Figure 7. IQ vs ILOAD APPLICATIO S I FOR ATIO W" />
參數(shù)資料
型號(hào)： LT1794CFE#TRPBF
廠商： Linear Technology
文件頁(yè)數(shù)： 3/20頁(yè)
文件大小： 0K
描述： IC AMP XDSL LINE DRIVER 20TSSOP
標(biāo)準(zhǔn)包裝： 2,500
類(lèi)型： 線路驅(qū)動(dòng)器，發(fā)射器
驅(qū)動(dòng)器/接收器數(shù)： 2/0
規(guī)程： xDSL
電源電壓： 18V
安裝類(lèi)型： 表面貼裝
封裝/外殼： 20-TSSOP（0.173"，4.40mm 寬）裸露焊盤(pán)
供應(yīng)商設(shè)備封裝： 20-TSSOP-EP
包裝： 帶卷 (TR)
11
LT1794
Figure 7. IQ vs ILOAD
APPLICATIO S I FOR ATIO
WU
UU
ILOAD (mA)
–240
–200
–160
–120
–80
–40
0
40
80
120
160
200
240
TOTAL
I
Q
(mA)
10
15
20
1794 F07
5
0
25
With no signal being placed on the line and the amplifier
biased for 10mA per amplifier supply current, the quies-
cent driver power dissipation is:
PDQ = 24V 20mA = 480mW
This can be reduced in many applications by operating
with a lower quiescent current value.
When driving a load, a large percentage of the amplifier
quiescent current is diverted to the output stage and
becomes part of the load current. Figure 7 illustrates the
total amount of biasing current flowing between the + and
– power supplies through the amplifiers as a function of
load current. As much as 60% of the quiescent no load
operating current is diverted to the load.
At full power to the line the driver power dissipation is:
PD(FULL) = 24V 8mA + (12V – 2VRMS) 57mARMS
+ [|–12V – (– 2VRMS)|] 57mARMS
PD(FULL) = 192mW + 570mW + 570mW = 1.332W
The junction temperature of the driver must be kept less
than the thermal shutdown temperature when processing
a signal. The junction temperature is determined from the
following expression:
TJ = TAMBIENT (°C) + PD(FULL) (W) θJA (°C/W)
θJA is the thermal resistance from the junction of the
LT1794 to the ambient air, which can be minimized by
heat-spreading PCB metal and airflow through the enclo-
sure as required. For the example given, assuming a
maximum ambient temperature of 85
°C and keeping the
junction temperature of the LT1794 to 140
°C maximum,
the maximum thermal resistance from junction to ambient
required is:
θJA MAX
CC
W
CW
()
–
.
./
=
°° =°
140
85
1 332
41 3
Heat Sinking Using PCB Metal
Designing a thermal management system is often a trial
and error process as it is never certain how effective it is
until it is manufactured and evaluated. As a general rule,
the more copper area of a PCB used for spreading heat
away from the driver package, the more the operating
junction temperature of the driver will be reduced. The
limit to this approach however is the need for very com-
pact circuit layout to allow more ports to be implemented
on any given size PCB.
Fortunately xDSL circuit boards use multiple layers of
metal for interconnection of components. Areas of metal
beneath the LT1794 connected together through several
small 13 mil vias can be effective in conducting heat away
from the driver package. The use of inner layer metal can
free up top and bottom layer PCB area for external compo-
nent placement.
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