THS6042_16 Datasheet, PDF (27/46 Pages) Texas Instruments – 350mA,12V ADSL CPE LINE DRIVERS

THS6042, THS6043
350 mA, ±12 V ADSL CPE LINE DRIVERS
SLOS264G − MARCH 2000 − REVISED DECEMBER 2001
APPLICATION INFORMATION
shutdown control
The THS6043 is essentially the same amplifier as the THS6042. The only difference is the added flexibility of
a shutdown circuit. When the shutdown pin signal is low, the THS6043 is active. But, when a shutdown pin is
high (≥ 2 V), the THS6043 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 GND is utilized. An external resistor,
such as a 3.3 kΩ, to GND may be added to help improve noise immunity within harsh environments. If no
external resistor is utilized and SHDN pin is left unconnected, the THS6043 defaults to a power-on state. A
simplified circuit can be seen in Figure 47.
+VCC
To Internal
Bias Circuitry
Control
SHDN
50 kΩ
GND
−VCC
GND
Figure 47. Simplified THS6043 Shutdown Control Circuit
One aspect of the shutdown feature, which is often over-looked, is that the amplifier does not have a large output
impedance while in shutdown mode. This is due to the R_F and R_G resistors. This effect is true for any amplifier
connected as an amplifier with gains >1. The internal circuitry may be powered down and in a high-impedance
state, but the resistors are always there. This allows the signal to flow through these resistors and into the ground
connection. Figure 48 shows the results of the output impedance with no feedback resistor and a typically
configured amplifier.
1000
VCC = ±5 V to ±15 V
Open Loop
100
10
1
0.1
Gain = −8
RF = 750 Ω
0.01
10 K 100 K
1 M 10 M 100 M
1G
f − Frequency − Hz
Figure 48. Output Impedance In Shutdown Mode
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