AN105 - Current Sense Circuit Collection Making …

1 Application Note 105AN105-1an105faDecember 2005 Current Sense Circuit CollectionMaking Sense of CurrentTim Regan, Jon MunsonGreg Zimmer, Michael StokowskiL, LT, LTC, LTM, Linear Technology, the Linear logo, Over-The-Top and TimerBlox are registered trademarks and Hot Swap is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. INTRODUCTIONS ensing and/or controlling Current flow is a fundamen-tal requirement in many electronics systems, and the techniques to do so are as diverse as the applications themselves.

2 This Application Note compiles solutions to Current sensing problems and organizes the solutions by general application type. These circuits have been culled from a variety of Linear Technology Organized by General ApplicationEach chapter collects together applications that tend to solve a similar general problem, such as high side Current sensing, or negative supply sensing. The chapters are titled accordingly. In this way, the reader has access to many possible solutions to a particular problem in one place.

3 It is unlikely that any particular Circuit shown will exactly meet the requirements for a specific design, but the sug-gestion of many Circuit techniques and devices should prove useful. To avoid duplication, circuits relevant to multiple chapters may appear in one location. Circuit Collection INDEXn Current Sense Basicsn High Siden Low Siden Negative Voltagen Unidirectionaln Bidirectionaln ACn DCn Level Shiftingn High Voltagen Low Voltagen High Current (100mA to Amps)n Low Current (Picoamps to Milliamps)n Motors and Inductive Loadsn Batteriesn High Speedn Fault Sensingn Digitizingn Current Controln Precisionn Wide RangeApplication Note 105AN105-2an105faThis chapter introduces the basic techniques used for sensing Current .

4 It serves also as a definition of common terms. Each technique has advantages and disadvantages and these are described. The types of amplifiers used to implement the circuits are SIDE Current SENSING (Figure 1) Current sensed in the ground return path of the power connection to the monitored load. Current generally flows in just one direction (unidirectional). Any switching is performed on the load-side of monitor. +ILOADISENSELOADOUTPUT ILOADDC VSUPPLYVCCRSENSELow Side Advantagesn Low input common mode voltagen Ground referenced output voltagen Easy single-supply designLow Side Disadvantagesn Load lifted from direct ground connectionn Load activated by accidental short at ground end load switchn High load Current caused by short is not detectedHIGH SIDE Current SENSING (Figure 2) Current sensed in the supply path of the power connection to the monitored load.

5 Current generally flows in just one direction (unidirectional). Any switching is performed on the load-side of monitor. +ILOADISENSELOADOUTPUT ILOADDC VSUPPLYRSENSE +ILOADISENSEOUTPUT ILOADDC VSUPPLYVCCRSENSELOADCURRENT Sense BASICSF igure 1. Low Side Current SensingFigure 3. Full-Range (High And Low Side) Current SensingFigure 2. High Side Current SensingHigh Side Advantagesn Load is groundedn Load not activated by accidental short at power con-nection n High load Current caused by short is detectedHigh Side Disadvantagesn High input common mode voltages (often very high)n Output needs to be level shifted down to system operating voltage levelsFULL-RANGE (HIGH AND LOW SIDE) Current SENSING (Figure 3)

6 Bidirectional Current sensed in a bridge driven load, or uni-directional high side connection with a supply side Advantagesn Only one Current Sense resistor needed for bidirec-tional sensingn Convenient sensing of load Current on/off profiles for inductive loads Full-Range Disadvantagesn Wide input common mode voltage swingsn Common mode rejection may limit high frequency accuracy in PWM applicationsApplication Note 105AN105-3an105faHIGH SIDEF igure 4. LT6100 Load Current MonitorFigure 5. Classic Positive Supply Rail Current SenseFigure 6.

7 Over-The-Top Current SenseThis chapter discusses solutions for high side Current sensing. With these circuits the total Current supplied to a load is monitored in the positive power supply Load Current Monitor (Figure 4)This is the basic LT6100 Circuit configuration. The internal circuitry, including an output buffer, typically operates from a low voltage supply, such as the 3V shown. The moni-tored supply can range anywhere from VCC + up to 48V. The A2 and A4 pins can be strapped various ways to provide a wide range of internally fixed gains.

8 The input leads become very Hi-Z when VCC is powered down, so as not to drain batteries for example. Access to an internal signal node (Pin 3) provides an option to include a filtering function with one added capacitor. Small-signal range is limited by VOL in single-supply F04 RSENSELT610081VS VS+ F3V65 FILTO LOAD+5V+ + +LT16375V200 200 2k0V TO TA02 VOUT = (2 )(ILOAD)Q12N3904 LOADILOAD +LT16373V TO 44V3VR1200 R22kVOUT(0V TO )Q12N39041637 TA06 LOADILOADVOUT(RS)(R2/R1)ILOAD = Classic Positive Supply Rail Current Sense (Figure 5)This Circuit uses generic devices to assemble a function similar to an LTC6101.

9 A rail-to-rail input type op amp is required since input voltages are right at the upper rail. The Circuit shown here is capable of monitoring up to 44V applications. Besides the complication of extra parts, the VOS performance of op amps at the supply is generally not factory trimmed, thus less accurate than other solutions. The finite Current gain of the bipolar transistor is a small source of gain Current Sense (Figure 6)This Circuit is a variation on the classic high side cir-cuit, but takes advantage of Over-the-Top input capability to separately supply the IC from a low voltage rail.

10 This provides a measure of fault protection to downstream circuitry by virtue of the limited output swing set by the low voltage supply. The disadvantage is VOS in the Over-the-Top mode is generally inferior to other modes, thus less accurate. The finite Current gain of the bipolar transistor is a source of small gain High Side Current Sense (Figure 7)This Circuit takes advantage of the microampere supply Current and rail-to-rail input of the LT1494. The Circuit is simple because the supply draw is essentially equal to the load Current developed through RA.