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參數(shù)資料
| 型號： | AND8020 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Termination of ECL Logic Devices with EF (Emitter Follower) OUTPUT Structure |
| 中文描述： | ECL邏輯器件的終止與英法（發(fā)射極跟隨器）輸出結(jié)構(gòu) |
| 文件頁數(shù)： | 3/18頁 |
| 文件大?。?/td> | 168K |
| 代理商： | AND8020 |
AND8020/D
http://onsemi.com
3
Output Open, Short, and Safe DC Current
Left open, an output will only swing a few millivolts due
to parasitic “minimum current” leakage paths.
Shorted to V
EE
, a maximum current will develop, limited
only by the output transistor 8 impedance, and
may cause
damage to the output. Worst case short circuit current risks
destruction of the devices
.
VOH
RINT
ISC
4 V
8
= 500 mA!
(eq. 1)
Where:
V
OH
= 4.0 V
V
CC
= 5.0 V
V
EE
= 0.0 V
R
int
= 8
The continuous safe output current, I
out
(continuous),
maximum limit is 50 mA under all spec operating
conditions. The continuous safe repetitive surge, I
out
(surge), maximum current limit is 100 mA for
10 milliseconds per second duty cycle, provided the device’s
total thermal limits are observed. Output current polarity
will always be sinking into the termination scheme during
proper operation.
Static Analysis of Termination Resistor R
E
The output continuous safe current limit, I
out
(cont),
determines R
E
minimum DC termination scheme resistance
to V
EE
although this will not provide a practical AC signal
termination as shown in Table A: Minimum R
E
Values.
RE
VOH
Imax
(eq. 2)
Table A. Minimum R
E
Values
Line
V
OH
R
E
(min)
PECL
4.0 V
80
LVPECL
2.4 V
48
LVEP PECL
1.6 V
32
A DC terminating resistor minimum, R
E
(min), of 80
while sufficiently limiting the output load current to V
EE
,
may generate insufficient PECL output LOW and HIGH
state transitions.
The R
E
maximum is effectively determined by the
application load capacitance, C
L
, since an RC network is
formed by R
E
and C
L
which limits the signal fall time,
discharging the line to the LOW state voltage level. A
sufficiently high value R
E
or C
L
can cause the signal fall
time to the V
OL
level to violate specification limits.
Designed R
E
or C
L
values may selectively eliminate
undesirable noise.
,
Dynamic Analysis of Termination Resistor R
E
The dynamic function of the termination resistor, R
E
is to
develop the voltage change, V, during a hightolow or
lowtohigh transition and present this to the transmission
medium such as coax, twisted pair, microstrip or stripline.
The
V signal propagates to the receiver and is either
reflected, dissipated, or a combination.
Since the reflection coefficient at the load is of opposite
polarity to that of the source, a reflection will travel back and
forth over the transmission changing polarity after each
reflection until critically damped by line impedance. Thus,
steps may appear in the signal V at the receiving gate input
due to impedance mismatch and consequent partial
reflections.
When R
E
is too large, steps appear in the trailing edge of
the propagating signal, V, at the input to the receiving gate,
slowing the edge speed and increasing the net propagation
delay. A reasonable negativegoing signal swing at the input
of the receiving gate results when the value of R
E
is selected
to produce an initial step of 75% of the expected V, or a
600 mV step for an 800 mV signal at the driving gate. For
a RSECL expected V swing of 400, a 300 mV initial step
is desired. Hence for a 600 mV initial step:
I(init) * Z0
( VOH
( Rt
VEE)
Z0)
* Z0
0.6
0.6
(eq. 3)
The value for R
E
is found in Table B: Recommended
Values of R
E
in Dynamic Functional Application. This table
lists recommended R
E
values for the various ECL devices by
Family Series according to the equation above. The table
assumes operation with various data sheet V
OH
values and
various V
CC
values driving a Z
0
= 50 line. Lowering the
value of R
E
will increase the voltage change, V, launched
into the transmission media. Raising the value of R
E
will
decrease the voltage change,
transmission media.
V, launched into the
Table B. Recommended Nominal Values of R
E
in
Dynamic Functional Application
Series
|V
CC
V
EE
|
R
E
(
NB
2.5
140
NB
3.3
250
10/100LVEP
2.5
50
10/100EP, 100LVEL
3.3
120
10/100EL, 10/100E
5.0
235
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