SGSC34119 Datasheet, PDF (4/9 Pages) SeCoS Halbleitertechnologie GmbH – Low Power Audio Amplifier

Elektronische Bauelemente
SGSC34119
Low Power Audio Amplifier
DESIGN GUIDELINES
GENERAL
The SGSC34119 is a low power audio amplifier capable of low voltage operation (V_CC = 2.0 V minimum) such as that
encountered in line-powered speakerphones. The circuit provides a differential output (V_O1-V_O2) to the speaker to
maximize the available voltage swing at low voltages. The differential 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≧80 dB (at f 100 Hz), and the closed loop gain is set by external resistor R_f and R_i. The
amplifier is unity gain stable, and has a unity gain frequency of approximately 1.5 MHz. In order to adequately cover the
telephone voice band (300 Hz to 3400 Hz), a maximum closed loop gain of 46 is recommended. Amplifier #2 is
internally set to a gain of-1.0(0dB).
The outputs of both amplifiers are capable of sourcing and sinking a peak current of 200 mA. The outputs can
typically swing to within≅ 0.4 V above ground, and to with≅1.3 V below V_CC, at the maximum current. See Figures 17
and 18 for V_OH and V_OL curves.
The output dc offset voltage (V_O1-V_O2) is primarily a function of the feedback resistor (R_f), and secondarily due to the
amplifiers' input offset voltages. The input offset voltage of the two amplifiers swill 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 V_IN (Pin 4) and
through Rf, forcing V01 to shift negative by an amount equal to [R_f x I_IB]. VO2 is shifted positive an equal amount. The
output offset voltage, specified in the Electrical Characteristics, is measured with the feedback resistor shown in the
Typical Application Circuit, and therefore takes into account the bias current as well as internal offset voltages of the
amplifiers. The bias current is constant with respect to V_CC.
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
Figures 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 a function of the capacitors, and the equivalent impedance looking into FC1 and FC2 (listed in the
Electrical Characteristics as R_FC1 and R_FC2).
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 50 kΩ and 125 kΩ resistors. The graph of Figure 1 indicates the turn-on
time upon application of V_CC of + 6.0 V. The turn-on time is ≅60% longer for V_CC = 3.0 V, and ≅20% less for V_CC = 9.0V.
Turn-off time is < 10 µs upon removal of V_CC.
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" (0 V to 0.8 V), the SGSC34119 is enabled for normal operation. When Pin 1 is at a Logic "1" (2.0 V to V_CC V),
the IC is disabled. If Pin 1 is open, that is equivalent to a Logic "0", although good design practice dictates that an input
should never be left open. Input impedance at Pin 1 is a nominal 90 kΩ. The power supply current (when disabled) is
shown in Figure 19.
Muting, defined as the change in differential gain from normal operation to muted operation, is in excess of 70 dB.
The turn-off time of the audio output, from the application of the CD signal, is <2.0 µs, and turn on-time is 12 ms-15 ms.
Both times are independent of C1,C2, and V_CC.
When the SGSC34119 is disabled, the voltages at FC1 and FC2 do not change as they are powered from V_CC. The
outputs, V_O1 and V_O2, change to a 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 V_CC and Ground.
17-Dec-2009 Rev. A
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