bd63847 ROHM Co. Ltd., bd63847 Datasheet - Page 9

• Manufacturer Part Number: bd63847
• Description: Micro Step 36v Stepper Motor Drivers
• Manufacturer: ROHM Co. Ltd.

●Current decay mode
Output current
© 2010 ROHM Co., Ltd. All rights reserved.
BD63847/63843EFV
www.rohm.com
CR voltage
The IC allows for a mixed decay mode in which the ratio of fast and slow decay can be optionally set.
The following diagrams show the operating state of each transistor and the regenerative current path during attenuation for
each decay mode:
The merits of each decay mode are as follows:
○SLOW DECAY
○FAST DECAY
Additionally, this IC allows for a mixed decay mode that can help improve upon problems that arise from using fast or slow
decay alone. In this mode, the IC switches automatically between slow and fast decay, improving the current control
characteristics without increasing the output current ripple. The ratio of fast to slow decay is set externally via the voltage
input to the MTH pin; therefore, the optimal mix of slow and fast decay can be achieved for each application. Mixed decay
mode operates by splitting the decay period into two sections, the first X%(t1-t2) of which operates the IC in slow decay
mode, and the remainder(t2-t3) of which operates in fast decay mode. However, if the output current (i.e., the voltage on the
RNF pin) does not reach the set current limit during the first X% (t1-t2) decay period, the IC operates in fast decay mode
only.
OFF→ON
ON→OFF
During current attenuation, the voltage between motor coils is small and the regeneration current decreases slowly,
decreasing the output current ripple. This is favorable for keeping motor torque high. However, due to fall-off of current
control characteristics in the low-current region, or due to reverse EMF of the output motors exhibited when using
high-pulse-rate half-step, eighth-step or sixteenth-step modes, the output current increases, distorting the output current
waveform and increasing motor vibration. Thus, this decay mode is most suited to full-step modes, or low-pulse-rate
half-step, eighth-step or sixteenth-step modes.
Fast decay decreases the regeneration current much more quickly than slow decay, greatly reducing distortion of the
output current waveform. However, fast decay yields a much larger output current ripple, which decreases the overall
average current running through the motor. This causes two problems: first, the motor torque decreases (increasing the
current limit value can help eliminate this problem, but the rated output current must be taken into consideration); and
second, the power loss within the motor increases and thereby radiates more heat. If neither of these problems is of
concern, then fast decay can be used for high-pulse rate half-step, eighth-step or sixteenth-step drive.
Fig.5 Relation between CR terminal voltage, MTH voltage, and output current during mixed decay
FAST DECAY
chopping period t
t1
M
SLOW DECAY
Fig.4 Route of regenerative current during current decay
chop
ON→OFF
t2
OFF
FAST DECAY
t3
9/19
ON→OFF
OFF→ON
SLOW DECAY
MTH voltage
Current limit value
1.0V
0.4V
GND
0
M
MTH voltage [V]
0.4~1.0
1.5~3.5
0~0.3
ON→ON
OFF→OFF
Output current path
Current decay path
Technical Note
2010.4 - Rev.A
Current decay mode
SLOW DECAY
FAST DECAY
MIX DECAY