Application Note, V 1.0, August 2001
ANPS063E
TLE 472x – Family Stepper
Motor Drivers
Current Control Method and
Accuracy
by Frank Heinrichs
Automotive Power
Never stop
-1-
thinking.
ANPS063E
TLE 472x stepper motor drivers
Current control method and accuracy
1. Abstract
3. Current Control Method
This Application Note is intended to provide
detailed technical information about the current
control method that is used in Infineons stepper
motor driver family TLE 472x. Special attention
is paid on accuracy considerations.
In general, three different methods are used to
control the current in the coil of a stepper motor:
2. Introduction
The TLE 472x stepper motor drivers are
intended to be used for bipolar, two-phase,
current-controlled stepper motors (see figure 1).
3.1.
Peak current control with fixed offtime
With this control method, the H-bridge is
switched into freewheel-condition as soon as
the output current reaches the desired value
Itarget. The bridge remains in off-state for a
fixed time toff and is switched on again
automatically after toff (see figure 2). As a
result, the peak value of the output current
equals the desired value.
Iout
Itarget
t
ton
toff
Figure 2: peak current control with constant
off-time
3.2.
Figure1: TLE 4729 block-diagram
Application Note
Peak current
frequency
control
with
fixed
With this control method, the H-bridge is
switched into freewheel-condition as soon as
the output current reaches the desired value
Itarget. In contrast to the previous method, the
off-time is not fixed, but the bridge is switched
ON after every time interval 1/f, f being the
chopper frequency as shown in figure 3. This
has the advantage that the chopper frequency is
fixed, i.e. independent of the load characteristics
and the supply voltage (in other words:
independent of the off-time toff). This control
method is used in the TLE 472x driver
family.
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ANPS063E
TLE 472x stepper motor drivers
Current control method and accuracy
Vs
Itarget
Iout
Source
Source
Iout
Load
Sink
+
t
ton
Rsense
1/f
Figure 3: peak current control with fixed
frequency
3.3.
Mean-value current control (currentmode control).
The control methods described so far have the
disadvantage that the effective, mean output
current is always smaller than the desired
current. By using suitable filters and
comparators, it is also possible to control the
current in such a manner that the mean value of
the output current is equal to the desired value
Itarget, as shown in figure 4.
Iout
Itarget
t
ton
1/f
Figure 4: current-mode control
However, for this method it is necessary to
adapt the filtering elements to the load
characteristics, so a fully integrated IC solution
is not possible. For that reason, current mode
control is not implemented in the TLE 472x
familly.
Vref
Osc
Figure 5: simplified schematic of current
control implementation
The oscillator sets the NOR-latch output to high
with the oscillator frequency fosc. By this, the
sink-transistor is turned ON every period 1/f.
According to the input signals IXX, a reference
voltage Vref is generated:
IX0 IX1
Symb.
min
typ
max
unit
H
L
Vch
40
70
100
mV
L
H
Vcs
410
450
510
mV
H
H
Vca
630
700
800
mV
This reference Voltage is compared to the drop
across the sense resistor Rsense. As soon as
Iout * Rsense equals Vref, the NOR-latch is set
to low. This turns OFF the sink transistor,
setting the H-bridge to freewheel (freewheel in
source transistors).
This means that the desired output current is:
Itarget = Vref / Rsense
5. Accuracy Considerations
4. Implementation
As just described, fixed frequency peak-current
control is used in the TLE 472x stepper motor
driver family. As simplified schematic of the
implementation is shown in figure 5.
The accuracy of the actual output current value
depends on the accuracy of :
•
the internal comparator reference voltage
Vref
•
the external sense resistor
•
the current ripple
5.1.
Accuracy of Vref and Rsense
The accuracy of Vref is given in the datasheet
and in above table. For Example, the Setpoint
Application Note
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V1.0, 2001-05
ANPS063E
TLE 472x stepper motor drivers
Current control method and accuracy
current has a nominal value of 450mV
variation of –9% and +13%. As the
current depends linearly on Vref, the
accuracy has to be expected for the
current.
and a
output
same
output
The same is true for the sense resistor. The
accuracy of Rsense contributes linearly to the
accuracy of Iout.
Current ripple
5.2.
As explained earlier, the control method leads to
an average current smaller that the desired
current Itarget because the peak current is
limited to Itarget. If DI is the current ripple, the
average current is
Iout,mean = Itarget – DI/2
DI
Iout
Itarget
Iout,mean
Vs = VsatuC + UL + R*Iout + Vsatl +
Rsense*Iout.
The parameter we are interseted in is the
current slew rate:
(di/dt)c = UL/L =
(Vs – VsatuC – Vsatl – Iout(R + Rsense))/L
Lets look at an example to carry out the
calculation:
Vs
Rsense
Iout
R
L = 4mH
=
=
=
=
12V
1.3Ohm
315mA
8Ohm
From the datasheet, we have (using the values
at Iout = 450mA):
VsatuC
Vsatl = 0.3V
=
1V
This leads to :
(di/dt)c = (12 – 1 - 0.3 – 2.92)V / 4mH =
1.95 A/ms
t
ton
1/f
Figure 6: Current ripple, peak- and mean
current
To determine, the current ripple, we have to
calculate the voltage drop across the load
inductivity.
5.2.2.
(di/dt)discharge
The same has now to be done for the falling
current slope (discharging of the inductivity,
frewheeling):
Source Transistor
5.2.1.
Source diode
(di/dt)charge
For the rising current slope (charging the
inductivity), we have:
L
R
Vs
We have
Source
- UL = R*Iout + VFu + VsatuD,
(di/dt)d = (R*Iout + VFu + VsatuD) / L
For our example, this leads to:
L
R
(di/dt)d =
UL/L = - (2.52 + 1 + 0.3)V/4mH = -0.96A/ms
Sink
5.2.3.
Rsense
Application Note
ton, toff and Ripple
Now, we can approximate toff and ton by
making the approximation of constant di/dt:
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TLE 472x stepper motor drivers
Current control method and accuracy
(di/dt)c
(di/dt)d
6. Conclusion
The general method to control the output
current that is used in the TLE 472x stepper
motor driver family was explained. An
approximative formula to determine the current
ripple and thus the difference between mean
output current and desired current is derived
and carried out for a representative example
Iout
DI
t
ton
toff
ton + toff = 1/fosc,
(di/dt)c * ton = (di/dt)d * toff = DI
This leads to:
ton = (1/fosc) / [{(di/dt)c / (di/dt)d} + 1],
toff = 1/fosc – ton,
DI = (di/dt)c * ton
For our example with fosc = 25kHz, this leads
to:
ton = 40µs / [{1.95 / 0.96} + 1] = 13.2µs
toff = 40µs – ton = 26.8µs
We see that the ON-time is much shorter than
the OFF-time. The reason is that the (higher)
supply-voltage is used to charge the coil, while
during freewheeling only the (smaller) drops
accross load resistance and power transistors
contribute to the di/dt.
This finally gives us the current ripple:
DI = (di/dt)c * ton = 25.7mA
Application Note
page 4 of 4
V1.0, 2001-05
ANPS063E
TLE 472x stepper motor drivers
Current control method and accuracy
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7. Disclaimer
Edition 2000-07-14
Published by Infineon Technologies AG,
St.-Martin-Strasse 53,
D-81541 München, Germany
© Infineon Technologies
All Rights Reserved.
AG
8/d/yy.
Published by Infineon Technologies AG
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Application Note
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