TLE2301 Datasheet, PDF (18/22 Pages) Texas Instruments – EXCALIBUR 3-STATE-OUTPUT WIDE-BANDWIDTH POWER OPERATIONAL AMPLIFIER

TLE2301
EXCALIBUR 3-STATE-OUTPUT WIDE-BANDWIDTH
POWER OPERATIONAL AMPLIFIER
SLOS131 – DECEMBER 1993
APPLICATION INFORMATION
series resistor
The series resistor, R_S, is included to limit the turn-on current, the amplifier’s offset current, and the signalling
current through the filter. With dual supply rails, there is always a potential problem of large turn-on currents as
the amplifier powers up. If one supply rail turns on before the other, the output of the TLE2301 amplifier could
saturate near to the applied supply rail, causing a large current to flow through the transformer winding
(R_winding = 0.1 Ω for the P2820 transformer). The power supply needs to be of sufficient rating to ensure that
its rails could rise to the minimum operating voltage of the amplifier, at which point the amplifier is ensured to
have returned to stable operation.
With a series resistor of 3.3 Ω and assuming the output saturates at the maximum peak-to-peak voltage
excursion of 3 V, this turn-on current is limited to less than the device’s 1-A rating ( I_transient = 3 V / 3.3 Ω
= 0.91 A). Further reduction of this turn-on current by raising the value of the series resistor deteriorates the
filter’s performance into low signalling impedances on the mains network.
Alternatively, this turn-on current could be blocked by means of a series capacitor, but for this frequency band
the capacitor has to be large in value ( ≥ 3.3 µF ) so as not to adversely affect the filter. A nonpolarized capacitor
of this value is relatively expensive, and the resistor is still required to fulfill other functions.
Another way of preventing overcurrent at power up is to use the TLE2301 3-state mode. As the TRS2 control
line is intended to be tied to the microprocessor’s 0-V rail, the TRS1 control line must be taken high to activate
the 3-state mode, which implies that the positive rail is required to turn on first. Other schemes could be devised
to take TRS2 below the 0-V rail until the power supply has stabilized if the negative rail turns on first. Instead
of relying on a definite power-supply sequence or elaborate control circuitry, it is simpler to limit the current either
with a series resistor or capacitor.
The second function of the series resistor is to limit the dc current flow through the transformer winding due to
the dc offset at the amplifier’s output, which is caused by its input offset voltage. For a worst case input offset
of 20 mV, the output offset is also 20 mV as the dc gain of the circuit is unity. Offsets due to input bias currents
are negligible since the values of the gain-setting resistors are low. The dc current through the transformer is
therefore less than 7 mA (20 mV/3.3 Ω). This low level of dc current does not appreciatively increase the power
dissipation of the amplifier or noticeably diminish the harmonic performance of the transformer.
The final function of the series resistor is to limit the signalling current in the event that the mains impedance
might appear as solely reactive; i.e., without a resistive component. As a rough estimate, the peak signal current
+ + ^{ǒ Ǔ} + ^{from the amplifier is:}
VO _(PP)
IOM
RS
5.5 V
2
3.3 Ω
833 mA
where:
+ VO _(PP)
+ IOM
Peak-to-peak output voltage swing
Peak-output-signalling current from amplifier
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