ST AN2650 Application note

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.

November 2007 Rev 1 1/24

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Contents AN2560

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

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AN2560 Contents

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

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

3/24

List of tables AN2560

List of tables

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

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

4/24

AN2560 List of figures

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

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Typical application schematic AN2560

1 Typical application schematic

Figure 1. Application schematic diagram

bat

121

QA 1 2

QA 2

QB 1 11

QB 2

13 24

PGNDGND

buffer

2n2

reg

100n

out

µC

out

6k8

100n

16 15

Vcc

STEP9

PWM8

DO7

DI5

CLK4

CSN6

RREF

TEST

GND

1817

L9942

PGND 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 C

, with a minimum value as

buffer

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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AN2560 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 C

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

The energy balance can be calculated from:

motor

buffer

⋅+ V

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

---

⋅⋅

bufferVs1

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

-- -

motor

buffer

motorImotor

⋅+=

2 s2

2 motor

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.

must be chosen carefully.

buffer

-- -

⋅⋅≈⋅⋅+

bufferVs2

buffer

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%.

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Typical application schematic AN2560

1.2 Low drop reverse polarity protection

Figure 2. Low drop reverse polarity protection

bat

100k

STD17NF03LT4

100k

100n

310 2216 15

VsCP Vs

buffer

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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