AN1625 datasheet(13/39 Pages) STMICROELECTRONICS

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AN1625 APPLICATION NOTE

Figure 13. Output Current Regulation Waveforms

Figure 14 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).

OFF

tON

tOFF

2.5V

Slow Decay

µs tBLANK

tRCRISE

SYNCHRONOUS RECTIFICATION

µs tBLANK

VRC

VSENSE

VREF

IOUT

VREF

RSENSE

D02IN1351

tOFF

µs tDT

tRCFALL

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