njm13775 New Japan Radio Co.,Ltd, njm13775 Datasheet - Page 7

• Manufacturer Part Number: njm13775
• Description: Dual Stepper Motor Driver
• Manufacturer: New Japan Radio Co.,Ltd

Current sense filtering
The clock oscillator generates a blanking pulse at turn-on to prevent flip-flops from resetting through the current sensing
comparators due to the current spike. The blanking pulse disables the comparators for a short time. Thereby any voltage
transient across the sensing resistor will be ignored during the blanking time.
The time is calculated as:
possible. Nominal value is 4 700 pF, which gives a blanking time of 1.0 µs.
current, will reach a slightly higher level than what is defined by the reference voltage. The filtering delay also limits the
minimum possible output current. As the output will be on for a short time each cycle, equal to the digital filtering blanking
time plus additional internal delays, an amount of current will flow through the winding. Typically this current is 1-10 % of
the maximum output current set by R
the comparator C input. In this case the digital blanking time should be as short as possible. The recommended filter
component values are 1 kohm and 820 pF. Lowering the switching frequency also helps reducing the minimum output
current.
Switching frequency
filter blanking time, the clock oscillator frequency is adjusted by R
frequency is approximately calculated as:
in higher current ripple, but may improve low level linearity. A higher clock frequency reduces current ripple, but increases
the switching losses in the IC and possibly the iron losses in the motor.
At turn-on a current spike occurs from the recovery of the recirculation diodes and the capacitance of the motor winding.
Choose the blanking pulse time to be longer than the duration of the switching transients by selecting a proper C
t
As the C
As the filtering action introduces a small delay, the peak value across the sensing resistor, and hence the peak motor
When optimizing low current performance, the filtering may be done by adding an external low pass filter in series with
To create an absolute zero current, the Dis input should be HIGH.
The frequency of the clock oscillator is set by the timing components R
f
Nominal component values of 12k and 4 700pF results in a clock frequency of 23.0 kHz. A lower frequency will result
Figure 8. Stepping modes
b
s
= 1 / ( 0.77 • R
= 210 • C
T
value may vary from approximately 2 200 pF to 33 000 pF, a blanking time ranging from 0.5 µs to 7 µs is
T
[s]
T
• C
T
)
S
.
T
. The value of R
T
and C
T
at the RC-pin. As C
T
is limited to 2 - 20 kohm. The
NJM13775
T
sets the digital
T
- 7 -
value.