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LT1684

Encoded PWM Signal Input Basics

The LT1684 accepts a user-supplied PWM carrier that

represents the desired output ring tone signal. This PWM

input is normalized by the LT1684 such that ring tone

output amplitudes can be accurately encoded into the

PWM input.

The LT1684 accepts a differential input to maximize rejec-

tion of system transients and ground noise. If no differen-

tial signal is readily available from the PWM controller, a

simple inverter/buffer block can be used to create the

differential signal required.

Each differential input is internally connected through a 5k

series resistor to back-to-back isolation-base diodes. These

devices internally clamp the differential input signal to

±100mV greater than the input comparator hysteresis

range. The input comparator toggles with a differential

hysteresis equal to that of a standard diode forward

voltage (0.7V nominal). As such, the differential imped-

ance of the input remains high throughout the input

hysteresis region, then reduces to a nominal value of 10k

(7k minimum) as the input is overdriven beyond the

comparator input threshold. A minimum differential input

of 1.6V is specified to assure valid switching.

The PWM signal can be visualized in terms of instanta-

neous ring tone amplitude, normalized to the LT1684

amplitude reference. For a given desired output voltage

VOUTN, the input pulse train required follows the relation:

VOUTN = 2 • VREF • (DC – 0.5), or

DC = [VOUTN / (2 • VREF)] + 0.5, where:

VREF = 1.25V normalized peak voltage

DC = PWM input duty cycle

A 10% to 90% duty cycle range is a practical limit for a

10kHz input carrier. This corresponds to normalized sig-

nal amplitude of

±1V.Dutycyclesexceedingthisrangecan

cause increased output signal distortion as signal energy

is lost due to finite rise and fall times becoming a signifi-

cant percentage of the signal pulses. The associated

reduction in the pulse energy manifests itself as a “soft

clipping” of the output signal resulting in an increase in

harmonic distortion.

The normalized PWM signal is amplified to the desired

output signal level by the active filter/amplifier stage.

Thus, dividing the desired peak output amplitude by the

peak normalized encoded amplitude (VOUT/VOUTN) yields

the required DC gain of the active filter.

System Considerations

Assuming use of a 10% to 90% maximum PWM range, the

peak normalized signal will be:

VPWM(pk) = ± 0.8 • VREF = ±1.0V, and:

VOUT(pk) = VPWM(pk) • Filter DC Gain

Thus, the DC gain of the output filter equals the desired

peak voltage of the output ring tone signal.

The frequency characteristics of the lowpass output filter

must reflect the allowable carrier ripple on the output

signal. For example, a 10kHz carrier system could use a

2-pole Butterworth lowpass with a cutoff frequency of

100Hz. This filter provides 40dB of input signal rejection

at 10kHz yielding 25mVP-P output ripple. If the DC gain of

the output filter/amplifier was 100, the output ripple volt-

age would be riding on a

±100V sine wave, and therefore

be about – 78dB relative to the output ring signal.

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