TS34119

5-11

2004/09 rev. A

Design Guideline

GENERAL

The TS34119 is a low power audio amplifier capable of low voltage operation (Vcc=2.0V minimum) such as that

encountered in line-powered speakerphones. The circuit provides a differential output (VO1-VO2) to the speaker to

maximize the available voltage swing at low voltages. The different gain is set by two external resistors. Pins FC1

and FC2 allow controlling the amount of power supply and noise rejection, as well as providing alternate inputs to the

amplifiers. The CD pin permits powering down the IC for muting purposes and to conserve power.

AMPLIFIERS

Referring to the block diagram, the internal configuration consists of two identical operational amplifiers. Amplifier #1

has an open loop gain of

≥80Db (at f≤100Hz), and the closed loop gain is set by external resistor Rf and Rj. The

amplifier is unity gain stable, and has a unity gain frequency of approximately 1.5MHz. In order to adequately cover

the telephone voice band (300Hz to 3.4kHz), a maximum closed loop gain of 46dB is recommended. Amplifier #2 is

internally set to gain of –1.0 (0dB). The outputs of both amplifiers are capable of souring and sinking a peak current

of 200mA.The outputs can typically swing to within

The output dc offset voltage (VO1-VO2) is primarily a function of the feedback resistor (Rf), and secondarily due to

the amplifiers’ input offset voltages. The input offset voltage of the two amplifiers will generally be similar for a

particular IC, and therefore nearly cancel each other at the outputs. Amplifier #1’s bias current, however, flows out of

equal amount. The output offset voltage, specified in the Electrical Characteristics is measured with the feedback the

feedback resistor shown in the Typical Application Circuit, and therefore takes into account the bias current as wells

FC1 AND FC2

Power supply rejection is provided by the capacitors (C1 and C2 in the typical Application Circuit) at FC1 and FC2.

C2 is somewhat dominant at low frequencies, while C1 is dominant at high frequencies, as shown in the graphs of

Figure 4 to 7. The required values of C1 and C2 depend on the conditions of each application. A line powered

speakerphone, for example, will require more filtering than a circuit powered by a well regulated power supply. The

amount of rejection is function of the capacitors, and the equivalent impedance looking into FC1 and FC2 (listed in

time of the circuit at power-up, since the two capacitors must charge up through the internal 50K and 125K resistors.

CHIP DISABLE

The chip Disable (pin 1) can be used to power down the IC to conserve power, or for muting, or both. When at a

disabled. If pin 1 is open, that is equivalent to Logic “0” although good design proactive dictates that an input should

never be left open. Input impedance at pin 1 is a nominal 90KΩ. The power supply current (when disabled) is shown

in Figure 15.

Muting, defined as the change in differential gain from normal operation to muted operation, is in excess of 70dB.

The turn-off time the audio output, from the application of the CD signal, is <2uS, and turn on-time is 12 mS-15mS.

change as they are powered from Vcc. The outputs, VO1 and VO2, change to high impedance condition, removing

the signal from the speaker. If signals from other sources are to be applied to the outputs (while disabled), they must

LAYOUT CONSIDERATIONS

Normally a snobbier is not needed at the output of the TS34119, unlike many other audio amplifiers, However, the

PC board layout, stray capacitances, and the manner in which the speaker wires are configured, may dictate

otherwise. Generally, the speaker wires should be twisted tightly, and not more than a few inches in length.