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SA01-6 Arkusz danych(PDF) 4 Page - Cirrus Logic

Numer części SA01-6
Szczegółowy opis  Pulse Width Modulation Amplifier
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Producent  CIRRUS [Cirrus Logic]
Strona internetowa  http://www.cirrus.com
Logo CIRRUS - Cirrus Logic

SA01-6 Arkusz danych(HTML) 4 Page - Cirrus Logic

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P r o d u c t I n n o v a t i o n F r o m
SA01 • SA01-6
4
SA01U
GENERAL
Please read Application Note 30 on "PWM Basics". Refer
to Application Note 1 "General Operating Considerations" for
helpful information regarding power supplies, heat sinking
and mounting. Visit www.Cirrus.com for design tools that help
automate pwm filter design;heat sink selection;Apex Precision
Power’s completeApplication Notes library;Technical Seminar
Workbook; and Evaluation Kits.
CURRENTLIMIT
The current limit function sets a peak limit on current flow in
pin 8 (Isense). This limits load current and also limits current
in the event of a short of either output to +Vs. This circuit can
trip anytime during the conduction period and will hold the
output transistors off for the remainder of that conduction period.
For proper operation the current limit sense resistor must
be connected as shown in the external connection diagram.
It is recommended that the resistor be a non-inductive type.
Load current flows in pin 8. No current flows in pin 10 (Shut-
down/filter) so no error will be introduced by the length of
the connection to pin 10. However, the voltage at pin 10 is
compared to GND (pin 4) and an error could be introduced if
the grounded end of R
LIMIT is not directly tied to pin 4. Good
circuit board layout practice would be to connect R
LIMIT directly
between pins 8 and 4.
Switching noise spikes will invariably be found at pin 8. The
amplitude and duration will be load dependent. The noise
spikes could trip the current limit threshold which is only 200
mV. R
FILTER and CFILTER should be adjusted so as to reduce the
switching noise well below 200 mV to prevent false current
limiting. The sum of the DC level plus the noise peak will de-
termine the current limiting value. Suggested starting values
are C
FILTER = .01µF, RFILTER = 5k.
The required value of R
LIMIT may be calculated by:
R
LIMIT = .2 V / ILIMIT
where R
LIMIT is the required resistor value, and ILIMIT is the
maximum desired current.
SHUTDOWN
The shutdown circuitry makes use of the internal current
limiting circuitry.The two functions may be externally combined
as shown below in Figure 1. R
LIMIT will normally be a very low
value resistor and can be considered zero for this application.
R
SD and RFILTER form a voltage divider for the shutdown signal.
After a suitable noise filter is designed for the current limit
adjust the value of R
SD to give 317 mV of shutdown signal at
pin 10 when the shutdown signal is high. This means pin 10
will reach the 200 mV trip point in about one time constant
withlowoutputcurrentand
less time as output current
increases. The voltage at
pin 10 is referenced to pin
4 (GND). C
FILTER will filter
both the current limit noise
spikes and the shutdown
signal. Shutdown and
current limit operate on
each cycle of the internal
switching rate. As long as
the shutdown signal is high the output will be disabled.
PROTECTIONCIRCUITS
There are two conditions which will latch all the output tran-
sistors off.The first of these conditions is activation of the high
side current limit. Specifically, current in pin 7 (+V
S) is moni-
tored. The DC trip level is about 35A and response time about
5us.As actual currents increase the response time decreases.
The external fault generally associated with this condition is
shorting one of the outputs to ground. However, a load fault
can also activate this high side current limit if the current rise
time is less than the response time of the filter discussed under
“Current Limit”.The second of these conditions is activation of
any of the four output transistor over-temperature sensors at
about 165°C.Ambient temperature, air flow, amplifier mounting
problems and all the previously mentioned high current faults
contribute to junction temperature. When either of these pro-
tection circuits are activated, the root fault must be corrected
and power cycled to restore normal operation.
DEADTIME
There is a dead time between the on and off of each out-
put. The dead time removes the possibility of a momentary
conduction path through the upper and lower transistors of
each half bridge output during the switching interval. During
the dead time all output transistors are off. Noise or flyback
may be observed at the outputs during this time due to the
high impedance of the outputs in the off state. This will vary
with the nature of the load.
ERRORAMPLIFIER
The internal error amplifier is an operational amplifier. For
highest loop accuracy it is best to configure the op amp as an
integrator (See Figure 2). Feedback can be adjusted with ap-
propriate poles and zeroes to properly compensate the velocity
loop for optimum stability.
The op amp is operated from a single supply voltage gener-
ated internally. The non-inverting input of the op amp does not
have a common mode range which includes ground. R2 and
R7 are used with the reference voltage provided at pin 5 to
bias the non-inverting input to +5 volts, which is approximately
half of the voltage supplied internally to the op amp. Similarly,
R1 and the parallel combination of R5 R6 are selected to bias
the inverting input also at +5 volts. Resistors R1 R2 must be
matched. Likewise the parallel combination of R5 R6 must
be matched with R7. The source impedances of the tach
and the signal source may affect the matching and should be
considered in the design.
10
8
SHDN/FILTER
ISENSE
RFILTER
RLIMIT
CFILTER
RSD
SHUTDOWN
SIGNAL
FIGURE 1.
REF OUT
R2
R1
R4
C2
R6
R7
R5
R3
C1
E/A OUT
–IN
+IN
1
2
3
5
TO TACH
SIGNAL
FIGURE 2.
.1µF


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