L6207Q datasheet(14/27 Pages) STMICROELECTRONICS

14 / 27 page

Circuit description

L6207Q

14/27

DocID018993 Rev 3

Figure 11 shows the magnitude of the OFF time tOFF versus COFF and ROFF values. It can be

approximately calculated from Equation 1 and Equation 2:

Equation 1

tRCFALL = 0.6 · ROFF · COFF

Equation 2

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

where ROFF and COFF are the external component values and tDT is the internally generated

deadtime with:

Equation 3

20 k

ROFF 100 k

0.47 nF

COFF 100 nF

tDT = 1 µs (typical value)

therefore:

Equation 4

tOFF(MIN) = 6.6 µs

tOFF(MAX) = 6 ms

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 is only an issue if the capacitor is not completely charged

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

depends on motors and supply parameters, must be bigger than tRCRISE to allow a good

current regulation by the PWM stage. Furthermore, the ON time tON can not be smaller than

the minimum ON time tON(MIN).

Equation 5

Figure 12 shows the lower limit for the ON time tON for having a good PWM current

regulation capacity. It should be mentioned 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.

Therefore, a small COFF value gives more flexibility to the applications (allows smaller ON

time and, therefore, higher switching frequency), but, the smaller the value for COFF, the

more influential the noises on the circuit performance.

tON tON MIN





tON tRCRISE tDT

–













1,5

styp



tRCRISE

600 COFF



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