Power Supply 1.2V Regulator - Bose Companion 5 Service Manual

Power Supply 1.2V Regulator

1.2V
The processing core of the DSP is the sole consumer of the 1.2 volt power supply. Its relatively high current
consumption requires efficient power conversion. The Texas Instruments TPS40055, U50, provides the PWM
control functions for a synchronous-rectifier power circuit. U51 is a dual N-channel FET used as switch transistor
and synchronous rectifier for the buck regulator circuit. Inductor L50 and filter capacitor C60 form the filter for the
+1.2V supply. R57 and R60 establish the DC operating voltage for the output. R56, R59, C62, C63 and C64
provide the compensation for the feedback path.
C59 filters the 10 volt reference internal to U50 which is used to control the switching of the transistor drive
signals. C57 provides the boot-strap (boost) potential to drive the high-side switch. C54 filters the 5 volt reference
also generated internally to U50.
R52 sets the operating frequency of the PWM circuit of approximately 100kHz. Resistor R52 provides a
reference to the input voltage which is used by U20 provide feed-forward input voltage compensation. C55
shapes the soft-start characteristics of the regulator.
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