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STA575
100+100W STEREO BASH POWER AMPLIFIER
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STA575February 2003 MONOCHIP BRIDGE STEREO AMPLIFIER
ON BASH® ARCHITECTURE 80+80W OUTPUT POWER @ RL = 4/8 Ω,
THD = 0.5% 100+100W OUTPUT POWER @ RL = 4/8 Ω,
THD = 10% HIGH DYNAMIC PREAMPLIFIER INPUT
STAGES EXTERNAL PROGRAMMABLE FEEDBACK
TYPE COMPRESSORS AC COUPLED INPUT TO CLASS AB BRIDGE
OUTPUT AMPLIFIER PRECISION RECTIFIERS TO DRIVE THE
DIGITAL CONVERTER ON-OFF SEQUENCE/ TIMER WITH MUTE
AND STANDBY PROPORTIONAL OVER POWER OUTPUT
CURRENT TO LIMIT THE DIGITAL
CONVERTER ABSOLUTE POWER BRIDGE OUTPUT
TRANSISTOR POWER PROTECTION ABSOLUTE OUTPUT CURRENT LIMIT INTEGRATED THERMAL PROTECTION POWER SUPPLY OVER VOLTAGE
PROTECTION FLEXIWATT POWER PACKAGE WITH 27 PIN
BASH® LICENCE REQUIRED
DESCRIPTION The STA575 is a fully integrated power module de-
signed to implement a BASH® amplifier when used
in conjunction with STABP01 digital processor.
PRODUCT PREVIEW100+100W STEREO POWER AMPLIFIER
BLOCK DIAGRAM
STA575 2/20
DESCRIPTION (continued)Notice that normally only one Digital Converter is needed to supply a stereo or multi-channel amplifier system,
therefore most of the functions implemented in the circuit have summing outputs
The signal circuits are biased by fixed negative and positive voltages referred to Ground. Instead the final stag-
es of the output amplifiers are supplied by two external voltages that are following the audio signal . In this way
the headroom for the output transistors is kept at minimum level to obtain a high efficiency power amplifier.
The Compressor circuits, one for each channel, performs a particular transfer behavior to avoid the dynamic
restriction that an adaptive system like this requires. To have a high flexibility the attack / release time and the
threshold levels are externally programmable. The tracking signal for the external digital converter is generated
from the Absolute Value block that rectifies the audio signal present at the compressor output. The outputs of
these blocks are decoupled by a diode to permit an easy sum of this signal for the multichannel application. The
output power bridges have a dedicated input pin to perform an AC decoupling to cancel the compressor output
DC offset. The gain of the stage is equal to 4 (+12dB). A sophisticated circuit performs the output transistor pow-
er detector that , with the digital converter, reduces the power supply voltage . Moreover, a maximum current
output limiting and the over temperature sensor have been added to protect the circuit itself. The external volt-
age applied to the STBY/MUTE pin forces the two amplifiers in the proper condition to guarantee a silent turn-
on and turn-off.
ABSOLUTE MAXIMUM RATINGSNote 1: VCD- must not be more negative than -Vs and VCD+ must not be more positive than +VS
Note 2: All pins withstand ±2KV ESD but not pin 11
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STA575
THERMAL DATA
OPERATING RANGE
PIN CONNECTIONNote: Slug connected to pins n. 1 and 27
STA575 4/20
PIN FUNCTION
5/20
STA575
ELECTRICAL CHARACTERISTCS (Test Condition: Vs+ = 28V, Vs- = -23V, VCD+ = 20V, VCD- = -20V, RL =8Ω, external components at the nominal value f = 1KHz, Tamb = 25°C unless otherwise specified This value is due to the thermal noise of the external resistors Rr and Ri.
STA575 6/20
ELECTRICAL CHARACTERISTCS (continued)
7/20
STA575
FUNCTIONAL DESCRIPTIONThe circuit contains all the blocks to build a stereo amplifier. Each single channel is based on the Output Bridge
Power Amplifier, and its protection circuit. Moreover, the compression function and a signal rectifier are added
to complete the circuit.
The operation modes are driven by The Turn-on/off sequence block. In fact the IC can be set in three states by
the Stby/mute pin:
Standby ( Vpin < 0.8V), Mute (1.6V < Vpin < 2.5V), and Play (Vpin > 4V).
In the Standby mode all the circuits involved in the signal path are in off condition, instead
in Mute mode the circuits are biased but the Speakers Outputs are forced to ground potential.
These voltages can be get by the external RC network connected to Stby/Mute pin.
The same block is used to force quickly the I.C. In standby mode or in mute mode when the I.C. dangerous
condition has been detected. The RC network in these cases is used to delay the Normal operation restore.
The protection of the I.C. are implemented by the Over Temperature, Unbalance Ground, Output Short circuit,
Under voltage, and output transistor Power sensing as shown in the following table:
Table 1. Protection ImplementationSee the POWER PROTECTION paragraph for the details
CompressionAn other important function implemented, to avoid high power dissipation and clipping distortion, is the Com-
pression of the signal input. In fact the preamplifier stage performs a voltage gain equal to 5, fixed by Ri and Rr
external resistor, but in case of high input signal or low power supply voltage, its gain could be reduced of 26dB.
This function is obtained with a feedback type compressor that , in practice, reduces the impedance of the ex-
ternal feedback network. The behavior of compression it's internally fixed but depends from the Audio input volt-
age signal level, and from the Threshold voltage applied to the Threshold pin. The attack and release time are
programmable by the external RC network connected to the Att_Rel pins.
The constraints of the circuit in the typical application are the following:
Vthreshold range = -5 to 0
Vin peak max = 8V
Vout peak max = 10V
STA575 8/20
Gain without compression (G) = 5
Max Attenuation ratio = 26 dB
The following graph gives the representation of the Compressor activation status related to the Vthreshold and
the input voltage. The delimitation line between the two fields, compression or not, is expressed by the formula :
Where G is the preamplifier gain without compression.
In the compression region the gain of the preamplifier will be reduced
(G = 2·Vthreshold/Vin) to maintain at steady state the output voltage equal 2*|Vthreshold| .
Instead in the other region the compressor will be off (G = 5).
The delimitation line between the two fields can be related to the output voltage of the preamplifier: in this case
the formula is :
Figure 1. Compressor activation fieldThe relative attenuation introduced by the variable gain cell is the following :
The total gain of the stage will be:
Gdb = 20log5 + Attenuation
The maximum input swing is related to the value of input resistor, to guarantee that the input current remain
under Iin_Max value (1 mA). Vthreshold 200mV+ ()⋅-------- --------------- ---------------- --------------- --------------------out 2 Vthreshold 200mV+ ()⋅=
Attenuation 20 2---log Vth 200mV+ () in_peak
--------------- --------------- ------------⋅=i in_peak in_max -------------------->
9/20
STA575
Figure 2. Compressor attenuation vs. input amplitude
ABSOLUTE VALUE BLOCKThe absolute value block rectifies the signal after the compression to extract the control voltage for the external
digital converter. The output voltage swing is internally limited, the gain is internally fixed to 14.
The input impedance of the rectifier is very high , to allow the appropriate filtering of the audio signal before the
rectification (between Out_pre and Trk pins).
OUTPUT BRIDGEThe Output bridge amplifier makes the single-ended to Differential conversion of the Audio signal using two
power amplifiers, one in non-inverting configuration with gain equal to 2 and the other in inverting configuration
with unity gain. To guarantee the high input impedance at the input pins, Pwr_Inp1 and Pwr_Inp2, the second
amplifier stages are driven by the output of the first stages respectively.
POWER PROTECTIONTo protect the output transistors of the power bridge a power detector is implemented (fig 3).
The current flowing in the power bridge and trough the series resistor Rsense is measured reading the voltage
drop between CD+1 and CD+. In the same time the voltage drop on the relevant power (Vds) is internally mea-
sured. These two voltages are converted in current and multiplied: the resulting current , Ipd, is proportional to
the instantaneous dissipated power on the relevant output transistor. The current Ipd is compared with the ref-
erence current Ipda, if bigger (dissipated power > 32W) a current, Iprot, is supplied to the Protection pin. The
aim of the current Iprot is to reduce the reference voltage for the digital converter supplying the power stage of
the chip, and than to reduce the dissipated power. The response time of the system must be less than 200μSec
to have an effective protection. As further protection, when Ipd reaches an higher threshold (when the dissipated
value is higher then 60W) the chip is shut down, forcing low the Stby/Mute pin, and the turn on sequence is
restarted.
STA575 10/20
Figure 3. Power Protection Block DiagramIn fig. 3 there is the power protection strategy pictures. Under the curve of the 32W power, the chip is in normal
operation, over 60W the chip is forced in Standby. This last status would be reached if the digital converter does
not respond quikly enough reducing the stress to less than 60W.
The fig.4 gives the protection current, Iprot, behavior. The current sourced by the pin Prot follows the formula:
for Pd < Pd_av_th the Iprot = 0
Independently of the output voltage, the chip is also shut down in the folowing conditions:
When the currentthrough the sensing resistor, Rsense, reaches 7A (Voltage drop (CD+) - (CD+1) = 700mV).
When the average junction temperature of the chip reaches 150°C.
When the ground potential differ from more than 5V from the half of the power supply voltage, ((CD+)-(CD-))/2
When the sum of the supply voltage |Vs+| + |Vs-| <20V
The output bridge is muted when the average junction temperature reaches 130°C.prot Pd_av_th)–( 5104–⋅⋅
1.25V ------------------------------------------------------------------≡
