AN2836 datasheet(21/57 Pages) STMICROELECTRONICS

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AN2836

Designing an application with L6225, L6226, L6227

Doc ID 15084 Rev 1

21/57

Figure 15.

PWM output current regulation waveforms (L6227 only)

Figure 16 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

deadtime with:

●

20 k

Ω ≤ R

OFF ≤ 100 kΩ

●

0.47 nF

≤ C

OFF ≤ 100 nF

●

tDT = 1 µs (typical value)

Therefore:

●

tOFF(MIN) = 6.6 µs

●

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 RCA (or RCB). 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 motor and supply parameters, has to be longer than tRCRISE in order to allow a

good current regulation by the PWM stage. Furthermore, the on-time tON cannot be shorter

than the minimum on-time tON(MIN).

Equation 10

Equation 11

tON tON MIN

()

>1.5

μs (typ. value)

tON tRCRISE tDT

–

⎩

⎨

⎧

RCRISE

600 C

OFF

⋅

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