AMIS-30522 Datasheet, PDF (8/29 Pages) AMI SEMICONDUCTOR – Micro-stepping Motor Driver

AMIS-30522
8.0 Functional Description
8.1 H-Bridge Drivers
A full H-bridge is integrated for each of the two stator windings. Each H-bridge consists of two low-side and two high-side N-type
MOSFET switches. Writing logic ‘0’ in bit <MOTEN> disables all drivers (high-impedance). Writing logic ‘1’ in this bit enables both
bridges and current can flow in the motor stator windings.
In order to avoid large currents through the H-bridge switches, it is guaranteed that the top- and bottom-switches of the same half-
bridge are never conductive simultaneously (interlock delay).
A two-stage protection against shorts on motor lines is implemented. In a first stage, the current in the driver is limited. Secondly,
when excessive voltage is sensed across the transistor, the transistor is switched off.
In order to reduce the radiated/conducted emission, voltage slope control is implemented in the output switches. The output slope
is defined by the gate-drain capacitance of output transistor and the (limited) current that drives the gate. There are two trimming
bits for slope control (see Table 27).
The power transistors are equipped with so-called “active diodes”: when a current is forced trough the transistor switch in the
reverse direction, i.e. from source to drain, then the transistor is switched on. This ensures that most of the current flows through
the channel of the transistor instead of through the inherent parasitic drain-bulk diode of the transistor.
Depending on the desired current range and the micro-step position at hand, the Rdson of the low-side transistors will be adapted
such that excellent current-sense accuracy is maintained. The Rdson of the high-side transistors remain unchanged, see Error!
Reference source not found. for more details.
8.2 PWM Current Control
A PWM comparator compares continuously the actual winding current with the requested current and feeds back the information to
a digital regulation loop. This loop then generates a PWM signal, which turns on/off the H-bridge switches. The switching points of
the PWM duty-cycle are synchronized to the on-chip PWM clock. The frequency of the PWM controller can be doubled and an
artificial jitter can be added (see Table 16). The PWM frequency will not vary with changes in the supply voltage. Also variations in
motor-speed or load-conditions of the motor have no effect. There are no external components required to adjust the PWM
frequency.
8.2.1. Automatic Forward and Slow-Fast Decay
The PWM generation is in steady-state using a combination of forward and slow-decay. The absence of fast-decay in this mode,
guarantees the lowest possible current-ripple “by design”. For transients to lower current levels, fast-decay is automatically
activated to allow high-speed response. The selection of fast or slow decay is completely transparent for the user and no additional
parameters are required for operation.
Icoil
Set value
0
T_PWM
Actual value
t
Forward & Slow Decay
Forward & Slow Decay
Fast Decay & Forward
PC20070604.1
Figure 6: Forward and Slow/Fast Decay PWM
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