L6227Q datasheet(14/27 Pages) STMICROELECTRONICS

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Circuit description

L6227Q

14/27

These values allow a sufficient range of tOFF to implement the drive circuit for most motors.

The capacitor value chosen for COFF also affects the rise time tRCRISE of the voltage at the

pin RCOFF. The rise time tRCRISE will only be an issue if the capacitor is not completely

charged before the next time the monostable is triggered. Therefore, the on time tON, which

depends by motors and supply parameters, has to be bigger than tRCRISE for allowing a

good current regulation by the PWM stage. Furthermore, the on time tON can not be smaller

than the minimum on time tON(MIN).

tRCRISE = 600 · COFF

Figure 12 on page 15 shows the lower limit for the on time tON for having a good PWM

current regulation capacity. It has to be said that t

ON is always bigger than t

ON(MIN) because

the device imposes this condition, but it can be smaller than tRCRISE - tDT. In this last case

the device continues to work but the off time tOFF is not more constant.

So, small COFF value gives more flexibility for the applications (allows smaller on time and,

therefore, higher switching frequency), but, the smaller is the value for COFF, the more

influential will be the noises on the circuit performance.

Figure 11.

tOFF versus COFF and ROFF

ON MIN

()

2.5

µs

TCRISE

–

⎩

⎨

⎧

0.1

100

1.10

Coff [nF]

Roff = 100kΩ

Roff = 47kΩ

Roff = 20kΩ

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