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Network Circuitry
TB9100 Reciter Service Manual
© Tait Electronics Limited January 2006
would conduct, and prevent the charge pump capacitors fully
charging.
With both Q902 and Q903 conducting, capacitors C921 and C922 are
connected in series and stacked on top of the +3.3V supply. The voltage on
the stacked capacitors is peak rectified by diode D911 and reservoir capacitor
C930. With ideal diodes and MOSFETs, the peak rectified voltage would
be 9.9V, ie. three times the +3V3 supply. In practice, the diode voltage drops
and MOSFET conduction losses reduce the attainable voltage to
approximately 8.0V.
5.9.5
+6V Regulator
A +6V supply is used for some of the audio and I/O buffer circuitry.
This has a relatively low current drain, so it is linearly regulated from the
auxiliary supply using U602. For lower supply, noise capacitor C970 filters
the regulator’s internal reference voltage, and output capacitor C923
provides additional smoothing on the output, to minimize noise coupling
into the audio circuitry.
5.9.6
Power Sequencing and Start-up
Both the +3.3V and +1.8V switching regulators incorporate soft-start
circuits to minimize the inrush current surge and hence minimize stresses on
the power components. During start-up, the output of the error amplifier is
also clamped by the soft-start circuit so that the allowable inductor current
rises slowly, minimizing the inrush current to the power supply. The rise
time is controlled by a constant current charging the soft-start capacitors
C913 and C911.
The soft-start circuits also play a role in the fault protection circuitry of
U900. If an over-current condition is detected, the soft-start capacitors are
progressively discharged, lowering the inductor peak current limit. Should
the capacitors discharge sufficiently, the switching regulators will shut down
completely and require that the input power be cycled before they will start
up again. This behavior may be undesirable, eg. during circuit debugging;
two resistors, R912 and R913 (each 2M21), may be optionally fitted to
disable this latching shut-down function.
With devices such as the MPC and DSP using dual supply voltages, it is
imperative that the correct sequencing of these voltages be maintained
during power-up and power-down. This is enforced by clamping diodes
D901, D902, D906 and D907. These prevent the +1.8V or +1.6V supplies
exceeding the +3.3V rail by more than a diode drop. They also limit the
maximum differences between the +3.3V supply and +1.6V and +1.8V
supplies to approximately 2.4V and 1.8V, respectively.
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