9

LT1684

For applications that are extremely output ripple sensitive,

additional carrier rejection can be accomplished by modi-

fying the output filter/amplifier characteristics such as

implementing elliptical filter characteristics with a lower

cutoff frequency or implementation of additional poles.

Filter Design and Component Selection

The ring tone information represented in the low fre-

quency component of the input PWM signal is retrieved

using an active filter. This filter also generates the appro-

priate low frequency gain required to produce the high

voltage output signal and references the output to ground

(or other system reference). The frequency and gain

characteristics of this circuit element are both configurable

by the appropriate choice of external passive filter ele-

ments. Because of the active tracking supply mode of

operation, conventional active filter topologies cannot be

used. Most amplifier/filter topologies can, however, be

“transformed” into active tracking supply topologies.

A conventional amplifier circuit topology can be “trans-

formed” into an active tracking supply amplifier circuit by:

a) Inverting the amplifier signal polarity (swap amplifier +

and – connections) and input source polarity.

b) Referencing all signals to the output except the feed-

back elements, which are referenced to ground (swap

output and ground).

A variety of amplifier/filter configurations can be realized

using the transformation technique. A 2-pole filter is

generally adequate for most ringer applications. Due to the

relative simplicity of infinite-gain Multiple Feedback (MFB)

configurations, these filters are good candidates for ringer

applications. Component selection and active tracking

supply transformation will be described for the following

2-pole MFB infinite-gain lowpass filter.

+

–

R1

R2

LOAD

+

–

R1

TRANSFORMATION

Conventional Amplifier Configuration

Active Tracking Supply Amplifier

Lowpass Mulitple Feedback Active Filter

Active Tracking Supply Lowpass

Multiple Feedback Filter

R2

LOAD

TRANSFORMATION

+

–

R1

R3

C1

LOAD

C2

R2

R3

R1

1684 F01

LOAD

C2

C1

R2

+

–

APPLICATIO S I FOR ATIO