Silicon Laboratories integrates a complete analog telephone interface into one low-voltage CMOS

The ProSLIC® from Silicon Laboratories integrates a complete analog telephone interface into one low-voltage CMOS device and offers extensive software programmability to meet many global telephony
requirements and customer specifications. In addition to performing all BORSCHT functions, the Si321x also dynamically generates and controls its own battery voltage, eliminating the need for external battery supplies. Two different battery generation architectures are supported: a BJT/inductor design offering a low-cost battery supply solution, and a MOSFET/transformer design offering increased power efficiency and a wider range of input voltages.

Si321x DC-DC Converter Description
The dc-dc converter dynamically generates the large negative voltages required to operate the linefeed interface. The Si321x acts as the controller for a buck-boost dc-dc converter that converts a positive dc voltage into the desired negative battery voltage. In addition to eliminating external power supplies, this allows the Si321x to minimize power dissipation by dynamically controlling the battery voltage to the minimum required for any given mode of operation.

Understanding the maximum power required by the ProSLIC linefeed circuitry to operate a worst-case specified load is the first step in determining the dc-dc converter design solution. Typically, the ringing state is the highest power consumption state for the SLIC, but in special cases the off-hook state can have the highest.

Undervoltage and Overcurrent Protection
The Si321x dc-dc converter is designed to operate under a specific input voltage and output loading condition. When the input voltage goes too low, there is not enough power from the input to deliver to the output; so, the dc-dc converter may try to draw excessive current in an attempt to deliver power to the output. A similar condition exists when the output ramps up too fast (during power up, transient loading), short loads, or unintentionally overloads the output. To prevent damage to the switching transistor during these abnormal conditions, the Si321x implements an undervoltage and overcurrent mechanism.


MOSFET/Transformer Design DC-DC Converter
The MOSFET/transformer dc-dc converter solution offers higher power efficiency than the BJT/inductor solutions and is the preferred solution for applications using low VDC input voltages.

Power Transformer
A transformer was designed for the Si321x dc-dc converter based on the TDK PC40EF12.6 E core. This transformer has three primary windings connected in series to handle all VDC voltage ranging from 3 V to 35 V (consult the factory for higher VDC). The secondary winding can generates up to 94.5 V VBAT at any in range VDC input.

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