SA8281 Mitel Networks Corporation, SA8281 Datasheet - Page 7

• Manufacturer Part Number: SA8281
• Description: Three-Phase PWM Waveform Generator
• Manufacturer: Mitel Networks Corporation

Power frequency selection
frequency range (defined in the initialisation register) by the 12-
bit power frequency select word, PFS, allowing the power
frequency to be defined in 4096 equal steps. As the PFS word
spans the two temporary registers R0 and R1 it is therefore
essential, when changing the power frequency, that both these
registers are updated before writing to R3.
where pfs = decimal value of the 12-bit PFS word and f
power frequency range set in the initialisation register.
Control Register Programming
initialisation register contains the basic operating parameters of
the SA828.
loaded into the three 8-bit temporary registers R0 - R2. When all
the data has been loaded into these registers it is transferred
into the 24-bit control register by writing to the dummy register
R3. It is recommended that all three temporary registers are
updated before writing to R3 in order to ensure that a conformal
set of data is transferred to the control register for execution.
The power frequency is selected as a proportion of the power
The power frequency ( f
The control register should only be programmed once the
As with the initialisation register, control register data is
FORWARD/
REVERSE BIT
0 = FORWARD
1 = REVERSE
PFS
F/R
0 = DISABLED
OVERMOD-
1 = ACTIVE
7
ULATION
PFS
OM
BIT
Fig. 11 Temporary register R0
Fig. 12 Temporary register R1
6
PFS
f
INH
POWER
PULSE DELETION
PULSE DELETION
t
5
pd
OUTPUT
INHIBIT BIT
0 = OUTPUTS DISABLED
1 = OUTPUTS ACTIVE
SELECT WORD
FREQUENCY
DON’T
= PULSE DELETION TIME
CARE
PFS
PFS 0
PWM SIGNAL
PWM SIGNAL
POWER
=
BITS 0-7
POWER
X
f
RANGE
4096
4
BEFORE
= LSB
AFTER
PFS
PFS
) is given by:
x pfs
11
3
SELECT WORD
FREQUENCY
PFS
<t
PFS 11
PFS
BITS 8-11
pd
Fig. 10 The effect of the pulse deletion circuit
POWER
10
2
= MSB
DELETED
PFS
PFS
PULSE
1
9
>t
PFS
pd
PFS
RANGE
0
8
>t
pd
=
>t
pd
Forward/ reverse selection
waveforms is controlled by the Forward/Reverse bit F/R. The
actual effect of changing this bit from 0 (forward) to 1 (reverse)
is to reverse the power frequency phase counter from
incrementing the phase angle to decrementing it. The required
output waveforms are all continuous with time during a forward/
reverse change.
yellow-blue and in the reverse mode the sequence is blue-
yellow-red.
Output inhibit selection
PWM outputs to the off (low) state. No other internal operation
of the device is affected. When the inhibit is released the PWM
outputs continue immediately. Note that as the inhibit is asserted
after the pulse deletion and pulse delay circuits, pulses shorter
than the normal minimum pulse width may be produced initially.
Overmodulation selection
of the amplitude word. When active (i.e., high) the output
waveform will be controlled in the 100% to 200% range by the
amplitude word.
Fig. 13 Current waveforms as seen at the motor terminals,
The phase sequence of the three-phase PWM output
In the forward mode the output phase sequence is red-
>t
When active (i.e., Iow) the output inhibit bit INH sets all the
The overmodulation bit OM is, in effect, the ninth bit (MSB)
The percentage amplitude control is now given by:
V
V
pd
showing the effect of setting the overmodulation bit
0
0
where A
>t
Overmodulated Amplitude = A
pd
<t
>t
pd
OVERMODULATION BIT NOT SET
POWER
pd
OVERMODULATION BIT SET
(100% MODULATION)
(200% MODULATION)
DELETED
PULSE
= the power amplitude
>t
pd
>t
pd
POWER
+ 100%
SA828
t
t
7