STA575 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
Manufacturer
STA575 Datasheet PDF : 20 Pages
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STA575
Interfacing STA575 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 (STA575 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 STA575 (track out) by taking the absolute value of the pre-amp's
output. The outputs of each channel and of each STA575 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 STA575 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 in-
crease 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 ohms) this will swamp the smaller offset currents. It
is helpful to put this kind of passive network on the board with the STA575 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:
Rth heatsink = T----j--m----a---x-P---–--d--T----a---m-----b- – Rth–j ca se – Rth case to heatsink
For example in the STA575 the Rth jc is 1°C / W. R case-to-heatsink with grease is about 0.5°C / W. The max-
imum operating junction temperature is 130°C, which for margin should be derated to 120°C
Buck Regulator Heatsink
The Buck regulator heatsink can be designed in a similar manner and does not change by varying power supply.
In general the efficiency will be in the order of 85%. The thermal impedances from the junction(s) to the heatsink
may be lower and the maximum operating temperature will be higher.
Usually either the sub or the remaining channels are tested at full power. The result is that usually the Buck
heatsink is about ¼ the size of the linear heatsink, but this can be strongly affected by the design.
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