THS6032_14 Datasheet, PDF (19/35 Pages) Texas Instruments – LOW-POWER ADSL CENTRAL-OFFICE LINE DRIVER

THS6032
www.ti.com....................................................................................................................................................... SLOS233F – APRIL 1999 – REVISED AUGUST 2009
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 maintain 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 with no
degradation in performance of the receive signal coming through the transmission line.
There are a few design considerations that must be observed 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 QS1 through QS4. 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 QS1 through QS4 transistors are not designed for 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, allowing proper transmission line termination.
+VCC(H)
IBIAS−S1
IBIAS−1
Active
Load
QS1
DGND
QS2
QS3
Q1
1 kΩ
+IN
Pin
To Internal
QS4
Output
Node
Q2
Q3
Q5
−IN
Pin
Q6
Q4
To Output
Drive
Circuitry
IBIAS−S2
IBIAS−2
Active
Load
Shut−Down Circuitry
−VCC(H)
Figure 44. Simplified THS6032 Input Stages
Because the DGND-pin voltage is effectively a noninverting terminal, any signal or voltage fluctuation at this
node is amplified by the THS6032. This could possibly cause a noisy output to appear during shutdown
operation. Figure 45 shows the frequency response of the THS6032 due to an input signal at the DGND terminal.
The maximum DGND voltage signal which the THS6032 will follow linearly during shutdown operation is less
than ±4 V. With this dynamic range capability, it is recommended that the DGND pin be as noise-free as possible
to ensure proper transmission line termination.
Copyright © 1999–2009, Texas Instruments Incorporated
Product Folder Link(s): THS6032
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