Applications Note AN36
Issue1 - JULY 2002
External PWM
Fixed speed applications
The ZXBM200x series are also capable of
being driven from an externally derived PWM
signal. This is particularly useful where the
device is to be used with existing system
controllers that provide such an output.
For applications where the speed control
function is not required the device can be set
to run at full speed by simply holding the SPD
pin at a voltage below 1V. The best way to do
this is by attaching a 10k⍀ resistor to Ground.
A direct link to Ground can cause the SPD pin
and its associated circuit not to function as
required.
The PWM control waveform can be in the
form of a digital signal. The main criteria
being that it should have a low level of less
than 1V and a high level greater than 2V (but
not exceeding 5V). A conventional 3.3V or 5V
based TTL or CMOS signal is ideal The signal
is applied to Pin 8, the CPWM pin, with Pin 4,
the SPD pin, being left open circuit. A low
level on the CPWM pin represents the winding
drive Off period and a high level the On
period.
Winding Drive
The driving of the two phases of the motor is
achieved using a pair of external power
transistors driven from Pin 10 and Pin 9, Ph1
and Ph2 respectively, of the ZXBM200x
controller. These transistors can be either of
the Bipolar or MOSFET type. Both
methodologies are similar and incorporate
clamping to restrict the level of winding
avalanche to be within the capabilities of the
transistors chosen. Each Transistor type will
be discussed in detail together with their
respective merits and limitations.
Figure 6 illustrates an external input
waveform on the CPWM pin together with the
resulting drive at one of the phase windings.
There is some delay through the ZXBM200x
device that ensures the drive reaches 100%
PWM slightly before the input reaches 100%
duty cycle.
Bipolar Power Transistors
Bipolar devices at one time were the
universal power driving method, however,
with MOSFETs becoming more readily
available bipolar use has diminished. In the
case of the ZXBM200x series it has been
found that bipolar devices are very suited to
the lower power, lower current end of the
applications range.
Figures 7 and 8 illustrate a bipolar driver
output stage with two methods of clamping.
In both Figures 7 and 8, R1 and R2 provide the
base drive to the Output transistors Q1 and
Q2. The value of these resistors should be
chosen to provide sufficient current to
maintain a good saturation voltage of the
driver transistors but not to exceed the Ph1
and Ph2 drive capabilities of the ZXBM200x
under worst case conditions.
Figure 6
AN 36 - 6
ZETEX [ ZETEX SEMICONDUCTORS ]
