With the right combination of parameters, designers have more options for high-side current sensing amplification
BY ADOLFO A. GARCIA, Vice President Marketing & Applications
Touchstone Semiconductor
www.touchstonesemi.com
Sensing and controlling supply current flow are a fundamental requirement in most all electronic systems from battery-operated, portable equipment to mobile or fixed-platform power management and dc motor control. High-side current-sense amplifiers (CSAs) are useful in these applications especially where power consumption is an important design parameter. New CSA developments offer even greater benefits in allowing engineers to save power without sacrificing performance.
With the right combination of small form factor, low supply-current operation, wide operating supply-voltage range, low input offset voltage (VOS ) and gain errors, and fixed-gain options, design engineers now have even more options for high-side current-sensing amplification. As a result, new CSA enhancements enable the next generation of battery-powered, handheld portable instruments addressing power management, motor control, and fixed-platform applications.
Unidirectional current-sense amps
The internal configuration of some unidirectional CSAs is based on a commonly-used operational amplifier (op amp) circuit for measuring load currents in the presence of high-common-mode voltages. In the general case, a CSA monitors the voltage across an external sense and generates an output voltage as a function of load current. Referring to the typical application circuit in Fig. 1 , featuring the Touchstone Semiconductor TS1100, the inputs of the op-amp-based circuit are connected across an external RSENSE . At the RS- terminal, the applied voltage is ILOAD x RSENSE .
Since the RS- terminal is the non-inverting input of the internal op amp, op-amp feedback action forces the inverting input of the internal op amp to the same potential (ILOAD x RSENSE ). Therefore, the voltage drop across RSENSE (VSENSE ) and the voltage drop across RGAIN (at the RS+ terminal) are equal. To minimize any additional error because of op-amp input bias current mismatch, both RGAIN resistors are the same value.
Fig. 1: A typical application for a high-precision unidirectional current-sense amplifier (Touchstone Semiconductor’s TS1100).
Bidirectional current-sense amplifiers
While unidirectional CSAs are primarily used in those applications where current is delivered to a load, there are many applications where it is necessary to measure current in both directions. Some applications where bidirectional current-sense monitoring/amplification are needed include: smart battery packs and chargers, portable computers, super capacitor charging/discharging devices, and general-purpose current-shunt measurements.
Prior to the advent of bidirectional CSAs, unidirectional CSAs were used; however, it was necessary to use two unidirectional CSAs in order to measure current in both directions. The RS+/RS- input pair of CSA #1 is wired normally for measuring current to the load whereas, for CSA #2, the RS+/RS- input pair would be wired anti-phase with respect to CSA #1 for measuring current back to the source. Significant disadvantages to using this configuration besides the cost of two CSAs are that the technique requires twice the printed-circuit-board (pcb) area, ties up two ADC inputs, and requires additional microcontroller coding and machine cycles.
To save on additional computing resources, pcb area, and component costs, a straight-forward modification to the unidirectional CSA configuration yields a bidirectional CSA as shown in Fig. 2 for Touchstone Semiconductor’s TS1101.
Facn_touchstone02_apr2012 Fig. 2: A typical application for a bidirectional high-precision current sense amplifier ((Touchstone Semiconductor's TS1101).
As shown in Fig. 2, the internal amplifier was reconfigured for fully differential input/output operation and a second low-threshold p-channel FET (M2) was added. The operation of this bidirectional CSA is identical to that of the unidirectional CSA previously discussed when VRS- > VRS+ . In the implementation shown in Fig. 2, when M1 is conducting current, the internal amplifier holds M2 off. When M2 is conducting current, the amplifier holds M1 off. In either case, the disabled FET does not contribute to the resultant output voltage.
For both types of unidirectional or bidirectional CSAs, gain error accuracy is a measure of how well-controlled the ratio of ROUT to RGAIN , especially over temperature. In a monolithic implementation, gain error accuracy can be
To achieve their very-low VOS performance over temperature, over wide VSENSE voltages, and over wide power supply voltages, higher-performance CSAs incorporate chopper stabilization into the input stage, a commonly-used technique to reduce significantly amplifier VOS . In reducing the CSAs’ VOS s to 30 µV (typically) or less, load currents can be resolved to 12-bit resolution or better for full-scale VSENSE voltages equal to and larger than 123 mV. When compared to similar CSAs that exhibit VOS s > 100 µV or more, load current measurements are 2 times more accurate using CSAs that have implemented chopper-stabilized input stages.
The CSA’s SIGN output comparator
As was shown in Fig. 2, the bidirectional CSA incorporated one additional feature — an analog comparator the inputs of which monitor the internal amplifier’s differential output voltage. While the voltage at its OUT terminal indicates the magnitude of the load current, the SIGN comparator output indicates the load current’s direction. The SIGN output is a logic high when M1 is conducting current (VRS+ > VRS ). Alternatively, the SIGN output is a logic low when M2 is conducting current (VRS+ RS-
).Note that, unlike other bidirectional CSAs, the SIGN comparator exhibits no “dead zone” at ILOAD switchover. With respect to a 50-mΩ external sense resistor, the load current transition band is less than ±0.2 mA. Other types of CSAs that also use an analog OUT/comparator SIGN arrangement exhibit a SIGN transition band that can range up to 2 mV (or 40 mA referred to a 50-mΩ sense resistor). On this attribute alone, low-transition band, bidirectional CSAs can be 200 times more sensitive.
Internal noise filters
To counter the effects of externally-injected differential and common-mode noise prevalent in any load current measurement scheme, it’s always been good engineering practice to add external low-pass filters (LPFs) in series with the CSA’s inputs. In the design of discrete CSAs, resistors used in the external LPFs were incorporated into the circuit’s overall design so errors because of any input-bias current-generated voltage and gain errors were compensated.
With the advent of monolithic CSAs, using external LPFs in series with the CSA’s inputs only introduces additional offset voltage and gain errors. To minimize/eliminate the need for external LPFs and to maintain low offset voltage and gain errors, higher-performance unidirectional and bidirectional CSAs incorporate internal LPFs to further save system cost and improve overall system performance.
Additional applications tips
For optimal VSENSE , all parasitic pcb track resistances to the sense resistor should be minimized. Kelvin-sense pcb connections between RSENSE and the CSAs’ RS+ and RS- terminals are strongly recommended. The pcb layout should be balanced and symmetrical to minimize wiring-induced errors. In addition, the pcb layout for RSENSE should include good thermal management techniques for optimal RSENSE power dissipation.
A 22 to 100-nF good-quality ceramic capacitor from the OUT terminal to GND forms an LPF with the CSAs’ ROUT and should be used to minimize voltage droop (holding VOUT constant during the sample interval). Using a capacitor on the OUT terminal will also reduce the CSAs’ small-signal bandwidth as well as band-limiting amplifier noise.
In conclusion, a new state of the art in CSA technology has been redefined. Novel CSAs are extremely easy to use, can resolve charging or discharging currents with 12-bit or better resolution, exhibit very low VOS and gain match errors, are self-powered, and consume very little supply current. These higher-performance CSAs mate their electrical performance with pc-board space-saving packages (such as SOT23-5 and SOT23-6), are specified to operate over wide or extended industrial temperature ranges, and can operate from 2 to 25-V (and higher) power supplies. ■
Fig. 2: A typical application for a bidirectional high-precision current sense amplifier (Touchstone Semiconductor's TS1101).
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