2 OUT V– LTC6103 R D SENSE SENSE ROUT 6103 F03a LOAD 6103 F02
Figure 2. Kelvin Input Connection Preserves Figure 3a. Shunt Diode Limits Maximum Input Voltage to Allow Accuracy Despite Large Load Current Better Low Input Resolution Without Overranging
6103f 8