L6280 datasheet(18/26 Pages) STMICROELECTRONICS

18 / 26 page

APPLICATION INFORMATION

An application circuit useful to test the perform-

ance of the L6280 can be formed as shown on

Figure 24: CH1 drives one unipolar stepper mo-

tor, CH2 drives a DC motor, CH3 drives one sole-

noid and the SMPS can supply continuously 0.5A.

If the Watch Dog and the Chip Select functions

are not of interest, pins 22 and 23 must be

grounded. Each sensing resistor would be ob-

tained by the parallel of two or more metal film re-

sistor of the same value to minimize their series

equivalent inductance.

Generally, optimum stability of the SMPS voltage

control loop, is achieved by a series network

made by 1nF and 39 K

Ω (see pin 15) and by us-

ing an output capacitor of 10

0µF having an

equivalent series resistance of 100 m

Ω (see pin

14): the most of the unexpensivealuminium elec-

trolithic capacitors can be right.

The snubber network at the secondary winding of

the step-down inductor can be saved by accept-

ing a not regulated voltage at the Charge Pump

input pin 13. This condition is not recommended

when the supply voltage and/or the SMPS output

current changes too much (for instance respec-

tively 20V + 30% and/or 100 to 800 mA).

The inductance value of the primary winding of T1

defines the peak-to peak current ripple that flows

throught itself, that is the minimum output current

that allows the correct behaviour in continuous

mode of the SMPS; nevertheless, the device is

not demaged if it is obliged to work in discontinu-

ous mode at a low current level.

Figure 25 shows the characteristics of the trans-

former T1 suitable to be used on the Application

of Figure 24:

The maximum output current is of 500 mA con-

tinuous but current peaks of 800 mA can be

sinked out without the risk of the core saturation.

To avoid the discontinuous mode, the minimum

SMPS output current must be of 70mA. The recti-

fied voltage trend for the high side gate drive at

pin 13 is as shown on Figure 26.

Not equally cheap, the choice of a toroidal core

for T1 can optimize the application.

Instead of this, another solution can be as in Fig-

ure 27a it is shown. This is a full wave rectifier of

the voltage at pin 3; Z1 and R1 clamp the positive

peak while the forward characteristic of the Zener

rectifies the negative peak and charges C1. The

recommended Zener voltage is of 12V.

Could happen that the VSS output voltage is not

requested because already available: in this case

and only if at least one unipolar stepper motor is

continuously driven, the solution shown in Figure

27b can be implemented. The step down output

components can be left out.

The connection of the network is as follows:

A: to pin 4 (or pin 5) when the unipolar motor is

driven via CH1;

to pin 41 (or pin 42) when the unipolar motor

is driven via CH2;

B: to pin 1

C: to pin 13

The SMPS switching frequency is the same of the

oscillator frequency that can be typically defined

by:

fosc

Referring to Fig. 24 it is calculated fosc = 82KHz.

CH3 is chopped at the same frequency. The out-

put diodes must be chosen according to the sole-

noid working current (50ns of reverse recovery

time or better): for a current less than 1 A, the

PLQ08 is a good choice.

Driving one unipolar stepper motor, output protec-

tion diodes (Transil) are recommended: CH1 in

Fig 24 uses four BZW04 - 48 diodes; when a low

current motor is driven or a Vs less than 20V is

supplied, four fast diodes and only one Zener di-

ode can be used as a protection of the outpus

(see Figure 28). The driving of DC motor needs

the connection as shown for CH2 (full bridge con-

figuration).

The drive of one bipolar stepper motor by using

CH1 and CH2 both in full bridge configuration al-

lows the use of a higher supply voltage level that

however cannot exceed the Absolute Maximum

Figure 23

111

Tristate left and right (see

X000 configuration of full

bridge).

L6280

18/26

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