ST AN2650 Application note

November 2007 Rev 1 1/24

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. Micro-

stepping 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.

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Contents AN2560

2/24

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1 Typical application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.1 Calculation of the buffer capacitor Cbuffer . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.2 Low drop reverse polarity protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2 Shorted coil detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3 SPI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.1 Fault bit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.2 SPI communication monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4 Decay modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.1 Slow decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.2 Fast Decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.3 Advanced decay modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.3.1 Mixed decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.3.2 Auto decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5 Stall detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5.1 Internal functionality (simplified) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5.2 How to determine the stall threshold at bench test . . . . . . . . . . . . . . . . . 17

6 Duty cycle for current regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

6.1 Minimum duty cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

6.2 Maximum duty cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

7 Power dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

7.1 Static ON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

7.2 Static freewheeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

7.3 Dynamic slew rate power dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

7.4 Power dissipation for one PWM phase . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

AN2560 Contents

3/24

8 PCB footprint proposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

9 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

List of tables AN2560

4/24

List of tables

Table 1. Phase counter values for fast decay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Table 2. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

AN2560 List of figures

5/24

List of figures

Figure 1. Application schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 2. Low drop reverse polarity protection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 3. Stepping modes (Auto decay mode, fast decay without delay time) . . . . . . . . . . . . . . . . . . 9

Figure 4. Auto decay, fast decay without delay time at phase 0 and 8 . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 5. SPI transfer timing diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 6. Slow decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 7. Fast decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 8. Mixed decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 9. Auto decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 10. Stall detection function overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 11. Cross current protection time and slew rate for maximum DC . . . . . . . . . . . . . . . . . . . . . . 18

Figure 12. Current flow and voltage drop during fast decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 13. Power SSO24 solder mask layout (all values in mm) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Figure 14. Power SSO24 solder mask opening (all values in mm) . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Typical application schematic AN2560

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1 Typical application schematic

Figure 1. Application schematic diagram

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 C

buffer

, 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.

100n

PGNDGND

C

buffer

SM

2n2

2n2

2n2

2n2

6k8

V

bat

100n

121

QA 1 2

3

CLK4

DI5

CSN6

DO7

PWM8

STEP9

10

QB 1 11

13 24

23

22

21

20

19

1817

16 15

14

QA 2

Vs

EN

RREF

Vcc

TEST

GND

CP

QB 2

PGND PGND

Vs

L9942

µC

out

out

out

out

out

in

in

5V

V

reg

Vs

AN2560 Typical application schematic

7/24

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 C

buffer

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 C

buffer

must be chosen carefully.

The energy balance can be calculated from:

From this equation, it is possible to conclude:

● 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.

Note: 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%.

1

2

---

C

buffer

V

s1

2

⋅⋅

1

2

-- -

L

motor

I

motor

2

1

2

-- -

C

buffer

V

s2

2

⋅⋅≈⋅⋅+

V

s1

2

L

motor

C

buffer

------------------ -

I

motor

2

⋅+ V

s2

2

=

V

s2

V

s1

2

L

motor

C

buffer

------------------ -

I

motor

2

⋅+=

Typical application schematic AN2560

8/24

1.2 Low drop reverse polarity protection

Figure 2. Low drop reverse polarity protection

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.

V

bat

100n

C

buffer

310 2216 15

VsCP Vs

Vs

L9942

STD17NF03LT4

100k

100k

L9942 Vreg