AND8020 Datasheet, PDF (3/18 Pages) Analog Devices – Termination of ECL Logic Devices with EF (Emitter Follower) OUTPUT Structure

AND8020/D
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 W impedance, and may cause
damage to the output. Worst case short circuit current risks
destruction of the devices.
ISC
+
VOH
RINT
+
4
8
V
W
(eq. 1)
= 500 mA!
Where:
V_OH = 4.0 V
V_CC = 5.0 V
V_EE = 0.0 V
R_int = 8 W
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
I _max
(eq. 2)
Table A. Minimum R_E Values
Line
V_OH
PECL
4.0 V
LVPECL
2.4 V
LVEP PECL
1.6 V
R_E(min)
80 W
48 W
32 W
A DC terminating resistor minimum, R_E (min), of 80 W,
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, DV, during a high−to−low or
low−to−high transition and present this to the transmission
medium such as coax, twisted pair, microstrip or stripline.
The DV 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 DV 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, DV, at the input to the receiving gate,
slowing the edge speed and increasing the net propagation
delay. A reasonable negative−going 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 DV, or a
600 mV step for an 800 mV signal at the driving gate. For
a RSECL expected DV swing of 400, a 300 mV initial step
is desired. Hence for a 600 mV initial step:
I(init) * Z0 u 0.6
( VOH * VEE )
( Rt ) Z0 )
*
Z0 y 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 Z₀ = 50 W line. Lowering the
value of R_E will increase the voltage change, DV, launched
into the transmission media. Raising the value of R_E will
decrease the voltage change, DV, launched into the
transmission media.
Table B. Recommended Nominal Values of R_E in
Dynamic Functional Application
Series
NB
|V_CC−V_EE|
2.5
R_E (W)
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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