SA56 Datasheet, PDF (6/7 Pages) Cirrus Logic – PULSE WIDTH MODULATION AMPLIFIER

SA56
OPERATING
CONSIDERATIONS
MODES OF OPERATION
2 QUADRANT DIGITAL MODE
The following chart shows the four modes of operation.
For sign/magnitude (2 quadrant) operation, two digital input
Mode CPWM PWM DIR
Aout
Bout
signals are required. A digital PWM signal to the PWM pin can
control the output duty cycle at one output pin with the other
2
GND Modu- High High
PWM
output pin held "HIGH". The digital input on the DIR pin will
Quad
lation
control direction by selecting the outputs that switch accord-
Digital
In
ing to the PWM input. If DIR is a logic "HIGH", the A output
2
Quad
Digital
4
Quad
GND
GND
Modu-
lation
In
High
Low
Modu-
lated
PWM
DIR
High
DIR
will be held "HIGH" and the B output will be switched as the
inverse of the PWM input signal. If DIR is logic "LOW", the B
output will be held "HIGH" and the A output will be switched.
Operating in 2 quadrant mode reduces switching noise and
power dissipation, but limits the control of the motor at very
low speed.
Digital
In
A braking function can be achieved by holding the PWM input
4
Add Drive Not Greater Greater
Quad Cap. to with used than 50% than 50%
"LOW", which will turn both of the upper MOSFETs on, rapidly
reducing the circulating current of the motor winding.
Analog set analog but high for a high for a
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quency
input
input
ANALOG INPUT OPERATION
The SA56 can operate with analog or digital inputs. In the
analog mode, the capacitor from CPWM to GND (SIG) sets
Y _{the frequency of an internal triangular ramp signal. An analog}
input at the PWM pin is compared to the ramp to generate
the duty cycle of the output. In Analog mode, the digital input
on the DIR pin is ignored, though this pin should never be
R _{left floating.}
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L OPERATING WITH DIGITAL INPUTS
Two and 4 quadrant operation are possible with the SA56
when driven with a digital PWM signal from a microcontroller or
E ^{DSP. When using a digital modulation signal, tie the CPWM pin}
to GND to disable the internal oscillator and ramp generator.
When operating in the digital mode, pulse widths should be
no less than 100ns and the switching frequency should remain
R _{less than 500KHz. This will allow enough time for the output}
MOSFETs to reach their full on/off state before receiving a
P ^{command to reverse state.}
  

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4 QUADRANT DIGITAL MODE
During 4 quadrant operation a single digital PWM input in-
cludes magnitude and direction information. The digital PWM
input signal is applied to the DIR pin and the PWM/INPUT pin
is tied to "HIGH". Both pairs of output MOSFETs will switch
in a locked anti-phase fashion from 0-100% duty cycle. With
a 50% duty cycle the average voltage of each output will be
half of Vs, and the differential voltage applied to the load will
be zero. Four quadrant operation allows smooth transitions
through zero current for position servos and low speed ap-
plications. Power dissipation is slightly higher since all four
output MOSFETs switch every cycle.
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