bd6709fs ROHM Co. Ltd., bd6709fs Datasheet - Page 30

• Manufacturer Part Number: bd6709fs
• Description: Dc Brushless Motor Drivers For Cooling Fans Speed Controllable Single-phase Full-wave Dc Brushless Fan Motor Drivers
• Manufacturer: ROHM Co. Ltd.

8) Soft start ＜ BD6722FS ＞
The soft start section is set at the rotation start or the LOCK protection release, etc.
The soft start time is decided by the constant circuit connected with VMIN terminal.
And VMIN terminal is used as the lowest rotational speed setting. Set VMIN to 1.75V or more (lowest rotational speed
DUTY ≦ 50%).
a) Soft start time of VTH setting DUTY ≧ 50%
It gradually rises to the DUTY set with VTH after it starts.
Fig.94 shows the soft start operation at VTH setting DUTY=70% and VMIN setting DUTY=20%.
b). Soft start time of VTH setting DUTY ≦ 50%
DUTY rises gradually, and rises to Duty=50% temporarily after it starts. The purpose of this is to secure the start torque
when DUTY set with VTH and VMIN is too low.
It gradually descends to the DUTY set with VTH and VMIN after it rotates by DUTY=50% for a fixed time.
Fig.95 shows the soft start operation at VTH setting DUTY=10% and VMIN setting DUTY=20%.
Because the lock protection function is turned off while soft start, soft start time can be set longer than the lock
detection time. The soft start function can be turned off by not connecting the capacitor to VMIN terminal.
Fig.92 Output current characteristic by soft start
WITHOUT SOFT START
SOFT START
VMIN=20%
VTH=70%
VMIN=20%
VTH=10%
50%
50%
Fig.94 Soft start time chart of DUTY ≧ 50%
Fig.95 Soft start time chart of DUTY ≦ 50%
30/40
TIME
Fig.93Soft start setting circuit
R1
R2
TIME
TIME
VREF
－
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