THS6032 Datasheet, PDF (22/31 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 function (continued)
The second design consideration is due to transistors Q₅ and Q₆. These transistors ensure the +IN to – IN
voltage separation is less than a V_BE drop (about 0.7 V). This protects the other transistors, Q₁ to Q₄, from
saturating during fast transients. Transistors Q₅ and Q₆ also enhance the slew rate capabilities of the THS6032.
When a fast transient is applied to the input, these transistors will quickly apply the currents to the active load
stages. A design issue with this setup is that while in shutdown mode, a large enough signal being applied to
the input pins may turn on these transistors. Once the input voltage differential between the +IN and – IN pins
reaches ± 0.7-V, transistors Q₅ and Q₆ turn on applying the difference signal to the rest of the amplifier circuitry.
Because these two transistors are designed for much higher performance levels than the shutdown circuitry
transistors (Q_S3 and Q_S4), they will become dominant and the difference input signal will be utilized instead of
the DGND signal. Because the external negative feedback resistor path is still connected around the amplifier,
this difference input signal will be amplified just like a normal amplifier is designed to do (see Figure 46). As long
as the +IN and – IN input signals are kept below ± 0.7 V, the isolation from input-to-output is very high as shown
in the Shutdown Isolation vs Frequency graphs (see Figures 30 and 31).
To ensure proper shutdown functionality of the THS6032, it is important to keep the DGND voltage noise-free.
Additionally, the +IN and – IN signals should be limited to less than ± 0.7 V during shutdown mode. This will
ensure proper line termination functionality while conserving power.
SHUTDOWN FEEDTHROUGH
7
6
G=5
5
4
G=2
G = +1;
G = –1
3
2
VCC(H)= ± 15 V
VCC(L)=± 5 V
1
RL = 25 Ω
VSD = 5 V
0
0
2
4
6
8
10
VIN – Input Voltage – V
slew rate
Figure 46
The slew rate performance of a current feedback amplifier, like the THS6032, is affected by many different
factors. Some of these factors are external to the device, such as amplifier configuration and PCB parasitics,
and others are internal to the device, such as available currents and node capacitance. Understanding some
of these factors should help the PCB designer arrive at a more optimum circuit with fewer problems.
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