AN45 Datasheet, PDF (1/14 Pages) Silicon Laboratories – DESIGN GUIDE FOR THE Si3210/15/16 DC-DC CONVERTER

AN45
DESIGN GUIDE FOR THE Si3210/15/16 DC-DC CONVERTER
Introduction
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. This application note gives
specific guidance in determining dc-dc converter power
requirements and selecting component values for each
of the dc-dc converter architectures.
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.
Input
+
V_DC
–
DC-DC
Converter
Output
+
TIP
+
V_BAT
V_TR
–
–
RING
Linefeed Circuitry
Twisted
Pair Line
R_LINE
2
R_LINE
2
Telephone
Hook SW.
Ringer
RDC
Figure 1. Linefeed Power Diagram
Power Output Requirement
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. Figure 1 defines the linefeed
circuit and load circuit in basic blocks of circuitry.
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. Guidance in
calculating each of these states is offered in this
section.
The ringer impedance of one telephone is defined as an
8 µF cap in series with a 6930 Ω resistor. This is
approximately the same impedance as 7000 Ω at 20 Hz
and is defined as 1 REN (ringing equivalence number).
Since there can be N number of telephones connected
in parallel to the TIP and RING lines, the equivalent
impedance of the parallel ringers can be computed as
the following (NREN is limited to 1 to 5):
R_NREN
=
---⁷---⁰----⁰---⁰----
NREN
During ringing, the TIP-to-RING peak voltage, V_{TR_PK},
is the sum of the rms voltage drop across the ringer
circuit, V_RINGrms, the line resistance, and the internal
source resistance of 160 Ω.
V_{TR_PK}
=
V_{-----R---I--N----G----r--m----s----}×_---------2_--
7000 ⁄ NREN
×


---⁷---⁰----⁰---⁰----
NREN
+
R_LINE
+
R_{s}
Rev. 0.5 7/03
Copyright © 2003 by Silicon Laboratories
AN45-050