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| 型号： | LT1794ISW#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 2/20页 |
| 文件大小： | 0K |
| 描述： | IC OPAMP 200MHZ DUAL 20-SOIC |
| 标准包装： | 1,000 |
| 类型： | 线路驱动器，发射器 |
| 驱动器/接收器数： | 2/0 |
| 规程： | xDSL |
| 电源电压： | 18V |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 20-SOIC（0.295"，7.50mm 宽） |
| 供应商设备封装： | 20-SOIC |
| 包装： | 带卷 (TR) |
10
LT1794
Power Dissipation and Heat Management
xDSL applications require the line driver to dissipate a
significant amount of power and heat compared to other
components in the system. The large peak to RMS varia-
tions of DMT and CAP ADSL signals require high supply
voltages to prevent clipping, and the use of a step-up
transformer to couple the signal to the telephone line can
require high peak current levels. These requirements
result in the driver package having to dissipate on the
order of 1W. Several multiport cards inserted into a rack
in an enclosed central office box can add up to many,
many watts of power dissipation in an elevated ambient
temperature environment. The LT1794 has built-in ther-
mal shutdown circuitry that will protect the amplifiers if
operated at excessive temperatures, however data trans-
missions will be seriously impaired. It is important in the
design of the PCB and card enclosure to take measures to
spread the heat developed in the driver away to the
ambient environment to prevent thermal shutdown (which
occurs when the junction temperature of the LT1794
exceeds 165
°C).
Estimating Line Driver Power Dissipation
Figure 6 is a typical ADSL application shown for the
purpose of estimating the power dissipation in the line
driver. Due to the complex nature of the DMT signal,
which looks very much like noise, it is easiest to use the
RMS values of voltages and currents for estimating the
driver power dissipation. The voltage and current levels
shown for this example are for a full-rate ADSL signal
driving 20dBm or 100mWRMS of power on to the 100
telephone line and assuming a 0.5dBm insertion loss in
the transformer. The quiescent current for the LT1794 is
set to 10mA per amplifier.
The power dissipated in the LT1794 is a combination of the
quiescent power and the output stage power when driving
a signal. The two amplifiers are configured to place a
differential signal on to the line. The Class AB output stage
in each amplifier will simultaneously dissipate power in
the upper power transistor of one amplifier, while sourc-
ing current, and the lower power transistor of the other
amplifier, while sinking current. The total device power
dissipation is then:
PD = PQUIESCENT + PQ(UPPER) + PQ(LOWER)
PD = (V+ – V–) IQ + (V+ – VOUTARMS)
ILOAD + (V – – VOUTBRMS) ILOAD
APPLICATIO S I FOR ATIO
WU
UU
Figure 6. Estimating Line Driver Power Dissipation
1794 F06
+
–
B
–IN
–
+
A
+IN
12V
20mA DC
SHDN
–12V
–2VRMS
17.4
24.9k – SETS IQ PER AMPLIFIER = 10mA
1:1.7
110
1000pF
110
1k
17.4
SHDNREF
100
3.16VRMS
ILOAD = 57mARMS
2VRMS
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