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Texas Instruments DRV8800, DRV8801 Design in Application notes
Application Report
SLVA322 – April 2009
DRV8800/DRV8801 Design in Guide
.....................................................................................................................................................
ABSTRACT
This document is provided as a supplement to the DRV8800/DRV8801 datasheet. It
details the steps necessary to properly interface the device in most motion control
applications powering a DC motor with conventional control signals such as PHASE
and ENABLE.
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Contents
General Description ................................................................................
1.1
H-Bridge Motor Driver .....................................................................
1.2
Control Signals .............................................................................
1.3
Proportional to Current Voltage Output (VPROPI) ....................................
Thermal Considerations ...........................................................................
2.1
Junction-to-Ambient Thermal Impedance (θJA) ........................................
Using VPROPI ......................................................................................
3.1
Selecting the SENSE Resistor ...........................................................
3.2
VPROPI Output ............................................................................
Micro Processor Interface .........................................................................
4.1
Pulse-Width Modulating ENABLE .......................................................
4.2
Pulse-Width Modulating PHASE .........................................................
2
2
2
2
2
2
3
3
4
8
9
9
List of Figures
1
2
3
4
5
6
7
SENSE Resistor Connection ......................................................................
RC Network in Series With the VPROPI Output ...............................................
PHASE PWM........................................................................................
Fast Decay: MODE1 = 0, MODE2 = 0 ...........................................................
Slow Decay: MODE1 = 1, MODE2 = X ..........................................................
VPROPI Limitations During Ground to Source Current Flow .................................
Micro Processor Interface .........................................................................
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3
4
5
6
7
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9
DRV8800/DRV8801 Design in Guide
1
General Description
1
General Description
1.1
H-Bridge Motor Driver
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The DRV8800/DRV8801 consists of a single H Bridge capable of delivering up to 2.8-A peak current to a
single DC motor load in both directions.
The H Bridge is protected against shorts to ground or power input by means of increased current
detection.
The device is also protected against increased die temperature by implementing a thermal shutdown
mechanism.
The SENSE pin can be used to set the maximum current so that the DC motor will drive at 100% duty
cycle. This terminal can be connected to ground for maximum current (peak current limited by Over
Current Protection circuitry).
1.2
Control Signals
The DRV8800/DRV8801 H Bridge offer PHASE and ENABLE control input lines which allow device users
to control direction (with PHASE) and speed (by applying a PWM to the ENABLE pin).
The device also offers an improved means to control current through the motor winding recirculation.
MODE 1 and MODE 2 input control signals grant three different degrees of slow/fast decay modes.
1.3
Proportional to Current Voltage Output (VPROPI)
The DRV8800/DRV8801 adds an important feature to motion control applications wanting to determine the
current level flowing through the DC motor winding. An analog voltage output pin, called VPROPI for
voltage proportional to current, supplies a signal which analog to digital converters and microcontroller
applications can use to accurately determine how much current is flowing through the controlled DC
motor.
2
Thermal Considerations
Although the DRV8800 and DRV8801 are rated at 2.8-A of current handling, the previous only holds true
as long as the internal temperature does not exceeds 170°C. In order to operate at this rate, the following
measures must be taken under consideration.
2.1
Junction-to-Ambient Thermal Impedance (θJA)
The DRV8800RTY/DRV8801RTY device, cased in a QFN package, has a θJA of 42°C/W. At any given
time during the steady state portion of the cycle, two FETs are enabled: A high side sourcing FET and a
low side sinking FET. Die temperature can then be computed by Equation 1.
Tdie = 42
2
°C
× I winding × RDSON
W
(1)
The DRV8800PWP/DRV8801PWP device, cased in a HTSSOP package, has a θJA of 33°C/W. At any
given time during the steady state portion of the cycle, two FETs are enabled: A high side sourcing FET
and a low side sinking FET. Die temperature can then be computed by Equation 2.
Tdie = 33
2
2
°C
× I winding × RDSON
W
DRV8800/DRV8801 Design in Guide
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Using VPROPI
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3
Using VPROPI
The analog output VPROPI offers SENSE current information as an analog voltage proportional to said
current. Said feature is only available when a resistor has been placed between the SENSE pin and
ground, and a voltage is being drawn across said resistor. The following sections detail the equations
required to properly select this resistor and extract the information from the VPROPI output.
3.1
Selecting the SENSE Resistor
A SENSE resistor must be connected between the SENSE pin and ground as shown in Figure 1.
100K
VDD
ANA_VPROPI
VBB
PHASE
5K
VCP
NFAULT
GND
(DRV8801) VPROPI
VDD
MODE / MODE 1
.1 uf
50V
1000 pf
GND
.1 uf
50V
CP2
CP1
VBB
M
RSENSE
SENSE
OUT-
OUT+
ENABLE
MODE 2 (DRV8801)
DRV8800/01
SLEEP
.1 uf
50V
100 uf
50V
Figure 1. SENSE Resistor Connection
A lack of a resistor (i.e. a direct connection to ground) yields a SENSE voltage equal to zero. In that case,
maximum current is 2.8 A and VPROPI outputs 0 V.
A resistor connected as explained before, will yield a VPROPI output as detailed in the next section. The
voltage across this resistor has to be smaller than 500 mV. Any voltage equal or larger to 500 mV will
signal the device to shut down as trip current has been reached. In this case, device will enter
recirculation as stipulated by the MODE input pin (for the DRV8800) or MODE 1 and MODE 2 input pins
(for the DRV8801).
Equation 3 shows the value of the resistor to a particular current setting.
Rsense =
500mV
I trip
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Using VPROPI
3.2
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VPROPI Output
The analog output VPROPI varies proportionally with the SENSE voltage according to Equation 4.
VPROPI = 5 × VSENSE
(4)
An RC network in series with the VPROPI output is recommended, if this voltage is to be sampled by an
analog to digital converter.
To M CU ADC
10K
VPROPI
ANA_VPROPI
•
1000 pf
Figure 2. RC Network in Series With the VPROPI Output
It is imperative to realize that VPROPI will decrease to 0 V while the H Bridge enters slow decay
recirculation.
4
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Figure 3. PHASE PWM
Channel 1 is PHASE pin.
Channel 2 is VPROPI analog output.
Channel 3 is SENSE pin.
Channel 4 is winding current.
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Using VPROPI
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Figure 4. Fast Decay: MODE1 = 0, MODE2 = 0
Channel 1 is PHASE pin.
Channel 2 is VPROPI analog output.
Channel 3 is SENSE pin.
Channel 4 is winding current.
6
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Figure 5. Slow Decay: MODE1 = 1, MODE2 = X
Channel 1 is PHASE pin.
Channel 2 is VPROPI analog output.
Channel 3 is SENSE pin.
Channel 4 is winding current.
VPROPI analog output will not offer winding current information while on slow decay mode (MODE1 = 1).
In slow decay m ode, the lower transistors (MODE2 = 0) or the upper transistors (MODE2 = 1) are
enabled. Winding current flow decays through this transistor pair and not through the SENSE resistor.
VSENSE = 0 V and VPROPI is 0 V as well.
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Micro Processor Interface
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Figure 6. VPROPI Limitations During Ground to Source Current Flow
Channel 1 is ENABLE pin.
Channel 2 is SENSE pin.
Channel 3 is VPROPI analog output.
Channel 4 is winding current.
VPROPI analog output is meaningful during current flow with positive polarity (from VBB to SENSE).
However, whenever the winding current flows from GND, through the SENSE resistor and into the source
(VBB), while it decays, current magnitudes larger than a threshold proportional to the SENSE resistor will
trigger parasitic current flow. As a result, winding current is a function of current flowing through the
SENSE resistor and current flowing through the parasitic path.
4
Micro Processor Interface
The DRV8800/DRV8801 can be easily controlled with most microcontrollers. At the most, seven I/O pins
are required. All of the digital control signals can be handled with conventional GPIO pins configured as
outputs. ENABLE and PHASE control signals can be pulse-width modulated by using timers, allowing for
proportional direction and speed control.
8
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Micro Processor Interface
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VDD
Microcontroller
DRV8800/01
GPIO
SLEEP
Timer GPIO
ENABLE
Timer GPIO
PHASE
MODE / MODE 1
GPIO
GPIO
MODE 2 (DRV8801)
VDD
GPIO
ADC
NFAULT
VPROPI (DRV8801)
GND
Figure 7. Micro Processor Interface
4.1
Pulse-Width Modulating ENABLE
The most common H-Bridge direction/speed control scheme is to use a conventional GPIO output for the
PHASE (selects direction) and pulse-width modulate ENABLE for speed control.
4.2
Pulse-Width Modulating PHASE
Another technique widely used is to use a speed/direction control scheme where ENABLE is connected to
a GPIO output and the PHASE is pulse-width modulated. In this case, both direction and speed are
controlled with a single signal. ENABLE is only used to disable the motor and stop all current flow.
When pulse-width modulating PHASE, a 50% duty cycle will stop the motor. Duty cycles above 50% will
have the motor moving on the clockwise direction with proportional control; 100% duty cycle represents
full speed.
Duty cycles below 50% will have the motor rotating with a counter clockwise direction; 0% duty cycle
represents full speed.
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