THS6032 Datasheet, PDF (14/31 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
ADSL (continued)
To ensure that the cutoff point does not introduce distortion into the system, the entire output stage is always
biased on. This constant biasing scheme will cause a decrease in the efficiency over hard switching class-G
circuits, but the very low distortion results tend to outweigh the efficiency loss. The biasing scheme used in the
THS6032 can be shown by the currents being supplied by the V_CC(L) power supplies in Figure 37. This graph
shows there is no discrete current transfer point between the V_CC(L) supplies and the V_CC(H) supplies. This was
done to ensure low distortion throughout the entire output range. By changing the V_CC(L) supply voltage, the
system efficiency can be tailored to suit almost any system with high crest factor requirements.
OUTPUT CURRENT DISTRIBUTION
vs
OUTPUT VOLTAGE
100
90
ICC(L)
Current Draw
80
VCC(H) = 15 V
VI = 1 MHz
RL = 25 Ω
70
60
VCC(L) = ±5 V
50
40
VCC(L) = ±7.5 V
30
20
10
0
0
1
2
3
4
5
6
7
RMS – Output Voltage – V
Figure 37
class-AB mode operation
The class-G architecture produces sizable power dissipation savings over traditional class-AB designs while
maintaining low distortion requirements. The only drawback to the class-G design is the requirement of 4 power
supply voltages, 2 more than a typical line driver requires. In certain instances, the addition of two separate
power supplies may be cost prohibitive or PCB space prohibitive. There are two options in this case, use a
traditional amplifier, such as a THS6012, or use the THS6032 in class-AB mode.
Using the THS6032 in class-AB mode will give several functional benefits over the THS6012. This includes
shutdown capability, low-impedance output while in shutdown state, and a slight reduction in quiescent current.
One important thing to remember is that the THS6032 running in class-AB mode, will be only about as efficient
as the THS6012. This means that the power dissipation of the THS6032 will increase dramatically and must
be accounted for. Failure to do so will result in a part which continuously overheats and may lead to failure.
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