Datasheet 2

AN120 – UNDERSTANDING MP6500 CURRENT CONTROL
AN120 Rev. 1.0 www.MonolithicPower.com 5
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These should be
the same level
Average
current
Figure 4: Conventional Current Regulation Waveform
In the waveform above, the motor moves more in the step after the peak current than in the step before
the peak current. This results in a position error and instantaneous speed variation. A similar jump occurs
as the current waveform crosses zero.
Sensing Bi-Directional Current
Most stepper drivers use an external sense resistor connected between the bottom of each H-bridge and
ground. They only measure the current during the PWM on time, when the sense voltage is positive.
During slow decay, the current recirculates through the H-bridge and does not pass through the sense
resistor, so the current cannot be measured. During fast decay, the current through the resistor is
reversed, generating a negative voltage. In most power IC processes, it is difficult or expensive to make
use of this negative voltage.
Many of the current regulation problems in stepper motor drivers can be improved if we monitor the
current during the decay period in addition to the PWM on time. However, this can be difficult when
measuring the current with an external sense resistor.
The MP6500 implements a better solution: sensing the current though the H-bridge internally instead of
using an external sense resistor. Internal current sensing allows for monitoring of the current at all times—
during both fast and slow decay, as well as during the PWM on time. Even though it adds complexity to
the driver IC, internal current sensing can actually lower the system cost since external low-ohm sense
resistors are not needed. These resistors are physically large and quite expensive—two of these resistors
can cost as much as the driver IC itself.
The MP6500 Stepper Driver IC
The MP6500 bipolar stepper motor driver with internal current sensing is designed to replace typical,
inexpensive, peak-current control bipolar stepper motor drivers. A block diagram of the MP6500 is shown
in Figure 5.