sa828 Mitel, sa828 Datasheet - Page 6

• Manufacturer Part Number: sa828
• Description: Three-phase Pwm Waveform Generator
• Manufacturer: Mitel

waveform frequency ( f
determined by the 3-bit FRS word. The value of m is determined
as shown in Table 5.
Pulse delay time
the rising edges of each of the outputs by an equal amount.
frequency and pdy , defined by the 6-bit pulse delay time select
word (PDY). The value of pdy is selected as shown in Table 6.
where f
(as set by FRS).
where pdy = 1- 64 (as set by PDY) and f
frequency.
Fig 8 shows the eftect of the pulse delay circuit.
pulse deletion circuit (see Fig. 2), the minimum pulse width
seen at the PWM outputs will be shorter than the pulse deletion
time set in the initialisation register. The actual shortest pulse
generated is given by t
6
SA828
DRIVE BOTTOM SWITCH
Table 5 Values of carrier frequency multiplicaion factor m
The power frequency range is a function of the carrier
The pulse delay time affects all six PWM outputs by delaying
The pulse delay time is a function of the carrier waveform
Value of pdy
The power frequency range, f
The pulse delay time, t
It should be noted that as the pulse delay circuit follows the
Value of m
FRS word
PDY word
OUTPUT SIGNAL TO
OUTPUT SIGNAL TO
DRIVE TOP SWITCH
INVERTER OUTPUT
Fig. 8 Effect of pulse delay on PWM pulse train
CARR
X
INVERTER ARM
INVERTER ARM
DON’T
CARE
REQUIRED AT
PWM SIGNAL
= carrier frequency and m = 1, 2, 4, 8, 16, 32 or 64
X
Fig. 7 Temporary register R2
PDY
110
Table 6 Values of pdy
64
f
111111
RANGE
t
pd
5
CARR
1
pdy
t
pdy
PDY
101
– t
pdy
32
=
=
) and a multiplication factor m ,
pdy
, is then given by:
f
4
CARR
384
f
SELECT WORD
CARR
.
100
PDY
PDY
111110
PDY
t
t
16
pdy
pdy
RANGE
pdy
PULSE
DELAY
2
x m
= PULSE DELAY TIME
x 512
3
5
0
= MSB
011
= LSB
PDY
, is then given by:
8
...etc...
...etc...
2
010
t
PDY
pdy
4
CARR
1
001
t
PDY
pdy
2
= carrier
000000
0
64
000
1
Pulse deletion time
through a pulse deletion circuit. The pulse deletion circuit
compares pulse widths with the pulse deletion time set in the
initialisation register. lf a pulse (either positive or negative) is
greater than or equal in duration to the pulse deletion time, it is
passed through unaltered, otherwise the pulse is deleted.
frequency and pdt , defined by the 7-bit pulse deletion time word
(PDT). The value of pdt is selected as shown in Table 7.
where pdt = 1-128 (as set by PDT) and f
waveform.
Counter reset
frequency phase counter is set to 0 degrees for the red
phase. It will remain at 0 degrees until the CR bit is released
(i.e., high).
Control Register Function
normally be modified during PWM cycles in order to control
the operation of the motor.
Power frequency (speed)
adjusted within the range specified in the initialisation register
Forward/reverse
changed by changing the phase sequence of the PWM
outputs.
Power frequency amplitude
maintaining their relative widths, the amplitude of the power
waveform is effectively altered whilst maintaining the same
power frequency.
Overmodulation
that a quasi-squarewave is produced. A combination of
overmodulation and a lower power frequency can be used to
achieve rapid braking in AC motors.
Output inhibit
generation continues internally. Useful for temporarily
inhibiting the outputs without having to to change other
register contents.
To eliminate short pulses the true PWM pulse train is passed
The pulse deletion time, t
Value of pdt
COUNTER
The pulse deletion time, t
Fig. 10 shows the effect of pulse deletion on a pure PWM
When the CR bit is active (i.e., Iow) the internal power
This 24-bit register contains the parameters that would
The parameters set in the control register are as follows:
Allows the power frequency of the PWM outputs to be
Allows the direction of rotation of the AC motor to be
By altering the widths of the PWM output pulses while
Allows the output waveform amplitude to be doubled so
Allows the outputs to be set to the low state while the PWM
PDT word
RESET
CR PDT
6
Fig. 9 Temporary register R0
PDT
1111111
Table 7 Values of pdt
5
t
1
pd
PDT
PULSE DELETION
SELECT WORD
=
PDT
pd
PDT
4
pd
f
, is a function of the carrier wave
1111110
, is then given by:
CARR
PDT
TIME
6
0
= MSB
= LSB
pdt
2
3
x 512
PDT
CARR
2
...etc...
...etc...
= carrier frequency.
PDT
1
PDT
0000000
0
128