AMIS−30511
http://onsemi.com
9
Functional Description
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
(Table 23: SPI Control Parameter Overview EMC[1:0]).
The power transistors are equipped with so−called “active
diodes”: when a current is forced through 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 Table 4: DC Parameters
for more details.
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 (Table 12: SPI Control Register 1). 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.
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.
Figure 5. Forward and Slow/Fast Decay PWM
Icoil
0
t
Forward & Slow Decay
Actual value
Set value
Fast Decay & Forward
T
PWM
Forward & Slow Decay
In case the supply voltage is lower than 2*Bemf, then the
duty cycle of the PWM is adapted automatically to >50% to
maintain the requested average current in the coils. This
process is completely automatic and requires no additional
parameters for operation. The over−all current−ripple is
divided by two if PWM frequency is doubled (Table 12: SPI
Control Register 1).
