ia3222 integration, ia3222 Datasheet - Page 26

• Manufacturer Part Number: ia3222
• Description: Ia3222/ia3223 Ez Daa? Chipset With Analog Interface
• Manufacturer: integration

IA3222/IA3223
Component Discussion
The application schematic (Figure 23) is intended as a high-density surface-mount solution. When using through-hole components,
some changes may be possible in the design. Please contact IAI support for these issues.
The main hook switch is composed of three parallel NPN transistors (Q2–Q4) and three emitter ballasting resistors (R5–R7). This
structure is necessary when using low cost, generic 300 V telephone application transistors in order to keep these transistors out of
their quasi-saturation region at high loop currents.
With the use of qualified transistors, it is possible to reduce the transistor count on the NPN hook switch but may not be cost
effective or reduce board area. Although 300V rated telephone hook-switch application transistors only cost pennies, they work best
as switches and tend to behave poorly when operated in their linear region at currents above 20 mA, with only a few volts drop from
collector to emitter. This is due to quasi-saturation effects that reduce their frequency response to only a few MHz. Even the SPICE
models for these devices designed over 30 years ago are often inadequate in these linear operating regions. If too few of these
transistors are used in the hook circuit, stability issues at higher loop currents may occur. As an alternative, more recently designed
high-voltage NPN transistors in larger packages (for power dissipation) can be used to construct either a double or single hook switch
but may easily cost more than the three-parallel NPN solution. Without extensive characterization, IAI does not recommend
alternative hook-switch transistor solutions except the one provided for legacy TBR21 support (see below).
High-density ceramic capacitors can cause both microphonic and distortion problems due to piezoelectric effects (mechanical strain)
and voltage coefficients (changes in capacitance with voltage). In general, the higher a ceramic’s dielectric constant, the greater its
voltage-dependency and piezoelectric effects. The lower the voltage rating on a ceramic capacitor, then the higher the voltage
gradient across the dielectric. This means that a capacitor of lower voltage rating and same dielectric material will have worse
voltage distortion and electromechanical effects.
IAI has identified which capacitors are most likely to be critical with respect to these issues on the application schematic. It is
possible to use different dielectric material for these capacitors if the devices are physically larger and the customer can evaluate
the board for possible increased distortion or electro mechanical effects in the end product.
Legacy TBR21 support is possible but not recommended since so few countries require it and the standard has been superseded.
Current limiting is now obsolescent if not obsolete. In order to meet TBR21 current limit, a DAA needs to dissipate 2 W safely. In the
IA3222/3223 application, about half of this power is dissipated in R3, R4, R10 and R11 and the other half in the NPN transistor.
Figure 25: IA3222/3223 evaluation board
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