Precision microampere high-side current measurements require a small sense resistor and an amplifier with a low offset voltage. The LTC2063 zero-drift amplifier has a maximum input offset voltage of only 5 V and consumes only 1With a current of 4 A, it is ideal for building a complete ultra-low-power precision high-side current-sensing circuit (as shown in Figure 1).
Figure 1Precision high-side current sense circuit based on LTC2063 zero-drift amplifier.
The circuit only needs 2A supply current of 3 A to 280 A senses a wide dynamic range current from 100 A to 250 mA. The LTC2063's very low offset voltage allows the circuit to operate with shunt resistors as low as 100m, resulting in a maximum shunt voltage limit of only 25 mV. This significantly reduces the power losses on the shunt resistor and greatly increases the available power of the load. The rail-to-rail input of the LTC2063 allows the circuit to operate at very small load currents, and its input common-mode is almost exactly on the power rail. The LTC2063's integrated EMI filter protects the device from RF interference in high-noise conditions.
For a given sense current, the voltage output of this circuit is:
A key metric of a current sensing solution is the zero point, or the equivalent error current that is generated when the output is not sensed and converted to the input. The zero point is usually determined by dividing the input offset voltage of the amplifier by RSENSE. The LTC2063 has a low input offset voltage typical of 1 V and a maximum of 5 V, and a low input bias and offset current typical of 1 pA to 3 Pa, resulting in a typical zero error current of only 10 A (1 V 0.) to the input1) The maximum value is 50 A (5 V 0.).1) This low error allows the detection circuit to maintain its linearity down to the minimum current (100 A) within its specified range, without flattening the linearity due to a fixed offset value over the low scale due to resolution loss (as shown in Figure 2). The resulting input current vs. output voltage curve is linear over the entire current sensing range.
Figure 2There is no fixed offset value at the low end, and iSense can be as low as 100 A.
Another ** of the zero point error is the drain current or IDSS of the output PMOS at zero gate voltage, i.e., PMOS is nominally off (|vgs|= 0). MOSFETs with high IDSS leakage current will produce a non-zero positive VOUT value in the absence of iSense.
The transistor used in this design is Infineon's BSP322P, which is located in |vds|The upper limit of IDSS at = 100 V is 1 A. A reasonable estimate of the typical IDSS for BSP322P in this application can be made at room temperature with VDS = 7At 6 V, the iIDSS is only 02 Na, thus yielding only an error output of 1 V, or the equivalent of an input current error of 100 Na when measuring an input current of 0 A.
lt1389-4.096 The voltage reference and bootstrap circuitry consisting of M2, R2, and D1 form an ultra-low-power isolated 3 V rail (4VTH of 096 V + m2, the latter is typically -1 V), LTC2063 prevents 5Absolute maximum supply voltage value of 5 V. Although series resistors can also meet the need to establish bias currents, the use of transistor M2 can provide a higher overall supply voltage while also limiting current consumption to only 280 A on the high side of the supply range.
The input offset voltage of the LTC2063 results in a fixed current error of 10 A (typical) converted to the input. In a 250 mA full-scale input, the resulting offset error is only 0004%。At the low end, 10 A out of 100 A represents a 10% error. Since the offset is constant, it can be calibrated. Figure 3 shows that the total offset caused by the LTC2063, the mismatched parasitic thermocouples, and all parasitic series input resistors is only 2 V.
Figure 3Adopt 4VIN to VOUT conversion over the entire iSense range at 5 V minimum supply. 200.The output offset of 7 V is divided by 10005 V voltage gain, which means that the RTI input bias is 2 V.
Figure 3 shows a gain of 10005 V, which is 4978 kω/50.4 ω = 98.77 V v) large 128 v/v。This error may be caused by a parasitic series resistance of about 500m between the input of the LTC2063 and RSENSE.
The main element of output uncertainty in this circuit is noise, so filtering with large capacitors in parallel is essential to reduce the noise bandwidth and thus the total combined noise. Use 1With a 5 Hz output filter, the LTC2063 increases the low-frequency noise converted to the input by about 2 V p-p. The output is averaged over the longest possible duration, further reducing errors due to noise.
Other sources of error in this current-sense circuit include a parasitic board-level resistor in series with RSENSE at the LTC2063 input, resistance tolerances between the gain-setting resistors RIN and ROUT, a temperature coefficient mismatch of the gain-setting resistor, and an error voltage at the op amp input caused by parasitic thermocouples. It is possible to detect the RSENSE4 pin sense resistor by using the Kelvin connection and by using the key gain settings with RIN and ROUT paths with a similar or lower temperature coefficient of 01% resistance to dramatically reduce the first three sources of error. To eliminate parasitic thermocouples at the input of the op amp, R1 should have the same metal terminals as the RINs. Asymmetric thermal gradients at the input should also be avoided as much as possible.
with full scale 2The 5 V output is used as a benchmark, and the total contribution of all error sources discussed in this section is up to 14% (as shown in Figure 4).
Figure 4The percentage error remains at 14% or less.
lt1389-4.096 and LTC2063 in minimum vsupply and isense (45 V and 100 A) requires a minimum supply current of 23 A, up to 280 A at max vSupply and iSense E (90 V and 250 mA) (as shown in Figure 5). In addition to the current consumed by the active components, the vSupplyy needs to provide an output current iDrive flowing through M1, which is proportional to the output voltage and ranges from 1200 nA at 0 mV output (at 100 A for iSense) to 2500 A at 5 V output (at 250 mA for iSense). Therefore, with the exception of iSense, the total supply current range is 25 A to 780 A. Set Rout to 5 K to get a reasonable ADC drive value.
Figure 5The supply current increases with the supply voltage, but does not exceed 280 A.
In this architecture, the maximum power supply depends on the maximum ||that the PMOS output can toleratevds|。The BSP322P is rated at 100 V, so 90 V is a suitable operating limit.
This design can drive 5K loads, making it suitable as a driver stage for many ADCs. It has an output voltage range of 0 V to 25 v。Since the LTC2063 has a rail-to-rail output, the maximum gate drive is limited only by the margins of the LTC2063. In this design, the typical value is 3 V, which is defined by the LT1389-4096 of 4096 V plus VTH typical for M2 1 V setting.
Because the output of this circuit is current, voltage, ground, or lead offset do not affect accuracy. Therefore, a long lead can be used between the output PMOS M1 and Rout, allowing Rsense to be located near the current to be sensed and Rout to be located near the ADC and other signal chain follow-ups. The disadvantage of long leads is that they increase EMI sensitivity. 100 NF C3 at both ends of the ROUT shunt harmful EMI before it reaches the next level.
Since the LTC2063 has a gain-bandwidth product of 20 kHz, it is recommended to use this circuit to measure signals at 20 Hz or lower. C2 at 22 F is connected in parallel with the load to filter the output noise to 15 Hz to improve accuracy and protect subsequent circuits from sudden current surges. This filtering comes at the cost of a longer settling time, especially at the lowest end of the input current range.
The LTC2063 features ultra-low input offset voltage, low IOFFSET and low IPIA, and rail-to-rail inputs to provide precision current measurements over the full range of 100 A to 250 mA. The circuit has a maximum supply current of 2 A, so it can operate well below 280 A over most operating ranges. The LTC2063's low supply current, as well as low supply voltage requirements, make it more than sufficient to be powered from a voltage reference.
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