THS6032 Datasheet, PDF (20/31 Pages) Texas Instruments – LOW-POWER ADSL CENTRAL-OFFICE LINE DRIVER

THS6032
LOW-POWER ADSL CENTRAL-OFFICE LINE DRIVER
SLOS233C – APRIL1999 – REVISED MARCH 2000
APPLICATION INFORMATION
shutdown control
There are two shutdown pins which control the shutdown for each amplifier of the THS6032. When the shutdown
pin signals are low, the THS6032 is active. But, when a shutdown pin is high (≥ 2 V), the corresponding amplifier
is turned off. The shutdown logic is not latched and should always have a signal applied to it. To help ensure
a fixed logic state, an internal 50 kΩ resistor to DGND is utilized. An external resistor, such as a 3.3 kΩ, to DGND
may be added to help improve noise immunity within harsh environments. If no external resistor is utilized and
SHDN_X pins are left unconnected, the THS6032 will default to a power-on state. A simplified circuit can be seen
in Figure 43.
+VCC(H)
To Internal
Bias Circuitry
Control
SHDNX
50 kΩ
DGND
–VCC(H)
DGND
Figure 43. Simplified THS6032 Shutdown Control Circuit
shutdown function
The THS6032 incorporates a shutdown circuit to conserve power. Traditionally when an amplifier is placed into
shutdown mode, the input and output circuitry are turned off. This conserves a large amount of power, but the
output impedance will be a very high, typically greater than several kΩ. This situation does not allow for proper
line termination resulting in a severe reduction of the receive signal coming through the transmission line (see
Figure 36).
The THS6032 eliminates this problem. When the SHDN_X pin voltage is greater than 2 V, the THS6032 enters
shutdown mode to conserve power. Unlike the traditional amplifier, the THS6032’s output impedance is typically
0.5 Ω at 1 MHz (see Figure 28). The shutdown mode function results in the proper termination of the line without
degradation in performance of the receive signal coming through the transmission line.
There are a few design considerations in order to fully achieve this type of functionality. To better understand
these design considerations, it is helpful to examine what is happening inside the THS6032. Figure 44 shows
the simplified shutdown components. Notice that there are two similar input stages; the normal input stage
consisting of transistors Q₁ through Q₄ and the shutdown input stage consisting of transistors Q_S1 through Q_S4.
When in shutdown mode, the I_{(BIAS – 1)} and I_{(BIAS – 2)} current sources are turned off. This turns off the normal
input stage of the amplifier. The I_{(BIAS – S1)} and I_{(BIAS – S2)} current sources are then turned on. The shutdown
input stage signals are then fed through the same internal circuitry which the normal input stage drove. This
allows for sinking and sourcing large amounts of current at the output of the THS6032 during shutdown
operation. The Q_S1 through Q_S4 transistors are not designed for the performance like the Q₁ through Q₄
transistors because their only function is to amplify the DC ground reference, DGND. A 1-kΩ resistor connects
internally to the output node of the amplifier, which provides a feedback loop in shutdown mode. This forces the
output impedance to become very small, making for proper transmission line termination.
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