admc331 Analog Devices, Inc., admc331 Datasheet - Page 16

• Manufacturer Part Number: admc331
• Description: Single Chip Dsp Motor Controller
• Manufacturer: Analog Devices, Inc.

ADMC331
where the subscript 1 refers to the value of that register during
the first half cycle and the subscript 2 refers to the value during
the second half cycle. The corresponding duty cycles are:
since for the completely general case in double update mode,
the switching period is given by:
Again, the values of T
zero and T
Similar PWM signals to those illustrated in Figure 6 and Fig-
ure 7 can be produced on the BH, BL, CH and CL outputs by
programming the PWMCHB and PWMCHC registers in a man-
ner identical to that described for PWMCHA.
The PWM controller does not produce any PWM outputs until
all of the PWMTM, PWMCHA, PWMCHB and PWMCHC
registers have been written to at least once. Once these registers
have been written, internal counting of the timers in the three-
phase timing unit is enabled.
Effective PWM Resolution
In single update mode, the same value of PWMCHA, PWM-
CHB and PWMCHC are used to define the on-times in both
half cycles of the PWM period. As a result, the effective resolu-
tion of the PWM generation process is 2 t
26 MHz CLKOUT) since incrementing one of the duty-cycle
registers by 1 changes the resultant on-time of the associated
PWM signals by t
period).
In double update mode, improved resolution is possible since
different values of the duty cycles registers are used to define the
on-times in both the first and second halves of the PWM period.
As a result, it is possible to adjust the on-time over the whole
period in increments of t
PWM resolution of t
26 MHz CLKOUT).
The achievable PWM switching frequency at a given PWM
resolution is tabulated in Table V.
Table V. Achievable PWM Resolution in Single and Double
Update Modes
Resolution Single Update Mode
(Bit)
8
9
10
11
12
Minimum Pulsewidth, PWMPD Register
In many power converter switching applications, it is desirable
to eliminate PWM switching signals below a certain width. It
takes a certain finite time to both turn on and turn off modern
d
d
AL
AL
=
=
T
T
T
T
AH
AL
S
S
=
T
=
(PWMTM
(PWMCHA
S
= (PWMTM
S
.
PWM Frequency (kHz)
50.7
25.4
12.7
6.3
3.2
1
+ PWMTM
1
+ PWMCHA
CK
(PWMTM
CK
in each half period (or 2 t
AH
in double update mode (or 38.5 ns for a
1
2
CK
and T
+ PWMTM
1
– PWMCHA
+ PWMTM
(PWMTM
2
. This corresponds to an effective
– PWMDT
AL
1
are constrained to lie between
2
1
+ PWMTM
)
1
– PWMCHA
– PWMDT
2
)× t
Double Update Mode
PWM Frequency (kHz)
101.5
50.7
25.4
12.7
6.3
CK
CK
2
2
)
2
)
– PWMDT
(or 76.9 ns for a
CK
for the full
1
– PWMDT
2
)
power semiconductor devices. Therefore, if the width of any of
the PWM signals goes below some minimum value, it may be
desirable to completely eliminate the PWM switching for that
particular cycle.
The allowable minimum on-time for any of the six PWM out-
puts over half a PWM period that can be produced by the PWM
controller may be programmed using the 10-bit read/write
PWMPD register. The minimum on-time is programmed in
increments of t
produced over any half PWM period, T
value in the PWMPD register by:
so that a PWMPD value of 0x002 defines a permissible mini-
mum on-time of 76.9 ns for a 26 MHz CLKOUT.
In each half cycle of the PWM, the timing unit checks the on-
time of each of the six PWM signals. If any of the times are
found to be less than the value specified by the PWMPD regis-
ter, the corresponding PWM signal is turned OFF for the entire
half period and its complementary signal is turned completely
ON.
Consider the example where PWMTM = 200, PWMCHA = 5,
PWMDT = 3, PWMPD = 10 with a CLKOUT of 26 MHz
and operation in single update mode. In this case, the PWM
switching frequency is 65 kHz and the dead time is 230 ns.
The permissible on-time of any PWM signal over one half of
any period is 384.6 ns. Clearly, for this example, the dead
time adjusted on-time of the AH signal over half a PWM
period is (5–3) × 38.5 ns = 77 ns. This is less than the permis-
sible value, so the timing unit will output a completely OFF
(0% duty cycle) signal on AH. Additionally, the AL signal will
be turned ON for the entire half period (100% duty cycle).
Switched Reluctance Mode
The PWM block of the ADMC331 contains a switched reluc-
tance mode that is controlled by the state of the PWMSR pin.
The switched reluctance (SR) mode is enabled by connecting
the PWMSR pin to GND. In this SR mode, the low side PWM
signals from the three-phase timing unit assume permanently
ON states, independent of the value written to the duty-cycle
registers. The duty cycles of the high side PWM signals from
the timing unit are still determined by the three duty-cycle regis-
ters. Using the crossover feature of the output control unit, it is
possible to divert the permanently ON PWM signals to either
the high side or the low side outputs. This mode is necessary
because in the typical power converter configuration for switched
or variable reluctance motors, the motor winding is connected
between the two power switches of a given inverter leg. There-
fore, in order to build up current in the motor winding, it is
necessary to turn on both switches at the same time. Typical
active LO PWM signals during operation in SR mode are shown
in Figure 8 for operation in double update mode. It is clear that
the three low side signals (AL, BL and CL) are permanently ON
and the three high side signals are modulated so that the corre-
sponding high side power switches are switched between the ON
and OFF states.
CK
so that the minimum on-time that will be
T
MIN
= PWMPD × t
MIN
CK
, is related to the