Texas Instruments | Operating an Engine-Grille Shutter Motor With DRV8872-Q1 | Application notes | Texas Instruments Operating an Engine-Grille Shutter Motor With DRV8872-Q1 Application notes

Texas Instruments Operating an Engine-Grille Shutter Motor With DRV8872-Q1 Application notes
Application Report
SLVA858 – December 2016
Operating an Engine-Grille Shutter Motor
With DRV8872-Q1
Ishtiaque Amin, Rick Duncan
ABSTRACT
The addition of an active grille shutter is a key aerodynamic improvement in recent model-year vehicles.
Depending on the ambient temperature of the environment, the shutters behind the engine grille can open
and close to increase the engine efficiency and reduce turbulent air from entering the engine, thereby
reducing drag. The shutters can be operated by a brushed DC (BDC) motor. A suitable device for such an
application is Texas Instruments' DRV8872-Q1 BDC motor driver. This application report provides a
detailed overview of how the DRV8872-Q1 device can be used to design a circuit to operate an enginegrille shutter motor.
Contents
1
Grille Shutter System Overview ...........................................................................................
2
Internal Current Regulation With DRV8872-Q1..........................................................................
3
Circuit Design for Grille-Shutter Application ..............................................................................
4
Grille Shutter Motor-Current Profile .......................................................................................
Appendix A
Glossary ..............................................................................................................
2
3
3
5
7
List of Figures
1
Air Flow With Grille Shutter Open and Closed ........................................................................... 2
2
Active Grille Shutter Operation............................................................................................. 2
3
Internal Current-Chopping Timing Diagram .............................................................................. 3
4
Typical Stall-Detection Circuit .............................................................................................. 4
5
Motor Phase-Voltage Profile
6
7
8
...............................................................................................
Motor-Current Profile ........................................................................................................
Forward Direction Motor Stalling ..........................................................................................
Reverse Direction Motor Stalling ..........................................................................................
4
5
5
6
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1
Grille Shutter System Overview
1
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Grille Shutter System Overview
When the engine-grille shutters open, air is allowed to flow through the radiator and into the engine
compartment for faster cooling of the engine. But when that cooling air is not needed, the shutters close,
as determined by the engine control unit (ECU), which reroutes air around the vehicle creating less
turbulence than what would otherwise exist if the air was to flow through the vehicle. The reduced
turbulence lessens the aerodynamic drag by up to 6%, increasing fuel economy and reducing carbondioxide emissions by up to 2%.
Figure 1. Air Flow With Grille Shutter Open and Closed
As an added advantage, the closed shutters also reduce engine warm-up time in cold weather by stopping
air from entering the engine on cold days. This advantage allows the engine to run more efficiently,
heating to the optimal operating temperature faster, and warming up the interior of car quicker. Figure 2
shows an active grille shutter typically found in most modern vehicles.
Figure 2. Active Grille Shutter Operation
The brushed DC motor used to operate the shutters would have a peak current rating of approximately 1
A. The DRV8872-Q1 motor driver is a suitable device support this requirement. The device can be used to
regulate motor current around 1 A by following the process described in Section 2.
2
Operating an Engine-Grille Shutter Motor With DRV8872-Q1
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Internal Current Regulation With DRV8872-Q1
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2
Internal Current Regulation With DRV8872-Q1
The DRV8872-Q1 device features current regulation that restricts the motor current to a preset maximum.
This restriction is achieved by appropriately sizing an external sense resistor (RSENSE) on the ISEN pin.
Use Equation 1 to calculate the current trip level (ITRIP).
Vtrip
ITRIP (A)
RSENSE
(1)
The voltage trip level (Vtrip) for the device is internally set to 0.35 V (nominal). Select the current-chopping
limit of 900 mA which is close to what a grille-shutter BDC motor would receive for peak current.
Vtrip 0.35 V
RSENSE
0.388 :
ITRIP
0.9 A
(2)
Use Equation 3 to calculate the power dissipation (PD) for the RSENSE resistor.
I2R
PD (RSENSE )
0.9
2
u 0.388
0.315 W
(3)
Therefore, a resistor with these specifications should be appropriate for regulating current in this
application. An industry-standard part could be a 0.39-Ω, ½ W resistor in a1206 or larger package.
When the ITRIP threshold is reached the device stops driving the motor and enforce slow decay through the
low-side MOSFETs. The motor will be in slow decay for typically 25 µs (tOFF time) and, after tOFF has
expired, the outputs will be re-enabled to operate the motor for a drive time (t(DRIVE)) until the ITRIP threshold
is reached. Figure 3 shows the timing diagram for this operation.
Motor Current (A)
ITRIP
Drive
Brake and Slow Decay
Drive
Brake and Slow Decay
t(DRIVE)
tOFF
t(DRIVE)
tOFF
Figure 3. Internal Current-Chopping Timing Diagram
The drive time of the motor (t(DRIVE)) primarily depends on the supply voltage, motor inductance, and the
back-EMF generated by the motor. For more details about the device, refer to DRV8872-Q1 Automotive
3.6-A Brushed DC Motor Driver With Fault Reporting (SLIS175).
3
Circuit Design for Grille-Shutter Application
The block diagram in Figure 4 shows a basic circuit that can be implemented to operate the grille shutters.
The DRV8872-Q1 device receives command from the microcontroller unit (MCU) through the IN1 and IN2
lines and drives the BDC motor. The value of the RSENSE resistor, connected to the ISEN pin, is selected
based on the calculations in Section 2. The Zener diode is optional but recommended to clamp the node
to 3.6 V.
If the source of the power supply is the vehicle battery, instances of load dump could occur for which the
Zener diode will be required, especially if the MCU has 3.3-V rated digital I/O pins. Without the Zener
diode, the MCU I/O pin can receive a higher voltage that can potentially damage the MCU.
NOTE: The resistor values for R1 through R4, in Figure 4, have been selected for a regulated power
supply (VBATT) of 13.5 V.
The phase voltages of the motor will be approximately 0 V and battery voltage depending on which
direction the motor is rotating. The R2 resistor is in parallel with the R3 and R4 resistors so the voltage
between at the node between R1 and R2 becomes 4.5 V. This voltage gets further scaled down by the
resistor divider network of R3 and R4 to bring the nSTALL node voltage to 3.15 V, as shown by
Equation 4.
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Circuit Design for Grille-Shutter Application
VnSTALL
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§
R2 || >R3 R4@
¨
¨ ªR1 R2 || >R3 R4@
©¬
·
¸ u 13.5 V u ¨§ R3 ¸·
º¸
© R3 R4 ¹
¼¹
3.15 V
(4)
If the power supply is significantly higher than 12 V, for example 24 V (which is within the operating supply
range of the device), the voltage at the nSTALL node, without the Zener diode, becomes 5.6 V. Such a
high voltage can damage the MCU and, therefore, the resistors, R1–R4, should be sized appropriately to
meet the maximum allowable voltage at the I/O pin of the MCU, in addition to placing the Zener diode.
VBATT
VCC
IN1
R1
10 k
IN2
R4
3k
M
MCU
nSTALL
DRV8872-Q1
nFAULT
nSTALL
R3
7k
R2
10 k
47 pF
3.6 V
ISEN
RSENSE § 0.39
, ½W
Copyright © 2016, Texas Instruments Incorporated
Figure 4. Typical Stall-Detection Circuit
When the motor is running, the voltage at each motor phase is monitored by the MCU, after some signal
conditioning, for a motor stall condition. The voltage profile at the nSTALL node will be similar to the
profile in Figure 5.
3.6 V
(maximum)
Motor stalled
(DRV8872-Q1 enters current regulation)
tRUN
nSTALL
0V
Figure 5. Motor Phase-Voltage Profile
When the MCU identifies the voltage profile at the nSTALL node to be similar to the highlighted portion in
Figure 5, the motor has stalled. One way to identify the voltage profile is to trigger an interrupt in the MCU
at the first falling edge of the nSTALL node after the motor has been running for the tRUN time. The MCU
can then measure the time interval between the first falling edge to the next rising edge and compare it to
the tOFF time of the device. If this time interval is less than tOFF equal to 25 µs (with about 10% tolerance), it
can be concluded that the motor has stalled. At this point, the MCU must shut off or change the rotational
direction of the motor.
The current profile of the motor moving in a given direction (for example, in reverse drive) during this
process will look similar to profile in Figure 6. For motor rotation in the opposite direction (for example,
forward drive) the profile is inverted. Section 4 includes application graphs for motor motion in both
clockwise and counter-clockwise directions.
4
Operating an Engine-Grille Shutter Motor With DRV8872-Q1
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Grille Shutter Motor-Current Profile
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Motor stalls at ~1 A
Motor starts
running
Motor runs normally
0A
Figure 6. Motor-Current Profile
An inrush of current occurs initially to start the motor. Following that inrush, the current flowing through the
motor windings decays to an rms value while the grille shutter is in motion. When the shutters are
completely open or closed, the motor stalls at a peak current of approximately 1 A.
4
Grille Shutter Motor-Current Profile
Figure 7 shows current flowing through the motor in forward drive, and the voltage profile at the nSTALL
node. The steady-state DC current is approximately 375 mA. As the current flowing through the motor
increases above 800 mA, the device starts to perform current regulation. The stall-detection circuit toggles
between logic levels 0 and 1 at the nSTALL node, indicating to the MCU that the motor has stalled. The
zoomed-in plots show the motor-current profile during start-up and stall.
Figure 7. Forward Direction Motor Stalling
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Grille Shutter Motor-Current Profile
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Figure 8 shows current flowing through the motor in reverse drive, and the voltage profile at the nSTALL
node. The peak current is limited by the RSENSE resistor to approximately 800 mA.
Figure 8. Reverse Direction Motor Stalling
Based on these plots, the DRV8872-Q1 device is concluded to be able to drive the BDC motor for an
engine-grille shutter. The stall-detection circuit (Figure 4) also provides accurate information to the host
MCU regarding motor stall which prevents the device from over driving the motor.
6
Operating an Engine-Grille Shutter Motor With DRV8872-Q1
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Appendix A
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Glossary
A.1
Nomenclature Used in this Document
The following acronyms and initialisms are used in ths document:
BDC — Brushed DC
ECU — Engine control unit
IC — Integrated circuit
MCU — Microcontroller unit
MOSFET — Metal-oxide semiconductor field-effect transistor
OCP — Overcurrent protection
PWM— Pulse width modulation
For a more comprehensive list of terms, acronyms, and definitions, refer to the TI Glossary (SLYZ022).
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