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STEPMO_EVK_20x
Evaluation Board Manual
EUDC59-U-002 - Rev. 1.3
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Current Limitation Value
The ROHM stepper motor driver ICs supported by this EVK have a current limitation function. This must not to be confused with over
current protection (OCP) which is another feature of the IC (check datasheet for details). Instead, the purposes of the current
limitation are:
Constant motor current operation independent from the supply voltage
Operation of motors with low impedance phase without exceeding the rated motor current
Operation with high supply voltages for faster current rise in the phase windings to achieve higher motor torque
Operation in microstepping mode
The current limitation is achieved by chopping the output current with pulse width modulation (PWM) as soon as the set limit is
reached. As described in the datasheet the set current limit depends on the current sense resistor RNF (resistors R4 and R5 in the
schematic) and the voltage applied to the VREF pin. In this EVK the current sense resistor is fixed but the voltage on the VREF pin
can be adjusted by the potentiometer R19. The current limit value will depend linearly on the potentiometer setting. For reference,
the approximate values for minimum and maximum current limit values are given in Table 5. With the corresponding values of VREF
and RNF the set current limit value can be calculated with the formula:
𝐼𝑂𝑈𝑇 (𝑝𝑒𝑟 𝑝ℎ𝑎𝑠𝑒) =
𝑉𝑅𝐸𝐹
5 · 𝑅𝑁𝐹
VREF can be accessed at test point TP1 for indirect measurement of the current limit.
Note:
The highest value which can be set by the potentiometer is limited to the maximum continuous current per phase as
allowed by the motor driver IC specifications.
Nevertheless, for high output currents (~1.5A and above) additional cooling or heat sinking will need to be applied to
the IC and PCB. Please always check the IC temperature!
Also please take care to set the current limit to a value not exceeding the rated maximum current per phase of your
connected motor.
Minimum Setting
Maximum Setting
Potentiometer Setting
EVK Model Versions
VREF / V
Current Limit / A
VREF / V
Current Limit / A
RNF
Value /
Ω
1, 4, 7, 9, C
0.11
~ 0.067
1.64
~ 1.0
0.33
1
F
0.16
~ 0.13
1.88
~ 1.5
0.25
1
5, A
0.1
~ 0.15
0.98
~ 1.5
0.13
1
2, 6, 8, E
0.11
~ 0.17
1.3
~ 2.0
0.13
1
3
0.25
~ 0.35
1.75
~ 2.5
0.14
1
Note 1: Including parasitic board resistance of ~0.03
Ω
Table 5: Current limit settings by potentiometer R19 with references to VREF and RNF values
