bd63847 ROHM Co. Ltd., bd63847 Datasheet - Page 8

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

●PWM Constant current control
© 2010 ROHM Co., Ltd. All rights reserved.
BD63847/63843EFV
www.rohm.com
1) Current control operation
2) Noise-masking function
3) CR Timer
Attach a resistor of at least 10 kΩ to the CR terminal (10 kΩ~200 kΩ recommended) as lower values may keep the CR from
reaching the VCRH voltage level. A capacitor in the range of 470 pF~1500 pF is also recommended. As the capacitance
value is increased, however, the noise-masking period (t
exceed the current limit threshold due to the internal L and R components of the output motor coil. Also, ensure that the
chopping period (t
average output current and yielding lower output rotation efficiency. The optimal value should reduce the motor drive noise
while keeping distortion of the output current waveform to a minimum.
When the output transistor is turned on, the output current increases, raising the voltage over the current sense resistor.
When the voltage on the RNF pin reaches the voltage value set by the internal 4-bit DAC and the VREF input voltage,
the current limit comparator engages and enters current decay mode. The output is then held off for a period of time
determined by the CR time constant connected to the CR pin. The process repeats itself constantly for PWM operation.
In order to avoid misdetection of output current due to RNF spikes that may occur when the output turns ON, the IC
employs an automatic current detection-masking period (t
after the output transistor is turned on. This allows for constant-current drive without the need for an external filter. This
noise-masking period defines the minimum ON-time for the motor output transistor.
The CR filter connected to the CR pin is repeatedly charged and discharged between the VCRH and VCRL levels. The
output of the internal comparator is masked while charging from VCRL to VCRH in order to cancel noise. (As
mentioned above, this period defines the minimum ON-time of the motor output transistor.) The CR terminal begins
discharging once the voltage reaches VCRH. When the output current reaches the current limit during this period (i.e.
RNF voltage reaches the decay trigger voltage), then the IC enters decay mode. The CR continues to discharge during
this period until it reaches VCRL, at which point the IC output is switched back ON. The current output and CR pin
begin charging simultaneously.
The CR charge time (t
formulas. The total of t
t
t
t
ONMIN
discharge
CHOP
Output current
RNF voltage
CR voltage
[s]≒t
[s]≒C･R'･R / (R'+R)･ln[(VCR-0.4)/(VCR-1.0)]
chop
[s]≒C･R･ln[(1+α)/0.4]
ONMIN
) is not set longer than necessary, as doing so will increase the output ripple, thereby decreasing the
VCR=V･R/(R'+R)
V: internal regulator voltage 5V(Typ.)
R': CR terminal internal impedance 5kΩ (Typ.)
α:See the right graph.
Minimum ON time t
ONMIN
+ t
Fig.4 Timing chart of CR voltage, RNF voltage and output current
ONMIN
discharge
) and discharge time (t
and t
Spike noise
discharge
yield the chopping period, t
ONMIN
Discharge time t
discharge
chopping period t
onmin
8/19
) also increases, and there is a risk that the output current may
ONMIN
) are set by external components, according to the following
discharge
), during which current detection is disabled immediately
chop
CHOP
.
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
500
C [pF]
1000
Current limit value
Current limit value
GND
VCRH(1.0V typ.)
VCRL(0.4V typ.)
GND
0mA
Technical Note
1500
2010.4 - Rev.A
2000