11

LT1684

–

+

+

–

1684 F4a

Active Filter Tuned Oscillator Block Diagram

The LT1684 can be implemented easily into a telephone

ringer circuit based on the active filter tuned oscillator

topology, eliminating the need for a user-supplied PWM

input signal. The LT1684’s active filter amplifier can be

used as a high-Q bandpass filter element by configuring

it as an active tracking supply bandpass. The LT1684’s

controlled output receiver/buffer is also convenient for

use as the hard limiter. Because the LT1684 receiver/

buffer requires a true differential input for proper opera-

tion, a dual comparator IC such as the LT1017 must be

bootstrapped along with the LT1684 to provide differen-

tial control signals. The LT1017 and LT1684 receiver/

buffer combine to create a high gain hard limiter whose

output is controlled to

±1.25V. The LT1684 active

bandpass filter is then connected as a positive feedback

element with the limiter component, which completes

the active filter tuned oscillator topology.

The active bandpass filter circuit is easily configured using

a basic MFB bandpass configuration, however, the active

tracking supply technique used by the LT1684 requires

“transformation” of this topology. This “transformation”

swaps the amplifier signal polarity, references all signals

to the output, and references all feedback elements to

ground as described previously in the Filter Design and

Component Selection section.

The design equations for the active tracking bandpass

filter are the same as the pretransformation MFB topology,

Example:

Conditions: Output peak voltage = 95V

Ring frequency = 20Hz

Bandpass Q = 9.4

A square wave with peak amplitude A has a fundamental

component with amplitude 4A/

π, where A = 1.25V. There-

fore, the desired filter’s bandpass gain

1.25/

π) ~ 60. Given capacitor values C = 0.22µF (a conve-

nient value) and desired filter characteristics of: Q = 9.4,

sponse time of the output signal can be adjusted by simply

This produces a high voltage, high quality 20Hz sine wave

at the filter output with a peak amplitude of 95V. Differen-

tial amplitude and frequency characteristics are achieved

by simply changing a few resistor values. The output of the

LT1684 is internally current limited to a minimum of

±100mA peak, allowing this ring tone generation circuit to

be used with loads up to 7 REN with no degradation of the

output waveform.

–

+

+

–

1684 F5b

Bandpass MFB Filter

Active Tracking Bandpass MFB Filter

1684 F03

APPLICATIO S I FOR ATIO