sta530 STMicroelectronics, sta530 Datasheet - Page 12

• Manufacturer Part Number: sta530
• Description: 4 X 50w Stereo Bash Power Amplifier
• Manufacturer: STMicroelectronics

STA530
System Description & Operating Rules
SAM261 is a BASH® 6.1 amplifier ( 6 x 50W, 1 x 100W) implementation utilizing the STA530 Integrated Circuit.
Specifically designed for multi-channel implementation in DVD - HTIB systems, Multi-Media systems, Mini and
Micro systems and Set Top boxes.
SAM261 is dimensioned to provide the maximum Output Power (THD=10 %) on two channels and instanta-
neously and 1/3 max Pout on the remaining Outputs, or 1/8 of max Pout continuous; this rule is important to
define the main Power Supply size (+50V).
Buck Regulator Description
The function of the buck regulator is to efficient convert efficiently an input voltage to a lower voltage by adjust-
ing the ratio of the switching transistor's on-time to off-time. The resulting waveform is averaged by the output
filter to recover an analog signal.
In the BASH amplifier this output is in effect split in half by centering it on the audio ground to provide CD+ and
CD- rails.
To avoid the need for a high side driver for the transistor switch in the buck regulator the buck circuit recom-
mended has the switch in the return path. Hence the gate drive circuit (part of the STPB01) is referenced to the
negative return of the main supply that provides power for the buck regulator.
Interfacing STA530 to STPB01 (Feedback circuit)
This circuit produces a control signal current that is fed back to the STPB01 digital controller. The network used
in this example compares the track signal (STA530 track out) to a fixed ratio of buck regulator's output (CD+)
using a transistor. This method is effective because the controller's reference is the negative of the main DC
supply, which is not referenced to audio ground.
The tracking signal is generated inside the STA530 (track out) by taking the absolute value of the pre-amp's
output. The outputs of each channel and of each STA530 are then tied together in a diode-oring arrangement.
This means that the highest of any given output is the output that determines the tracking signal.
The absolute value circuit inside the STA530 has gain. This makes it possible to use an RC network and a re-
sistor divider to create a phase shift in the tracking signal at higher frequencies. This is also useful in optimizing
the alignment of the buck regulator's output with the output signal of the bridge amplifier at high frequency
This circuit first converts the buck switch current to a peak voltage. The control current is then converted to a
voltage (using a resistor) and added to the peak voltage. By doing this, the buck is better able to maintain the
desired headroom over a wide load range and output level.
Centering Network for CD + & CD- Rails
The power rail of a bridge amplifier has no current flowing through the ground node, as the load is not connected
to ground. However there are several different small sources of dynamic and continuos ground currents flowing
from either CD+ or CD- to support the function of various things such as the control signal to the STABP01 con-
troller. The centering network prevents these currents from shifting the CD+/- rails away from center i.e. away
from a symmetric split of the buck's output about ground. This is critical, even a small centering error requires
an increase in headroom which results in a significant drop in output losses. In its simplest form the centering
network could be a resistor divider from CD+ to CD- with its center tied to ground. As long as the impedance is
low enough (for example 200 ) this will swamp the smaller offset currents. It is helpful to put this kind of passive
network on the board with the STA530 devices to help when testing this board on its own.
Power Amplifier Heatsink requirements
The heatsink requirements are dependent on several design goals. However there are two common references:
Pink noise at 1/8 of full power, all channels loaded. This would approximate a system with all channels repro-
ducing music at full volume with clipping occurring only occasionally. The second would be full power at 1kHz
for 5 minutes after a one hour pre-soak at 1/8 power.
The worse of these two is the full power test. A conservative approach is to assume that the heatsink would
come to thermal equilibrium after 5 minutes. Thus the Rth of the heatsink can be determined by:
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