MA838 Datasheet, PDF (12/13 Pages) Zarlink Semiconductor Inc – SINGLE PHASE PULSE WIDTH MODULATION WAVEFORM GENERATOR

MA838
HARDWARE INPUT/OUTPUT FUNCTIONS
Set Output Trip (SET TRIP input)
The SET TRIP input is provided separately from the
microprocessor interface in order to allow an external source
to override the microprocessor and provide a rapid shutdown
facility. For example, logic signals from overcurrent sensing
circuitry or the microprocessor ‘watchdog’ might be used to
activate this input.
When the SET TRIP input is taken to a logic high, the output
trip latch is activated. This results in the TRIP output and the
PWM outputs being latched low immediately. This condition
can only be cleared by applying a reset cycle to the RST input.
It is essential that when not in use this pin is tied low and
isolated from potential sources of noise; on no account should
it be left floating.
SET TRIP is latched internally at the master clock rate in
order to reduce noise sensitivity.
Output Trip Status ( TRIP output)
The TRIP output indicates the status of the output trip latch
and is active low.
Reset (RST input)
The RST input performs the following functions when active
(low):
1. PWM outputs are forced low (if not already low) thereby
turning off the drive switches.
2. All internal counters are reset to zero (this corresponds to 0°
for the red phase output).
3. The rising edge of RST reactivates the PWM outputs
resetting the output trip and setting the TRIP output high –
assuming that the SET TRIP input is inactive (i.e. Iow).
Clock (CLK input)
The CLK input provides a timing reference used by the
MA838 for all timings related to the PWM outputs. The
microprocessor interface, however, derives all its timings from
the microprocessor and therefore the microprocessor and the
MA838 may be run either from the same or from different
clocks.
Waveform segment
Sample number
0°- 30°
0 - 127
30·23°- 60°
128 - 255
60·23°- 89·77°
256 - 383
Table 8 90° of the 360° cycle is divided into 384 8-bit
samples
Fig. 18 90° sample of typical power waveform
WAVEFORM DEFINITION
The waveform amplitude data used to construct the PWM
output sequence is read from the internal 384 x 8 ROM. This
contains the 90° span of the waveform as shown in Fig. 18.
Each successive 8-bit sample linearly represents the
instantaneous amplitude of the waveform. It is assumed that
the waveform is symmetrical about the 90°, 180° and 270°
axes.The MA828 reconstructs the full 360° waveform by reading
the 0°-90° section held in ROM and assigning negative values
for the second half of the cycle.
The 384 8-bit samples are regularly spaced over the 0° to
90° span, giving an angular resolution of approximately 0·23°.
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