TS34119

Low Power Audio Amplifier

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Version: B13

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

≈ 0.4V above ground, and to

within

≈1.3V below VCC, at the maximum current. See Figure 18 and 19 for VOH and VOL curves. 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 Vin (pin 4) and through Rf, forcing VO1 to shift negative by an amount equal to (Rf

× IIB), Vo2 is shifted posited

an 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 internal offset voltages of the amplifiers. The bias current is constant with respect to VCC.

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 the Electrical Characteristics as RFC1 and RFC2). In addition to providing filtering, C1 and C2 also

affect the turn-on time of the circuit at power-up, since the two capacitors must charge up through the internal 50K

and 125K resistors. The graph of Figure 1 indicates the turn-on time upon application of VCC of +6V. The turn-on

time is

≈ 60% longer for VCC =3V, and ≈ 20% less for VCC =9V. Turn-off time is <10uS upon removal of VCC.

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

Logic “0” (0V to 0.8V), the TS34119 is enabled for normal operation. When pin 1 is a Logic ”1” (2V to VCC), the IC

is 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. Both times are independent of C1, C2, and VCC. When the TS34119 is disabled, the

voltage at FC1 and FC2 do not 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 be within the range of VCC and Ground.

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.