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| EVAL6208N Datasheet, PDF (12/27 Pages) STMicroelectronics – DMOS DRIVER FOR BIPOLAR STEPPER MOTOR | |||
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L6208
Figure 11. Output Current Regulation Waveforms
IOUT
VREF
RSENSE
tOFF
tON
tOFF
VSENSE
VREF
0
VRC
5V
1µs tBLANK
Slow Decay
Fast Decay
tRCRISE
1µs tBLANK
Slow Decay
Fast Decay
tRCRISE
2.5V
tRCFALL
tRCFALL
ON
OFF
SYNCHRONOUS OR QUASI
SYNCHRONOUS RECTIFICATION
D01IN1334
B
1µs tDT
C
DAB
1µs tDT
C
D
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⦠⤠ROFF ⤠100Kâ¦
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).
12/27
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