rev. 1.0 7/14 copyright ? 2014 by silicon laboratories AN840 AN840 redefining a new state-of -the-art in microampere current-sense amplifiers 1. introduction sensing and controlling supply current flow are a fundamental requireme nt in most all electronic systems from battery-operated, portable equipment to mobile or fix ed-platform power management and dc motor control. high- side current-sense amplifiers (or ?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 sacrif icing performance. with the right combination of small form factor, low supp ly-current operation, wide oper ating supply-voltage range, low input offset voltage (vos) and gain errors, and fixed gain options, design engineers now have even more options high-side current-sensing amplification. as a re sult, new csa enhancements enable the next generation of battery-powered, hand-held portable instruments addre ssing power management, motor control, and fixed- platform applications. 2. unidirectional current sense amplifiers 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 ex ternal sense and generates an output vo ltage as a function of load current. referring to the typical application circuit in figure 1, feat uring the ts1100, the inputs of the op-amp-based circuit are connected across an external rsense. at the rs - terminal, the applied vo ltage is iload x rsense. figure 1. typical application for a high-precision unidirectional current-sense amplifier (ts1100) since the rs- terminal is the non-inverting input of t he internal op amp, op-amp feedback action forces the inverting input of the internal op amp to the same potent ial (i load 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 resi stors are the same value.
AN840 2 rev. 1.0 3. bidirectional cu rrent sense amplifiers while uni-directional csas are primarily used in those ap plications where current is delivered to a load, there are many applications where it is necessary to measure cu rrent in both directions. some applications where bi- directional current-sense monitoring/amplification are needed include: smart battery packs and chargers, portable computers, super capacitor charging/discharging device s, and general-purpose current-shunt measurements. prior to the advent of bi-directional csas, uni-directiona l csas were used; however, it was necessary to use two uni-directional csas in order to measure current in bot h 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 addi tional microcontroller coding and machine cycles. to save on additional computing resources, pcb area, and component costs, a straight-f orward modification to the uni-directional csa configuration yields a bi-direc tional csa as shown in figure 2 for the ts1101. figure 2. typical application for a bidirectional high-precision current sense amplifier (ts1101) as shown in figure 2, the internal amplifier was reconfigur ed for fully differential input/output operation and a second low-threshold p-channel fet (m2) was added. the oper ation of this bidirectional csa is identical to that of the unidirectional csa previously discussed when vrs? > vrs+. in the implementation shown in figure 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 re sultant output voltage. for both types of unidirectional or bidirectional csas, gain error accuracy is a measure of how well controlled is the ratio of rout to rgain, especially over temperature. in a monolithic impl ementation, gain error accuracy can be <0.5% using novel circuit techniques. in a discrete csa circ uit, it would be quite difficult to achieve this level of performance over temperature with standard 1% tolera nce and 100 ppm/c temperature coefficient resistors. while some csas are only available with fixed-gain optio ns, many csas offer different gain options tailored to specific application requirements.
AN840 rev. 1.0 3 to achieve their very-low vos perfo rmance over temper ature, over wide vsense voltag es, and over wide power supply voltages, higher-per formance csa incorporate chop per stabilization into the in put stage, a commonly-used technique to reduce significantly amplifier vos. in r educing the csas? voss to 30 v (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 vo ss > 100 v or more, load current measurements are two times more accurate using csas that have implemented chopper- stabilized input stages. 3.1. the csa?s sign output comparator as was shown in figure 2, the bi-directional 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 comp arator 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+ < vrs-). note that, unlike other bi-directional csas where hyst eresis was purposely introduced to prevent comparator output voltage chatter, sign comparat or exhibits no ?dead zone? at iload switchover. with respect to a 50 mv external sense resistor, the load curren t transition band is less than 0.2ma. other types of csas that also utilize an analog out/ comparator sign ar rangement exhibit a sign transition band that can range up to 2 mv (or 40 ma referred to a 50 mv sense resistor). on this attribute alone, low-transition band, bidirectional csas can be 200 times more sensitive. 3.2. internal noise filters to counter the effects of externally-injected differenti al and common-mode noise prevalent in any load current measurement scheme, it?s always been good engineering practi ce to add external low-pass filters (lpfs) in series with the csa?s inputs. in the design of discrete csas, resi stors 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, utilizing 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 uni-di rectional and bi-directional csas incorporate internal lpfs to further save system cost and improve overall system performance. 3.3. additional applications tips for optimal vsense, all parasitic pcb tr ack 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 te chniques for optimal rsense power dissipation. a 22 nf to 100 nf good-quality ceramic capacitor from the out terminal to gnd forms a lpf with the csas? rout and should be used to minimize voltage droop (holdi ng vout constant during the sample interval). using a capacitor on the out te rminal 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-b it 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 pcb-space saving pack ages (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. see the documentation for the ts1100 and ts1101 current-sense amplifiers. for additional information, contact silicon labs.
AN840 4 rev. 1.0 c ontact i nformation silicon laboratories inc. 400 west cesar chavez austin, tx 78701 tel: 1+(512) 416-8500 fax: 1+(512) 416-9669 toll free: 1+(877) 444-3032 please visit the silicon labs technical support web page: https://www.siliconlabs.com/support/pages/contacttechnicalsupport.aspx and register to submit a technical support request. patent notice silicon labs invests in research and development to help our cust omers differentiate in the market with innovative low-power, s mall size, analog- intensive mixed-signal soluti ons. silicon labs' extensive patent portfolio is a testament to our unique approach and world-clas s engineering team. silicon laboratories and silicon labs are trademarks of silicon laboratories inc. other products or brandnames mentioned herein are trademarks or registered trademarks of their respective holders. the information in this document is believed to be accurate in all respects at the time of publ ication but is subject to change without notice. silicon laboratories assumes no responsibili ty for errors and omissions, and disclaim s responsibility for any consequences resu lting from the use of information included herein. additionally, silicon laboratories assumes no responsibility for the functioning of und escribed fea- tures or parameters. silicon laboratories reserves the right to make changes without further notice. silicon laboratories makes no warran- ty, representation or guarantee regarding t he suitability of its products for any par ticular purpose, nor does silicon laborato ries assume any liability arising out of the application or use of any product or circuit, and specif ically disclaims any and all liability, in cluding without limitation consequential or incidental damages . silicon laboratories products are not designed, intended, or authorized for use in applica tions intend- ed to support or sustain life, or for any other application in which the failure of the silicon laboratories product could crea te a situation where personal injury or death may occur. should buyer purchase or us e silicon laboratories products for any such unintended or unaut horized application, buyer shall indemnify and hold silicon laboratories harmle ss against all claims and damages.
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