Full-Scale Current Adjustment Using a Digital-to … Current Adjustment Using a Digital-to-Analog Converter (DAC) 3 VDAC Calculation..... For the DRV8884/5/6/6AT, when the device reaches

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Full-Scale Current Adjustment Using a Digital-to-Analog Converter (DAC)

Application ReportSLVA872–March 2017

Full-Scale Current Adjustment Using a Digital-to-AnalogConverter (DAC)

Luis Riveros-Luque

ABSTRACTThe ability to dynamically control current in an inductive load system is very important for stepper motordesigns where different levels of torque control are desired. This adjustment feature can also be used toimprove system efficiency by reducing the motor current in low-load situations, achieving a longer batterylife.

This application report is provided as a supplement to the data sheet for the DRV8884, DRV8885,DRV8886 and DRV8886AT motor drivers. The goal of this document is to show how to achieve accuratecurrent regulation in normal and low-power modes using different methods. This document also describesdifferent sources of error in these configurations, how to minimize these errors, and the key factors toconsider when doing a design.

Contents1 Introduction .................................................................................................................. 22 Full-Scale Current Adjustment ............................................................................................ 3

2.1 TRQ Selection ...................................................................................................... 32.2 IFS Adjustment Using MCU DAC ................................................................................ 3

3 Various Sources of Error.................................................................................................... 43.1 VRREF, ARREF, and RREF Error ...................................................................................... 43.2 VDAC Error............................................................................................................. 5

4 Application-Specific Error Calculations ................................................................................... 75 Bench Data Correlation .................................................................................................... 8

5.1 Test Setup ........................................................................................................... 85.2 Normal Current Mode—1 A ....................................................................................... 95.3 Low-Current Mode—200 mA .................................................................................... 11

6 Considerations for Accurate Measurements............................................................................ 14

List of Figures

1 Current Chopping Waveform............................................................................................... 22 Controlling RREF With a DAC ............................................................................................. 33 GUI Setup—Current at 1 A ................................................................................................. 94 1-A Current at 24 V ....................................................................................................... 105 1-A Current at 24 V (Zoomed In)......................................................................................... 106 200-mA Current at 24 V ................................................................................................... 117 200-mA Current at 24 V (Zoomed In) ................................................................................... 128 GUI Setup—400-mA TRQ 50% (200 mA) .............................................................................. 139 400-mA Current at 24 V—TRQ 50% (200 mA) ........................................................................ 1310 400-mA Current at 24 V—TRQ 50% (200 mA) Zoomed In .......................................................... 14

List of Tables

1 Torque Scaling Settings .................................................................................................... 32 DRV8885 Data Sheet Values .............................................................................................. 4

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RREFFS

A (kA )I (A) TRQ (%)

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Introduction www.ti.com

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3 VDAC Calculation .............................................................................................................. 44 Worst Case Calculation—IFS Error at 1 A................................................................................. 45 Worst Case Calculation—IFS Error at 400 mA ........................................................................... 56 Worst Case Calculation—IFS Error at 200 mA ........................................................................... 57 Worst Case Calculation—VDAC 3% and 10%, IFS Error at 1 A .......................................................... 58 Worst Case Calculation—VDAC 3% and 10%, IFS Error at 400 mA..................................................... 69 Worst Case Calculation—VDAC 3% and 10%, IFS Error at 200 mA..................................................... 610 Values For DRV8885 VVM= 24-V........................................................................................... 711 IFS Error at 1 A, VDAC Fixed and Application Values ..................................................................... 712 IFS Error at 400 mA, VDAC Fixed and Application Values ................................................................ 713 IFS Error at 200 mA, VDAC Fixed and Application Values ................................................................ 714 VDAC 3%, VRREF and ARREF for 24-V Application at 1 A.................................................................... 815 VDAC 3%, VRREF and ARREF for 24-V Application at 400 mA .............................................................. 816 VDAC 3%, VRREF and ARREF for 24-V Application at 200 mA .............................................................. 817 Measured Values at 24 V for 1 A ........................................................................................ 1118 Measured Values at 24 V for 200 mA ................................................................................... 1219 Measured Values at 24 V for 400 mA, 50% TRQ (200 mA).......................................................... 14

TrademarksAll trademarks are the property of their respective owners.

1 IntroductionCurrent regulation through the motor windings is achieved by an adjustable fixed-off-time PWM currentregulation circuit. When the motor driver is enabled, current through the windings start to rise until thecurrent chopping threshold is met. For the DRV8884/5/6/6AT, when the device reaches this threshold, theH bridge of the motor driver enters decay mode for a fixed 20 μs. After the off time expires, the H bridge isre-enabled and current starts to rise again. This process repeats which is how current regulation isachieved.

Figure 1. Current Chopping Waveform

The PWM chopping current is set by a comparator which compares the voltage across the current senseparallel with the low-side drivers. The current sense MOSFETs are biased with a reference current that isthe output of a current-mode sine-weighted DAC whose full-scale reference current is set by the currentthrough the RREF pin. An external resistor is placed from the RREF pin to ground to set the referencecurrent.

Use Equation 1 to calculate the chopping current (IFS) when the RREF resistor is connected to ground.

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RREF RREF DACFS

RREF

A (kA ) [V (V) V (V)]I (A) TRQ (%)

V (V) RREF (k )

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u :

DAC

MCU

RREF

IREF

DVDD

Analog Input


RREF

www.ti.com Full-Scale Current Adjustment




where• ARREF is the transimpedance gain.• TRQ is the current scaling control. (1)

2 Full-Scale Current AdjustmentIn applications where is required for the current to be modified in real time, two different methods to adjustthe output current are available: TRQ setting and/or using a DAC function. Current adjustment featureis very important for designs where lower hold currents, lower motor torque, or both are required.Additionally, the ability to dynamically adjust the output current helps improve the efficiency of the system.

2.1 TRQ SelectionAs shown in Equation 1, the TRQ value dynamically changes the current output if desired. The setting forscaling the output current depends on the state of the TRQ pin. Table 1 lists the current scaling dependingon the TRQ setting.

Table 1. Torque Scaling Settings

TRQ Current Scalar (TRQ)0 100%Z 75%1 50%

For a 500 mA current at 100% TRQ, the full-scale current output could be easily set to 375 mA or 250 mAat 75% or 50%, respectively, without having to change any components in the design.

2.2 IFS Adjustment Using MCU DACThe second optimal solution is to use a DAC function which can be programmed to a target value as theapplication requires. Therefore a stepper motor design can achieve torque control with different holdingtorque and running torque values. It is important to note that whether using a DAC or an external supplyvoltage, they must have current sinking capabilities of at least (VRREF – VDAC) / RREF

Figure 2. Controlling RREF With a DAC

Use Equation 2 to calculate the chopping current as controlled by a DAC.

where

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RREFmax RREFmax DACminFSmax

RREFmax min

A (kA ) [V (V) V (V)]I (A) TRQ (%)

V (V) RREF (k )

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RREFmin RREFmin DACmaxFSmin

RREFmin max

A (kA ) [V (V) V (V)]I (A) TRQ (%)

V (V) RREF (k )

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u :

Various Sources of Error www.ti.com




• VRREF is the voltage reference measured from the RREF pin to ground.• VDAC is the voltage reference measured from the DAC output to ground. (2)

NOTE: In this case the RREF resistor is connected from the DAC to the RREF pin.

Equation 2 shows that both methods, TRQ and DAC adjustment, can be combined to better suit to aspecific application.

3 Various Sources of ErrorWhen performing a design error calculation, the different variables that contribute the most to the errormust be considered. To do so, first consider the typical values extracted from DRV8885 data sheet whichare listed in Table 2 with a 20-kΩ 1% resistor .

Table 2. DRV8885 Data Sheet Values

Parameter Minimum Typical MaximumARREF 28100 30000 31900VRREF 1.18 1.232 1.28RREF 19800 20000 20200

Using Equation 2 and knowing the desired output current, the VDAC value can be obtained. For example,the DRV8885EVM, which has a 20-kΩ resistor for RREF, was selected to operate at a 1-A, 400mA, and200 mA current. Table 3 lists the calculated VDAC values using typical ARREF and VRREF data sheet values

Table 3. VDAC Calculation

Parameter Minimum Typical MaximumIFS 1 0.4 0.2ARREF 30 000 30 000 30 000VRREF 1.232 1.232 1.232RREF 20 000 20 000 20 000VDAC 0.4107 0.9035 1.0677

Next, use Equation 3 and Equation 4 to calculate the worst case value for the minimum and maximum fullscale current, respectively.

(3)

(4)

These two equations show that error contributions come from VDAC, ARREF, VRREF, and RREF. The nextsections will show how these different error contributors, affect the overall IFS error and how they can beimproved.

3.1 VRREF, ARREF, and RREF ErrorTo observe how VRREF, ARREF, and RREFVRREF affect the IFS error , Equation 3 and Equation 4 are usedwith the data sheet values from earlier while VDAC voltage remains constant. Table 4, Table 5, and Table 6list the results at different current levels (1 A, 400 mA, and 200 mA, respectively).

Table 4. Worst Case Calculation—IFS Error at 1 A

Parameter Minimum Typical MaximumVDAC 0.4107 0.4107 0.4107ARREF 28100 30000 31900

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Table 4. Worst Case Calculation—IFS Error at 1A (continued)

Parameter Minimum Typical MaximumVRREF 1.18 1.232 1.28RREF 19800 20000 20200IFS (mA) 906.95 1000 1094.21Error (%) –9.30 9.42

Table 5. Worst Case Calculation—IFS Error at 400 mA

Parameter Minimum Typical MaximumVDAC 0.9035 0.9035 0.9035ARREF 28100 30000 31900VRREF 1.18 1.232 1.28RREF 19800 20000 20200IFS (mA) 326.00 400 473.93Error (%) –18.50 18.48

Table 6. Worst Case Calculation—IFS Error at 200 mA


These tables show that as the IFS current level decreases, the overall error percentage increases due toincreasing offset error from the internal signal chain. It is worthy to clarify that the VRREF and ARREF valuesin these tables are data sheet values which represent the characterization data variation across a widerange of temperatures and voltages with additional margin. For information on how to further minimize thispercentage of error based on targeted characterization data for VRREF and ARREF, see Section 4.

3.2 VDAC ErrorUsing the same methodology along with Equation 3 and Equation 4, the VDAC error contribution to IFS canbe shown. This is done by removing the error from VRREF, ARREF, and RREF. The following examples showthe VDAC error value with a 3% and 10% variation.

Table 7. Worst Case Calculation—VDAC 3% and 10%, IFSError at 1 A

Parameter Minimum Typical Maximum3% ERRORVDAC 0.3983 0.4107 0.423ARREF 30000 30000 30000VRREF 1.232 1.232 1.232RREF 20000 20000 20000IFS (mA) 985.08 1000 1015.07Error (%) –1.50 1.5010% ERRORVDAC 0.3696 0.4107 0.4517

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Various Sources of Error www.ti.com




Table 7. Worst Case Calculation—VDAC 3% and 10%, IFSError at 1 A (continued)

Parameter Minimum Typical MaximumARREF 30000 30000 30000VRREF 1.232 1.232 1.232RREF 20000 20000 20000IFS (mA) 950.08 1000 1050.07Error (%) –5.00 5.00

Table 8. Worst Case Calculation—VDAC 3% and 10%, IFSError at 400 mA

Parameter Minimum Typical Maximum3% ERRORVDAC 0.8764 0.9035 0.9306ARREF 30000 30000 31 900VRREF 1.232 1.232 1.232RREF 20000 20000 20000IFS (mA) 367.18 400 433.17Error (%) –8.25 8.2510% ERRORVDAC 0.8131 0.9035 0.9938ARREF 30000 30000 30000VRREF 1.232 1.232 1.232RREF 20000 20000 20000IFS (mA) 290.19 400 510.16Error (%) –27.48 27.48

Table 9. Worst Case Calculation—VDAC 3% and 10%, IFSError at 200 mA

Parameter Minimum Typical Maximum3% ERRORVDAC 1.0357 1.0677 1.0998ARREF 30000 30000 30000VRREF 1.232 1.232 1.232RREF 20000 20000 20000IFS (mA) 161.22 200 239.20Error (%) –19.48 19.4810% ERRORVDAC 0.9610 1.0677 1.1745ARREF 30000 30000 30000VRREF 1.232 1.232 1.232RREF 20000 20000 20000IFS (mA) 70.23 200 330.19Error (%) –64.92 64.92

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These tables show that as the variation in VDAC increases, the error percentage increases. Also, for verylow currents, the error percentage increases greatly because of the VDAC proximity to the VRREF voltage.

4 Application-Specific Error CalculationsAs described in the previous analysis, it is possible to obtain a tighter error calculations by using values forVRREF and ARREF for the specific application use case. The data sheet parameters represent limits based ondesign and characterization data across a wide range of temperatures and voltage with additional margin.For the following example, the operational voltage is limited to VVM = 24 V, a common operating point forthe DRV8884, DRV8885, DRV8886, and DRV8886AT.

Considering this use case, Table 10 provides updated values for VRREF and ARREF.

Table 10. Values For DRV8885 VVM= 24-V

Parameter Minimum Typical MaximumARREF 28800 30000 31200VRREF 1.207 1.232 1.257RREF 19800 20000 20200

Using values above and maintaining VDAC constant, the error percentage is reduced as shown in thefollowing tables.

Table 11. IFS Error at 1 A, VDAC Fixed and Application Values


Table 12. IFS Error at 400 mA, VDAC Fixed and Application Values


Table 13. IFS Error at 200 mA, VDAC Fixed and Application Values


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By keeping VDAC value fixed or close to be fixed, yields much less error variation. The same calculationcan be made using a VDAC value with a ±3 % variation to compare error percentage difference as shown inthe following tables.

Table 14. VDAC 3%, VRREF and ARREF for 24-V Applicationat 1 A


Table 15. VDAC 3%, VRREF and ARREF for 24-V Applicationat 400 mA


Table 16. VDAC 3%, VRREF and ARREF for 24-V Applicationat 200 mA


Table 14, Table 15, and Table 16 show values closer to the typical values for both VDAC, ARREF, and VRREF.From all these calculations, the error percentages for the 200 mA current are higher because at those verylow values, the minimum change greatly affects the full current equation. One method to improve the low-value current accuracy is to use a combination of the MCU DAC and TRQ pin. This method can helpimprove the error by reducing the need to use only the DAC voltage to achieve the low full-scale current.An example of this method is to achieve 200 mA using the 400 mA DAC setting and the 50% TRQ setting.

5 Bench Data CorrelationHaving the calculation data for all these cases, the results are validated using the DRV8885EVM at 24 V,1 A, 100% TRQ and at 24 V, 400 mA, 100% TRQ and 50% TRQ (200 mA). The DRV8885EVM was usedfor this setup using a 1-mH inductor for load.

5.1 Test SetupThe test setup included:• Board: DRV8885EVM• Device: DRV8885

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• Digital meter: Tektronix DMM 4040• Oscilloscope: Tektronix DPO 7054• Power supply: Chroma 62012P-100-50• Supply voltage: 24 V• Load: 1-mH inductor• Current: 400 mA

5.2 Normal Current Mode—1 AThe board was connected using a microUSB to use the DRV8885EVM GUI and the current was set to 1A. Figure 3 shows the GUI set up for this test.

Figure 3. GUI Setup—Current at 1 A

Figure 4 shows the step mode setup to be at 1/8 step with the default current value during startingcondition which is 71% based on DRV8855 data sheet.

Figure 4 and Figure 5 show the trip current at 1 A. The maximum current value for Figure 4 is 756 mA. Asrecorded, this maximum current includes undesired switching spikes which cause calculation errors.

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Figure 4. 1-A Current at 24 V

Figure 5 shows the reading focused at the switching transition. The current when the voltage switches offis 727.02 mA. This measurement is more accurate because the noise is ignored from this scope grab.

Figure 5. 1-A Current at 24 V (Zoomed In)

The current measured with the current probe is 71% of the output current. Table 17 lists the measuredvalues at 1 A.

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Table 17. Measured Values at 24 V for 1 A

Parameter ValueVDAC 0.4091VRREF 1.241RREF 19989IFS 1.024ARREF 30535

Comparing the full current value with the one calculated from, the error percentage is well within thecalculated variation at 2.34%.

5.3 Low-Current Mode—200 mAFollowing the same setup described in Section 5.2, the current is set to 200 mA. Figure 6 and Figure 7show the results.

Figure 6. 200-mA Current at 24 V

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Figure 7. 200-mA Current at 24 V (Zoomed In)

Table 18 lists the measured values for the 200 mA current at 24 V.

Table 18. Measured Values at 24 V for 200 mA


Comparing the full current value with the one calculated from, the error percentage is well within thecalculated variation at 4.94%.

Another way to obtain the low 200 mA current with better accuracy is to set the TRQ value to 50% anduse the 400-mA current setting as shown in Figure 8.

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Figure 8. GUI Setup—400-mA TRQ 50% (200 mA)

Figure 9. 400-mA Current at 24 V—TRQ 50% (200 mA)

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Considerations for Accurate Measurements www.ti.com




Figure 10. 400-mA Current at 24 V—TRQ 50% (200 mA) Zoomed In

Using the TRQ setting, the values for VDAC change slightly. This change is significant enough to see animproved error percentage. Table 19 lists the values measured when using the TRQ feature.

Table 19. Measured Values at 24 V for 400 mA, 50%TRQ (200 mA)


Comparing the full current value with the one calculated from, the error percentage is now 2.81%. Usingthe TRQ setting is recommended when low current values are desired.

6 Considerations for Accurate MeasurementsLow current measurements are not trivial, especially when low current limits are sought after, becausemultiple small errors can easily be introduced or overlooked which can drastically change the desiredoutput. Some of the measurement procedures taken with this application report are listed as follows:• High-precision voltmeter: Using at least a 6½ precision-calibrated digital meter is required to measure

at the milliamp level to achieve an accurate reading.• Grounding procedure: Measuring the values as direct and as close to the board design as possible is

important. While extracting data for this report, a millivolt difference occurred when sharing the groundfrom the digital multi-meter with the power supply versus connecting the ground of the meter directly tothe ground plane of the board. Although the difference was very small, it still had a noticeable impacton the final result.

• High-value inductor: Use a high-value inductor to obtain a long rising slope to obtain better reading. A1-mH inductor was used for this application report.

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Full-Scale Current Adjustment Using a Digital-to … Current Adjustment Using a Digital-to-Analog Converter (DAC) 3 VDAC Calculation..... For the DRV8884/5/6/6AT, when the device reaches