STMicroelectronics AN2650 L9942 stepper motor driver for bipolar stepper motor Datasheet

AN2650
Application note
L9942 stepper motor driver for bipolar stepper motors
Introduction
The L9942 is an integrated stepper motor driver for bipolar stepper motors. The device is
designed for automotive applications, such as headlamp leveling, steerable lights and
adaptive front lighting. Other applications, such as ventilation and air conditioning flap and
throttle positioning are also possible uses for the L9942.
The device drives bipolar stepper motors with high-efficiency and smooth operation. Microstepping is the preferred mode to provide low-noise operation since this technique
eliminates the effects of mechanical resonances, which can lower the motor torque.
A motor stall detection capability allows position alignment without an external sensor, while
its step counter is addressable via an SPI as well as by a separate input, to prevent the SPI
overloading when running multiple motors simultaneously.
December 2013
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Contents
AN2650
Contents
1
2
Typical application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1
Calculation of the buffer capacitor Cbuffer . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2
Low drop reverse polarity protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Shorted coil detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1
3
4
5
6
7
8
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Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
SPI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1
Fault bit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2
SPI communication monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Decay modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1
Slow decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2
Fast Decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.3
Advanced decay modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.3.1
Mixed decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.3.2
Auto decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Stall detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.1
Internal functionality (simplified) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.2
How to determine the stall threshold at bench test . . . . . . . . . . . . . . . . . 19
Duty cycle for current regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.1
Minimum duty cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.2
Maximum duty cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Power dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.1
Static ON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.2
Static freewheeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.3
Dynamic slew rate power dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.4
Power dissipation for one PWM phase . . . . . . . . . . . . . . . . . . . . . . . . . . 22
PCB footprint proposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
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Contents
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
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List of tables
AN2650
List of tables
Table 1.
Table 2.
4/26
Phase counter values for fast decay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
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List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Application schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Low drop reverse polarity protection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Stepping modes (Auto decay mode, fast decay without delay time) . . . . . . . . . . . . . . . . . 10
Auto decay, fast decay without delay time at phase 0 and 8 . . . . . . . . . . . . . . . . . . . . . . . 11
SPI transfer timing diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Slow decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Fast decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Mixed decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Auto decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Stall detection function overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Cross current protection time and slew rate for maximum DC . . . . . . . . . . . . . . . . . . . . . . 20
Current flow and voltage drop during fast decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Power SSO24 solder mask layout (all values in mm) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Power SSO24 solder mask opening (all values in mm) . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
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Typical application schematic
1
AN2650
Typical application schematic
Figure 1. Application schematic diagram
Vbat
Vs
Vreg
5V
100n
Cbuffer
100n
µC
19 16
Vcc CP
out
9
STEP
out
21
EN
in
8
PWM
in
7
DO
out
5
DI
out
4
CLK
out
6
CSN
20
RREF
6k8
3
15 22
Vs
10
Vs
QA 1
2
QA 2
23
2n2
2n2
L9942
GND TEST PGND
17 18 1 12
GND
QB 1
11
QB 2
14
2n2
PGND
13 24
SM
2n2
PGND
The L9942 is driven by a microcontroller via the SPI (DO, DI, CLK, and CSN), STEP and EN
pins. Additional information is provided from the PWM pin.
The stepper motor driver is supplied from a 5 V voltage regulator and the reverse polarity
protected Vs. It is necessary to use a stabilization capacitor (with a minimum value of
minimum 100 nF) as close as possible at the Vcc pin. For the stabilization of the Vs supply
pin and to absorb motor energy, an electrolytic capacitor Cbuffer, with a minimum value as
calculated in Section 1.1, must be used.
Because the motor currents are supplied via the Vs-pins, all Vs-pins must have a low ohmic
connection to the supply voltage Vs. For the same reason, all GND and PGND pins must
have a low ohmic connection to the system ground. A star ground concept with separate
lines for GND and PGND is recommended.
At the charge pump pin, a capacitor with 100 nF to Vs is recommended.
To improve the EMI behavior, it is recommended to have 2.2 nF capacitors as close as
possible to the motor output pins, Qxy. Short motor connection wires also improve the EMI
behavior.
The internally-used reference voltage depends upon the value of the reference resistor,
positioned between the pin RREF and GND. Consequently, the precision of the L9942
depends upon the value of the reference resistor. One possible value for this resistor is
6.8 kΩ.
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Typical application schematic
Due to the structure of the BCD process, the slug of the device is connected internally to
PGND and must also be connected externally to PGND.
1.1
Calculation of the buffer capacitor Cbuffer
The stepper motor driver L9942 is usually designed in an environment similar to that shown
in Figure 1.
During motor operation, electrical energy is stored in the motor coils. If the motor shuts
down, this energy is fed back to the supply voltage Vs. Thus, there is a voltage increase at
Vs, which may cause an electrical overstress of the L9942. To avoid damage to the L9942,
the value of the buffer capacitor Cbuffer must be chosen carefully.
The energy balance can be calculated from:
2
2
2
1
1
1
--- ⋅ C buffer ⋅ V s1 + --- ⋅ L motor ⋅ I motor ≈ --- ⋅ C buffer ⋅ V s2
2
2
2
L motor 2
2
2
V s1 + ------------------- ⋅ I motor = V s2
C buffer
V s2 =
L motor 2
2
V s1 + ------------------- ⋅ I motor
C buffer
From this equation, it is possible to conclude:
Note:
•
The voltage Vs2 must not exceed the maximum rating of the L9942
•
If an over voltage shut down must be avoided, the voltage Vs2 must not exceed the
minimum over voltage threshold.
As a general recommendation, STMicroelectronics recommend a minimum buffer capacitor
of 47 µF. The ripple at Vs during normal motor operation should be between 5% and 10%.
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Typical application schematic
1.2
AN2650
Low drop reverse polarity protection
Figure 2. Low drop reverse polarity protection
Vbat
100k
STD17NF03LT4
100k
100n
16
CP
3
10
Vs
Vs
15 22
Vs
Cbuffer
L9942
Vreg
L9942
As shown in Figure 2 the charge pump pin can be used for a low drop reverse polarity
protection. The charge pump pin can also be used for other devices in the same application.
Because of the additional gate capacity, the charge pump ramps up more slowly than
without the additional MOSFET gate.
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2
Shorted coil detection
Shorted coil detection
During free-wheeling time, the L9942 can use several decay modes that are programmable
by the SPI.
However, only on Auto decay, fast decay without delay time it is possible to detect
shorted coil because, during free-wheeling time, both opposite transistors (HS and LS) are
switched on and high current can rise. During free wheeling the current in the output stage is
monitored, but not regulated, with PWM. Because of this, the short overcomes the over
current filter time and can be detected.
In other decay modes, the decay can periodically change during wheeling of the motor (for
example, every 200 ms) to Auto decay, fast decay without delay time for few microseconds
and than change it back to previous value. This can be done in one clock step.
The decay mode cannot be changed at all values of the phase counter because fast decay
is not active during the whole period but only during decreasing current phase in the coil
(during the rising current phase, the coil is active slow decay mode).
A detailed explanation of Auto decay, fast decay without delay time for all step modes is
given in Figure 3.
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Shorted coil detection
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Figure 3. Stepping modes (Auto decay mode, fast decay without delay time)
Full-Step Mode, DIR=0
Phase Counter
0
8
24
Phase Counter
24
16
0
8
Current
Driver A
8
0
Slow Decay Fast Decay
Current
Driver A
0
8
Slow Decay Fast Decay
Slow Decay Fast Decay
Current
Driver B
8
0
8
Slow Decay
Fast Decay
0
4
Current
Driver A
0
4
Half-Step Mode, DIR=0
8
12
8
4
16
20
24
Slow Decay Fast Decay
0
4
8
Current
Driver A
4
Slow Decay Fast Decay
Fast Decay
8
Slow Decay
4
0
4
8
4
0
Current
Driver B
4
Slow Decay Fast Decay
2
4
6
Mini-Step Mode, DIR=0
8
Slow Decay Fast Decay
0
8
0
8
Slow Decay
Fast Decay
28
Half-Step
Mode,
DIR=1
20
16
12
8
24
4
0
4
0
Slow Decay Fast Decay
4
8
4
0
4
8
Slow Decay Fast Decay
Fast Decay
4
Slow Decay
0
4
8
4
0
4
8
Slow Decay Fast Decay
Mini-Step Mode, DIR=1
Phase Counter 30 28 26 24 22 20 18 16 14 12 10
10 12 14 16 18 20 22 24 26 28 30
Slow Decay Fast Decay
0
2
4
6
8
6
4
2
0
2
4
6
8
6
4
Current
Driver A
2
Slow Decay Fast Decay
Fast Decay
8
6
4
2
Slow Decay
0
2
4
6
8
6
4
2
0
2
4
Current
Driver B
6
Slow Decay Fast Decay
Step CLK
8
6
4
2
0
8
6
4
2
0
Slow Decay Fast Decay
2
4
6
8
6
4
2
0
2
4
6
Slow Decay Fast Decay
Fast Decay
6
4
2
0
Slow Decay
2
4
6
8
6
4
2
0
2
4
8
Micro-Step Mode, DIR=1
Micro-Step Mode, DIR=0
HSA2
Slow Decay Fast Decay
0 1 2 3 4 5 6 7 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 7 6 5 4 3 2 1
Slow Decay Fast Decay
Fast Decay
HSA1
HSB2
Slow Decay
8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7
HSB1
Slow Decay Fast Decay
Step CLK
Phase Counter 313029282726252423222120191817161514131211109 8 7 6 5 4 3 2 1 0
Current
Driver A
Current
Driver B
Slow Decay Fast Decay
HSA2
1 2 3 4 5 6 7 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 7 6 5 4 3 2 1 0
HSA1
Slow Decay Fast Decay
Fast Decay
Slow Decay
HSB2
7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8
HSB1
Slow Decay Fast Decay
Step CLK
Table 1 shows Phase Counter values where fast decay is active.
Table 1. Phase counter values for fast decay
Step Mode
Bridge
DIR=0
DIR=1
A
(8) (24)
(16(1)) (0(1))
B
(0) (16)
(24(1)) (8(1))
A
(8, 12) (24, 28
(20, 16(1)) (4, 0(1))
B
(0, 4) (16, 20)
(28, 24(1)) (12, 8(1))
Full step
Half step
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Slow Decay Fast Decay
Step CLK
Phase Counter 0 1 2 3 4 5 6 7 8 9 10111213141516171819202122232425262728293031
Current
Driver B
0
Step CLK
Phase Counter 0
Current
Driver A
8
Slow Decay Fast Decay
Phase Counter
28
Step CLK
Current
Driver B
0
Step CLK
Phase Counter
Current
Driver A
8
Slow Decay Fast Decay
Current
Driver B
0
Step CLK
Current
Driver B
Full-Step Mode, DIR=1
16
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AN2650
Shorted coil detection
Table 1. Phase counter values for fast decay (continued)
Step Mode
Bridge
DIR=0
DIR=1
A
(8, 10,12,14) (24,26,28,30)
(22,20,18,16(1)) (6, 4, 2, 0(1))
B
(0, 2, 4, 6) (16,18,20,22)
(30,28,26,24(1)) (14,12,10, 8(1))
A
(8,9,10,11,12,13,14,15)
(24,25,26,27,28,29,30,31)
(23,22,21,20,19,18,17,16(1))
(7, 6, 5, 4, 3, 2, 1, 0(1))
B
(0, 1, 2, 3, 4, 5, 6, 7)
(16,17,18,19,20,21,22,23)
(31,30,29,28,27,26,25,24(1))
(15,14,13,12,11,10, 9, 8(1))
Mini step
Micro step
1. Current profile_0 must be greater than 0
2.1
Conclusion
To detect shorted coil in both bridges (A and B) and for all step modes (Full, Half, Mini and
Micro), it is necessary to change decay mode to Auto decay, fast decay without delay time
at phase counter values of 0 and 8 (or 16 and 24). For the opposite direction, DIR=1 can
also use 0 and 8 (or 16 and 24) but the amplitude of current profile_0 must be greater than
0. This configuration is illustrated in Figure 4.
Figure 4. Auto decay, fast decay without delay time at phase 0 and 8
200ms
phase 0 1 2 3 4 5 6 7 89 10111213141516171819202122232425262728293031 0 1 2 3 4
1516171819202122232425262728293031
01 2 3 4 5 6 7 89 1011121314151617
0 1 2 3 4 5 6 7 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 7 6 5 4 3 2 1 0 1 2 3 4
1 0 1 2 3 4 5 6 7 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 7 6 5 4 3 2 1 0
8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 7 6 5 4
7 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8
Auto decay, fast decay without delay time at phase 0 and 8 (DIR=0)
Note:
Another possibility is to change the decay mode for Micro, Mini and Half Step mode at
phase counter values of 4 and 12 and for Full Step mode at phase counter values of 0 and
8.
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SPI
AN2650
3
SPI
3.1
Fault bit
Figure 5. SPI transfer timing diagram
t
CSN_HI,min
CSN high to low: DO enabled
CSN
time
CLK
0
1
2
3
4
5
6
7
8
9
10 11 12
13 14 15
0
1
time
DI: data will be accepted on the rising edge of CLK signal
actual data
DI
A2
new data
A1 A0 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
A2
A1
time
DO: data will change on the falling edge of CLK signal
status information
DO
D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
time
fault bit
Control and Status Register
CSN low to high: actual data is
transfered to registers
fault bit
old data
actual data
time
The first three bits of an SPI write frame are the register address. Thus, during this time, it is
not clear which register has to be written and read. This time is used at the DO-pin to
monitor the or-function of all diagnostic functions.
The register read out is started with the third falling edge of the SPI CLK. The read out is
finished 13 falling edges later and during the remaining time, until CSN is set to high, the orfunction of all diagnostic functions is monitored again.
Using this method, the failure status of the device can be checked without an SPI
communication. CSN is only pulled to low for a short while.
3.2
SPI communication monitoring
SPI communication monitoring is described in the specification.
However, for register 0 the following two points should be considered.
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1.
During SPI communication, monitoring the STEP-pin must not be used. This could
cause the Phase Counter to be modified.
2.
When not using the microstepping step mode, not all Current Profile Registers are
used. When sending a command to the Control Register 0 for SPI communication
monitoring with a Phase Counter value that not used in the selected step mode, the
device will correct the Phase Counter value itself. In this case, SPI communication
monitoring will fail.
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4
Decay modes
Decay modes
During the ON phase the current in the motor coil increases. After an ON phase, an OFF
phase follows that always starts with the cross current protection time (tcc).
The cross current protection time is automatically chosen with the slew rate and is typically
in the range from 0.5 to 4 µs.
After the cross current protection time, a programmed decay mode follows.
The basic decay modes of the stepper motor driver L9942 are:
4.1
•
Slow decay
•
Fast decay
•
Advanced decay modes, which are combinations of the slow and fast decay modes.
Slow decay
The slow decay mode realizes a minimum loss of energy in the motor coil. This means the
current decrease in the motor coil is “slow”.
Slow decay is illustrated in Figure 6.
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Decay modes
AN2650
Figure 6. Slow decay
IL
tCC
cross current
protection
ON
Vs
OFF
OFF
ON
OFF
IL
Vs
UBE
Vs
slow
decay
OFF
ON
OFF
OFF
IL
URon
„ON“
IL increase
t
ON
IL
OFF
gnd
4.2
gnd
OFF
gnd
Fast Decay
The fast decay mode realizes a maximum loss of energy in the motor coil. This means the
current decrease in the motor coil is “fast”.
Fast decay is illustrated in Figure 7.
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Decay modes
Figure 7. Fast decay
IL
tCC
cross current
protection
Vs
OFF
OFF
ON
OFF
IL
Vs
UBE
Vs
fast
decay
OFF
OFF
OFF
ON
IL
URon
„ON“
IL increase
ON
t
ON
IL
OFF
gnd
4.3
gnd
OFF
gnd
Advanced decay modes
With the stepper motor driver L9942, it is possible to combine the basic decay modes, slow
and fast decay.
4.3.1
Mixed decay
From the current point of view, for stepping down it is necessary to reduce the current in the
motor coil quickly. Therefore, a mostly slow decay is not useful because there is the danger
that the current in the motor coil does not reach the new (lower) current target. With fast
decay, the current undershoot may be stronger than necessary; this generates more EMI
than necessary.
A better result is obtained by mixing fast decay with slow decay. Mixed decay starts with fast
decay and switches to slow decay; the point for switching between the decay modes is
programmable. Mixed decay is shown in Figure 8.
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Decay modes
AN2650
Figure 8. Mixed decay
IL
ITarget
I
II
tCC
4.3.2
tMD
t
•
Mixed decay, fast decay until current undershoot
In Figure 8, this behavior is shown with graph “I”. Fast decay is driven until the motor
coil current has undershot the target current and switches to slow decay until the end of
the phase.
•
Mixed decay, fast decay until tMD > 4(8) µs
In Figure 8, this behavior is shown with graph “II”. The time tMD is started after the cross
current protection time tcc is over. After tMD is finished the stepper motor driver
switches to slow decay. tMD is programmable to 4µs or 8µs.
Auto decay
If the current in the motor coil is required to increase from step to step, it is sensible to save
the current in the motor coil. Therefore, the best decay mode is slow decay (see
Section 4.1).
If, on the other hand, the current in the motor coil should decrease from step to step, it is
sensible to reduce the current in the motor coil in a controlled way, as described in
Section 4.3.1. Therefore, the best decay mode is mixed decay.
The combination of slow decay for increasing current from step to step and mixed decay for
decreasing current from step to step is auto decay. A combination of slow decay and pure
fast decay is also possible but this option usually increases the EMI emission.
In Figure 9, auto decay is shown for one of the motor coils with micro stepping. For each
step, the appropriate current profile register is also shown.
Internally, a pointer moves from a current profile register to the next current profile register
with each StepCLK pulse. This start from the register 0 and goes step by step to register 8
and then back to register 0...
When the pointer is going in the direction from register 0 to 8, the L9942 uses slow decay.
When the pointer is going from register 8 to 0, mixed or fast decay is selected.
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Decay modes
Figure 9. Auto decay
IL
auto decay
slow decay
current profile register
fast or mixed
decay
0 1 2 3 4 5 6 7 8 7 6 5 4 3 2 1
t
0 1 2 3 4 5 6 7 8 7 6 5 4 3 2 1
slow decay
fast or mixed
decay
VStepCLK
auto decay
t
•
Auto decay, fast decay without delay time
Slow decay for current profile register going up (0  8) plus pure fast decay (see also
fast decay in Figure 7) for current profile register decreasing (8  0).
•
Auto decay, fast decay until tMD > 4(8) µs:
Slow decay for current profile register going up (0 8) plus mixed decay (see also
mixed decay in Figure 8, graph “II”) for current profile register decreasing (8 0).
•
Auto decay, fast decay until current undershoot
Slow decay for current profile register going up (0  8) plus mixed decay (see also
mixed decay in Figure 8, graph “I”) for current profile register increasing (0 8).
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Stall detection
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Stall detection
The stall detection function of the L9942 uses the reference drive of a stepper motor
system, such as that usually used in the start up phase of a front light levelling system.
5.1
Internal functionality (simplified)
The back EMF of the permanent magnetic rotor of the stepper motor is the effect that is
used for the stall detection function.
If the motor is turning “fast”, the back EMF is “high”. Thus, the voltage drop at the motor coils
is “low”. Combined with the inductance of the motor coils, it takes a “long” time to reach the
target current. Consequently, this means a “long” duty cycle of the pulse width modulation
(PWM).
If the motor is stopped mechanically, the back EMF is zero. Thus, the voltage drop at the
motor coils is “high”. Combined with the inductance of the motor coils, it takes a “short” time
to reach the target current. This means a “short” duty cycle of the PWM.
As is shown in Figure 10, an internal counter counts the duty cycle of the current regulation
PWM. This value is compared with a value given from the microcontroller via the SPI. If this
value is less than the one supplied by the microcontroller, the stall detection bit is set.
Figure 10. Stall detection function overview
IL
duty cycle
for cur. reg.
PWM
PWM current
regulation
t
counting
t
001000
comp.
From SPI
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5.2
Stall detection
How to determine the stall threshold at bench test
There are four steps to determine the stall threshold.
1.
Drive motor in an environment with parameters as in a possible stall situation.
2.
Run the motor so that it is turning continually and increase the stall threshold step by
step until the stall bit is set. This stall threshold value is called the “high value”.
3.
With the same electrical conditions as before, stop the motor mechanically. Decrease
the stall threshold from the “high value”. After some steps the stall bit is reset. This stall
threshold value is called the “low value”.
4.
The stall threshold is the middle value between the “high value” and the “low value”.
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Duty cycle for current regulation
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Duty cycle for current regulation
The duty cycle for current regulation is always switched on for a minimum time and has a
maximum time of less than 100%.
6.1
Minimum duty cycle
The minimum duty cycle is the sum of the “Glitch filter delay time”, the “Slew rate” and the
“Cross current protection time”. The “Slew rate” and the “Cross current protection time” are
programmed with the same bits.
6.2
Maximum duty cycle
The maximum duty cycle is less then 100%. The negligible off time is related to the “Cross
current protection time”. “Cross current protection time” is illustrated in Figure 11.
Figure 11. Cross current protection time and slew rate for maximum DC
OUT
ON
Slew Rate B
Slew Rate A
OFF
t
TCC
t
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7
Power dissipation
Power dissipation
The calculation of the power dissipation depends upon the selected slew rate and decay
mode of the device. In Figure 12 the set up was selected with a fast decay.
Figure 12. Current flow and voltage drop during fast decay
Vs
ON
OFF
OUT-B
OFF
OUT-A
OUT-B
ON
ON
IL
URon
Vs
ON
OUT-A
IL
OFF
gnd
OFF
gnd
ON
FW fast decay
VOUT-A
Vs
VOUT-B
t
tr/f
PWM
tPulse
t
tD
For a rough calculation of the power dissipation, three different phases are selected:
7.1
1.
Static ON
2.
Static free wheeling (FW), fast decay
3.
Dynamic on and off with slew rate.
Static ON
Only the RDS,on of the high and low side switches are used for generating power
dissipation.
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Power dissipation
7.2
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Static freewheeling
Only the RDS,on of the high and low side switches are used for free wheeling and
generating power dissipation.
7.3
Dynamic slew rate power dissipation
During the H-bridge switching, the voltage and the current chang in a triangular form with
the defined slew rate.
7.4
Power dissipation for one PWM phase
The temporary power dissipations, explained above, are summed for the complete PWM
phase.
 t Pulse – t r ⁄ f
 t D – t Pulse – t r ⁄ f
 2 ⋅ t r ⁄ f
P = P ON ⋅  ------------------------------- + P FW ⋅  ------------------------------------------- + 2 ⋅ P SR ⋅  -----------------
t
t




 tD 
D
D
 t D – 2 ⋅ t r ⁄ f
 t r ⁄ f
P = P ON ⋅  ---------------------------- + 2 ⋅ P SR ⋅  2 ⋅ ---------
tD
tD 



 t r ⁄ f
2  t D – 2 ⋅ t r ⁄ f
P = 2 ⋅ R DSON ⋅ I ⋅  ---------------------------- + 2 ⋅ U ⋅ I ⋅  ---------
tD


 tD 
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PCB footprint proposal
PCB footprint proposal
Figure 13. Power SSO24 solder mask layout (all values in mm)
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Figure 14. Power SSO24 solder mask opening (all values in mm)
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Revision history
Revision history
Table 2. Document revision history
Date
Revision
Changes
2-Nov-2007
1
Initial release.
22-Sep-2013
2
Updated Disclaimer.
05-Dec-2013
3
Updated equation into Section 7.4: Power dissipation for one PWM
phase
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