L6285 datasheet(10/16 Pages) STMICROELECTRONICS

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Oscillator For Clock and Reset Generation

The oscillator block provides for two functions:

1)Generate an internal reset signal when the

voltage at pin osc/res is below 1V. The reset

signal switches off all the outputs and resets

the logic registers.

2)Generate, when the pin osc/res is left free two

syncro signals p1 and p2 for the clock of the

PWM Current Control of the two stepper

driver blocks

The oscillator operates like the 555 concept in

which the capacitor voltage oscillates between

1/3VS 2/3VS (Fig. 6). The oscillator frequency is 2

times the chopping frequency in order to generate

the two syncro signals at operative 20KHz PWM.

The tCH = charge time of Cosc is defined by Rosc,

VTH1 and VTH2 ( threshold voltages)and Cosc.

The discharge time Tdsc is practically only defined

by Cosc and the internal discarge resistor Rdsc.

The tdsc is also the time lockout during which the

RS FF cannot read the Comparator output (see

Fig. 4)

Figure 6: Oscillator Concept

L6285

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