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| AN1305 APPLICATION NOTE CURRENT MEASUREMENT AND LIMITATION IN TD340 BASED MOTOR CONTROL SYSTEMS by J. CAMIOLO and J-F GARNIER 1. INTRODUCTION In motor control applications, it is very useful to monitor the current through the motor. The current value is an image of the motor torque which is of great interest for motion control. The over current monitoring can be used to detect end of motion conditions and thus avoid the use of a position sensor. It is also useful to provide security functions like motor stall detection and short circuit protection. Two main configurations can be used: - sense resistor between the bottom of the H-bridge and the Gnd line (low side sensing). - or sense resistor connected between the Vbat line and the top of the H-bridge (high side sensing) Each configuration has its own advantages and disadvantages, and the choice of the right method depends on the application This application note describes some solutions that can be used with the TD340, a quad MOSFET driver in H-bridge configuration. 2. SOLUTION WITH THE SENSE RESISTOR CONNECTED IN THE GND WIRE 2.1 Description Figure 1 shows the operating principle. The dual operational amplifier TS922A (U2A and U2B) is powered by the 5V coming from the TD340. The TS922A is a rail to rail device with a low input offset voltage (0.9mV max.) and is well suited for this application. The voltage across the shunt resistor (Vsens) is amplified with the op-amp U2A. The voltage at the op-amp output is: V(I reading) = Vsens * (R13+R14) / R13 The R11, C5 network is used to remove the PWM frequency (25kHz typ.). Cut frequency is about 1kHz, providing a response time of a few milliseconds for the current information. The second op-amp U2B is used to implement an overcurrent security. U2B is used as a comparator and provides an emergency signal to the C when an overcurrent occurs. October 2000 The threshold is set by resistors R17 and R18 from the 5V supply. The R15, C3 network acts as a filter to reject short overcurrent conditions, e.g. at motor startup. Figure 1 : Low Side Sensing VcarBat 4 C3 1nF 6 5 R17 10K U2B 7 Overcurrent Out + TS922A 8 5V R16 560K R18 10K Qreverse STP60NE-06L R15 10K STP60NE-06L Q2H Q1H U2A TS922A + 1 Out I Reading R14 47K 5V 3 2 R13 10K Rsens I Vsens C5 100nF R11 10K Q2L Q1L STP60NE-06L 2.2 Full Schematic Figure 2 shows the complete application schematic. The current reading signal is connected to a A/D input port of the micro-controller (C), and the overcurrent signal (active low) is wired to an interrupt input. Furthermore, the overcurrent signal immediately brakes the motor by pulling low the IN1/IN2 lines of the TD340 with diodes D3, D4. This feature provides a fast hardware current limitation. As soon as the current drops below the threshold (including the hysteresis), the IN1/IN2 lines are released and the TD340 is controlled again by the C. 2.3 Comments Advantages: -easiest method, -precise, low offset, no calibration needed. Disadvantage: -unable to detect load shorted to ground. 1/6 AN1305 - APPLICATION NOTE Figure 2 : Low Side Sensing, Full Application Schematic VcarBat Vcarbat 1 D2 1N4148 C4 10f 5V RStBy CVout 10K 220nF Vbat RReset 10K 1 2 3 Cwd 4 Watchdog 5 6 Temp Reading Speed Direction Current reading Overcurrent Gnd U2A 1 + Out TS922 D4 1N4148 D3 1N4148 3 2 R26 R25 2K2 2K2 7 8 9 10 Vbat Vout Reset Cwd Wd Stby Temp In1 In2 Cf U1 Osc Cb1 H1 S1 Cb2 H2 S2 L2 L1 Gnd 20 19 18 17 16 15 14 13 12 11 RQ2L RQ1L Cb2 47nF Cb1 47nF R2 5K6 RQ1H + C1 1N4148 100pF D1 R1 680K Vboost Qreverse STP60NE-06L Controller ST6xx Vcc Stby Reset RQ2H STP60NE-06L Q2H Q1H Q2L Q1L STP60NE-06L TD340 R11 10K C5 100nF R13 5V R15 100K R17 10K 10K Gnd 1 Rsens R14 47K R16 560K 5V 8 5 6 C3 47nF 7 + Out U2B TS922 4 - R18 10K 2/6 AN1305 - APPLICATION NOTE 3. SOLUTION WITH THE SENSE RESISTOR CONNECTED IN THE VBATT WIRE 3.1 Description If a fifth MOSFET is used to protect against battery connection reversal (see the TD340 datasheet), it can be used as a sense resistor. In normal mode the Q transistor is always on and has a very small Rdson resistance. An additional advantage from using the MOS resistance is that it provides a lower current limitation at higher temperature as the Rdson is increasing. Figure 3 shows the operating principle. The voltage at the op-amp output is: V(Ireading) = Vsens * R22 / (R21+R22) * R14 / (R11 + R21//R22) ESD diode 5V R12 220K R19 47K IR13 V1 V2 R21 47K Figure 4 : Op-Amp Protection VcarBat=60V VsensVBat Rsens + 2mA R13 20K R11 20K - R14 220K R20 15K R22 15K Rx//Ry means Rx parallel with Ry. Conditions are: R19=R21, R20=R22, R11=R13 and R12=R14. Figure 3 : High Side sensing VcarBat I Qreverse VBat Voltage range To achieve a good common mode rejection, precision resistors (at least 0.1%) should be used. If the common voltage range is 12V to 16V, the common voltage range after the resistor bridge is 3V to 4V. A 0.1% mismatch between each arm of the resistor bridge produces a change of 1mV differential voltage, i.e. a 2% systematic error (1mV relative to 50mV). Moreover, the 1V common mode voltage change seen by the op-amp also produces an error due to the finite CMRR of the op-amp: 1 mV for a standard 60dB rejection ratio. This shows that this method only works with limited common voltage range. Offset The total offset is due to the intrinsic offset of the op-amp, plus the offset due to the various resistor mismatch. As this offset is not negligible, it must be corrected by software. The C must proceed to an auto zero calibration when the motor is off (at each power-up for example). However, the offset can be positive or negative, so the zero current level at the input of the C should not be 0V, but a small positive value. This is the purpose of R23 and R24 that provide a shift of about 500mV. 3.2 Full schematic The full schematic is showed in figure 5 and includes an overcurrent protection built with a second op-amp used as a comparator. 3.3 Comments Advantages: -detection of load shorted to ground, -use of the 5th MOSFET as sensing resistor. Disadvantages: -need of high precision resistors, -need of a software auto-zero calibration. 3/6 Vsens R12 220K U2A TS922 Ireading 1 + Out R14 220K R16 560K 5V 8 5 6 C3 1nF R18 10K R15 10K R17 10K R20 15K R22 15K Q2L Q1L 3 2 R13 20K R11 20K 5V STP60NE-06L Q2H Q1H R19 47K R21 47K Overcurrent 7 + Out U2B TS922 4 High common mode voltage The major problem with this configuration is the high common mode voltage at the sense resistor. Moreover, in automotive applications, the Vbatt voltage can rise to more than 40V during 'load dump' transients. The TD340 can sustain such voltage (up to 60V), but the op-amp has to be protected as well. This can be done with resistor bridges at each side of the sense resistor to lower voltages to acceptable level. Resistor ratio of 1:3 provides a 1/4 attenuation of common voltage that can then rise near to 20V. Greater voltages are not functional but are harmless for the op-amp because the voltages at input pins are clamped by diodes to the 5V supply and the serial resistors limit the current into the input pins to acceptable levels (see figure 4). Of course, differential voltage is reduced as well, a typical 200mV shunt voltage gives only 50mV after the resistor bridge. AN1305 - APPLICATION NOTE Figure 5 : High Side Sensing with Resistor Bridge, Full Schematic VcarBat Vcarba 1 D2 1N4148 C2 470f 5V RStBy 10K Controller ST6xx Reset Cwd 4 Watchdog 5 6 Temp Reading Speed Direction Current reading Overcurrent Gnd R23 2.2M U2A TS922A 1 Out R14 R24 D4 1N4148 D3 1N4148 R16 560K 5V 8 5 6 C5 47nF R15 100K 220K 2.2M + 3 2 R13 20K R11 20K R12 220K 5V R26 R25 2K2 2K2 7 8 9 10 3 Vcc Stby CVout 220nF Vbat RReset 10K 1 2 Vbat Vout Reset Cwd Wd Stby Temp In1 In2 Cf U1 Osc Cb1 H1 S1 Cb2 H2 S2 L2 L1 Gnd 20 19 18 17 16 15 14 13 12 11 RQ2L Cb2 47nF Cb1 47nF R2 5K6 RQ1H + C1 1N4148 100pF D1 R1 680K Vboost Qreverse STP60NE-06L Q2H RQ2H Q1H STP60NE-06L Q2L RQ1L Q1L Gnd 1 5V TD340 V1 V2 R19 47K R21 47K R20 15K R17 10K C3 47nF R22 15K C4 47nF 7 + Out U2B TS922A 4 - R18 10K R11- R14, R19-R22: 0.1% 4/6 AN1305 - APPLICATION NOTE 4. ALTERNATE SOLUTION WITH THE SENSE RESISTOR CONNECTED IN THE VBATT WIRE 4.1 Description Using the sense resistor in high side position has a lot of advantage, but is usually difficult to handle. The following schematic (figure 6) shows an alternate solution using a floating op-amp near the Vbatt line. Figure 6 : High Side Sensing VcarBat 4.3 Comments Advantages: -detection of load shorted to ground, -precise, low offset, no calibration needed. Disadvantages: -2 independent op-amps ICs -more discrete parts. 5. CONCLUSION Measuring the current in motor control application is often required. Choosing the right method for current measurement and limitation requires careful reviewing of the advantages and disadvantages of each configuration. For automotive applications, a high side sensing will often be preferred, whereas industrial or cost sensitive applications will use simpler low side sensing. I Qreverse STP60NE-06L R10 20K C3 1nF R11 1K Operation The principle is to use a op-amp supplied near to Vbatt to measure the differential voltage across the shunt. This op-amp is supplied between Vbatt and an intermediate voltage at about 5V below Vbatt. The op-amp operates close to the upper rail, and converts the shunt voltage into a current by the means of resistor R11 and transistor Q3. Resistor R12 converts back this current into a voltage near the ground level where it can be easily used by the C or additional circuit. The I reading voltage is: V(I reading) = Vsens * R12 / R11 4.2 Full schematic The full schematic (figure 7) includes overcurrent protection build with a independent op-amp used as a comparator. The intermediate voltage for U3 is built with Zener diode Z1 and transistor Q4. If the application have to withstand 'load dump' transients, Q1 and Q4 should be rated at least to 60V. Vsens 2 Q2H Q1H 3 + U3A Out TS922A 1 Q3 PNP STP60NE-06L VcarBat-5V Q2L Q1L R12 10K Ireading 5/6 AN1305 - APPLICATION NOTE Figure 7 : High Side Sensing with Floating Op-Amp, Full Schematic VcarBat Vcarbat 1 D2 1N4148 C2 470 f 5V Vbat Vcc Stby ST6xx Res et Cwd 4 Watchdog 5 6 Temp Rea ding Speed Direction Current readi ng Overcurrent Gnd U2A TS922A 1 Out R26 R25 2K2 2K2 7 8 9 10 3 RStBy CVout 10K 220nF RRese t 10K 1 2 Vbat Vout Reset Cwd Wd Stby Temp In1 In2 Cf U1 Osc Cb1 H1 S1 Cb2 H2 S2 L2 L1 Gnd 20 19 18 17 16 15 14 13 12 11 RQ2L Cb2 47nF Cb1 47nF R2 5K6 RQ1H + C1 1N4148 100pF D1 R1 680K Vboost Qreverse STP60NE-06L R10 20K 2 3 + U3A Out TS922A 1 Q3 PNP 6V C3 1nF R11 1K Z1 Controller Q4 Q2H Q1H RQ2H STP60NE-06L R14 10K Q2L RQ1L Q1L PNP Gnd 1 TD340 + 3 2 R13 22K D4 1N4148 D3 1N4148 R16 560K 5V 8 5 6 C5 47nF R15 100K 5V R12 10K R17 10K C4 47nF 7 + Out U2B TS922A 4 - R18 10K R11- R14, R19-R22: 0.1% Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibil ity for the consequences of use of such information nor for any infring ement of patents or other righ ts of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change witho ut notice. This publ ication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life suppo rt devices or systems withou t express written approval of STMicroelectronics. (c) The ST logo is a registered trademark of STMicroelectronics (c) 2000 STMicroelectronics - Printed in Italy - All Rights Reserved STMicroelectronics GROUP OF COMPANIES Australia - Brazil - China - Finland - France - Germany - Hong Kong - India - Italy - Japan - Malaysia - Malta - Morocco Singapore - Spain - Sweden - Switzerland - United Kingdom (c) http://www. st.com 6/6 |
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