AN1625 datasheet(14/39 Pages) STMICROELECTRONICS

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AN1625 APPLICATION NOTE

14/39

tRCRISE = 600 · COFF

2.11.1 Off-time Selection and minimum on-time

Figure 14 also 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 con-

stant.

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 14. Off-time selection and minimum on-time.

2.11.2 Slow Decay Mode

Figure 15 shows the operation of the bridge in the Slow Decay mode during the Off Time. At any time only two

legs of the three-phase bridge are active, therefore only the two active legs of the bridge are shown in the figure

and the third leg will be off. At the start of the Off Time, the lower power MOS is switched off and the current

ON MIN

()

1.5

µs (typ. value)

RCRISE

–







0.1

100

1 . 10

Coff [nF]

0.1

100

Coff [nF]

R = 20 k

Ω

R = 47 k

Ω

R = 100 k

Ω

AN1625 Datasheet(PDF) 14 Page - STMicroelectronics