AN1451 datasheet(30/43 Pages) STMICROELECTRONICS

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AN1451 APPLICATION NOTE

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APPLICATION EXAMPLE

Application Data

Motor Data

Rotation Speed:

300 rpm (fCK = 1kHz)

Winding Resistance:

6.6

Ω

Winding peak Current: 1A

Winding Inductance:

7.9mH

Maximum Ripple:

50mA

Step Angle:

1.8°/step

Supply Voltage:

24V ±5%

Maximum BEMF at 300rpm: 15V

Sequence:

Wave Mode

3.1 Decay mode, sensing resistors and reference voltage.

The first step is choosing the decay type. Let's suppose to implement slow decay, which allows lower power

dissipation, lower ripple and avoids voltages below GND at output pins during recirculation. Referring to approx-

imated formulae in Figure 25, it's possible to calculate the Duty-Cycle (D), the Switching Frequency (fSW), the

Current Ripple (

∆I). With a 15 µs off-time, we will have:

≅ 63%, fSW ≅ 25kHz, ∆I ≅ 29mA. The on-time is tON = D / fSW ≅ 25µs, which is far from the minimum allowed

(1.5µs), so slow decay can be used.

The bulk capacitor need to withstand at least 24V + 5% + 25%

≅ 32V. A 50V capacitor will be used. Allowing a

voltage ripple of 200mV, the capacitor ESR should be lower than 200mV / 1A = 200m

Ω; the AC current capa-

bility should be about 1A.

Providing a reference voltage of 0.5V, 0.5

Ω sensing resistor are needed. In slow decay mode the resistors pow-

er rating is about PR ≅ Irms

2 · RSENSE · D ≅ 0.32W. Two 1Ω - 0.25W - 1% resistors in parallel are used. The

charge pump uses recommended components (1N4148 diodes, ceramic capacitors and a 100

Ω resistor to re-

duce EMI).

R = 18k

Ω, C = 1.2 nF are connected to the RC pins, obtaining tOFF ≅ 16µs. On the EN pin a 5.6nF has been

placed, and the pin is driven by the µC through a 100k

Ω resistor. With these values, in case of short circuit be-

tween two OUT pins or an OUT pin and GND, the PowerDMOS turns off after about 1µs, and tDISABLE ≅ 240µs.

Figure 33. Application Example.

With Wave Drive selected, referring to Figure 24, Figure 25, Figure 26, the dissipating power is about 1.36 W.

If the ambient temperature is lower than 50°C, with 4cm2 of copper area on the PCB and a SO24 package, the

estimated junction temperature is about 123°C. Using more copper area or a PowerDIP package will reduce the

junction temperature.

VS = 24 V

Vref = 0.5 V

CONTROL

HALF / FULL

RESET

CLOCK

CW / CCW

VrefA

VrefB

RCA

RCB

GND

SENSEA

SENSEB

VSA

VSB

VBOOT

OUT1B OUT2B

OUT1A OUT2A

1N4148

100 k

Ω

1.2 nF

Ceramic

18 k

Ω

100

Ω

0.25W

5.6 nF

Ceramic

47nF

Ceramic

18 k

Ω

1.2 nF

Ceramic

100nF 50V

Ceramic

220nF 25V

Ceramic

10nF 50V

Ceramic

100µF 50V

ESR<200m

Ω

L6208

µC

Custom Logic

2-Phase

Stepper Motor

4 X

Ω, 0.25 W,

2 k

Ω

0.25 W 1%

18 k

Ω

0.25 W 1%

Logic Supply

5 V

AN1451 Datasheet(PDF) 30 Page - STMicroelectronics

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