L6229 datasheet(11/25 Pages) STMICROELECTRONICS

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L6229

Figure 12 shows the magnitude of the Off Time tOFF versus COFF and ROFF values. It can be approximately

calculated from the equations:

tRCFALL = 0.6 · ROFF · COFF

tOFF = tRCFALL + tDT = 0.6 · ROFF · COFF + tDT

where ROFF and COFF are the external component values and tDT is the internally generated Dead Time with:

20K

Ω ≤ ROFF ≤ 100KΩ

0.47nF

≤ COFF ≤ 100nF

tDT = 1µs (typical value)

Therefore:

tOFF(MIN) = 6.6µs

tOFF(MAX) = 6ms

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 13 shows the lower limit for the On Time tON for having a good PWM current regulation capacity. It has

to be said that tON is always bigger than tON(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 12. tOFF versus COFF and ROFF.

ON MI N

()

>2.5

µs (typ. value)

RCRISE

–

⎩

⎨

⎧

0.1

100

1.10

Coff [nF]

Roff = 100kΩ

Roff = 47kΩ

Roff = 20kΩ

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