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LUCL7590CAE

Telephone Ringing Driver

Agere -> LSI Corporation 

L7590C Telephone Ringing Driver

Data Sheet

June 1998

Functional Description

Ringer Requirements

This discussion highlights some of the requirements in

the United States and Germany. Beginning with the

telephone set ringer to be powered, the United States

requires 40 Vrms to be delivered to the equivalent of

five parallel telephone sets, or 5 REN (Ringer Equiva-

lence Number), that translates to 1386 Ω + 40 µF. In

Germany, 30 mApeak must be delivered to four parallel

telephone sets. In Germany, 4 REN have an imped-

ance of 450 Ω + 3.4 µF. The difference results from the

impedance of the telephone-set ringers of the two

countries.

Ringing Requirements

In Germany, both single-ended ringing and balanced

ringing can be used. This is not so in the United States,

where some ringers on older equipment have a ground

connection at the telephone set, and the signal lead is

either the Tip or Ring wire of the twisted pair. This has

led to the recommended use of single-ended ringing.

Both administrations allow ringing with a negative dc

level, known as battery-backed ringing, but in the

United States, there is a constraint that this level can-

not be more negative than –75 V.

With the exception of the United States, most world-

wide administrations require an injection of a pure sine

wave. Depending on the country, the distortion require-

ment is in the range of 3% to 5%. In the United States,

the wave does not need to resemble a sine wave, but it

cannot be a square wave. The shape requirement of

the wave in the United States is expressed as a crest

factor (c.f.). This is the ratio of the peak to the rms volt-

age of the signal. The requirement is 1.2 < c.f. < 1.6.

Operation

On the L7590C, the prefiltered output is a high-

voltage replica of the PWM digital signal that is applied

to the CLK input. The signal at OUT is produced by

level shifting and control of the high-voltage transistors.

The level is determined by the magnitude of the nega-

tive dc voltage applied to the output stage transistors.

The output, ideally, should be an exact time replica of

the CLK input to faithfully reproduce the modulating

waveform. This high-voltage replica of the CLK input is

filtered to leave only the low-frequency, high-voltage

sine wave ringing signal. Figure 3, parts A and B, illus-

trates the PWM signal with relation to the modulating

signal frequency. The PWM signal is at a 100 kHz rate

and modulated at the desired ringing signal frequency,

typically 20 Hz to 30 Hz. Laboratory equipment genera-

tion of the required signal is shown in Figure 5. Propa-

gation delays and switching parameters associated

with the control circuitry and output stage transistors

can cause skewing which will produce a distorted out-

put of the modulating waveform. Skewing is illustrated

in Figure 4, parts A, B, and C. The skewing can be

seen as an increase in the duty cycle of the L7590C

output compared to the input signal duty cycle.

Using the test arrangement as shown in Figure 5, the

harmonic distortion of the filtered output of the L7590C

was measured with two different loading circuits. With

the North American 5 REN (1386 Ω + 40 µF) load, the

THD was typically 2.0% to 2.5%. With the Germany

4 REN (450 Ω + 3.4 µF) load, the THD was typically

1.5% to 2.0%. The 25 Hz output of the HP* 3311A

generator showed a typical THD of 1.5% when supply-

ing a signal to the HP 8116A.

A 20 Ω resistor (RILIM) must be added between L1 and

OUT for 5 REN loads.

* HP is a registered trademark of Hewlett-Packard Company.

4

Lucent Technologies Inc.

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