Datasheet
LTC6957-1/LTC6957-2/
LTC6957-3/LTC6957-4
25
6957f
For more information www.linear.com/LTC6957-1
that only the current source is cut off during shutdown.
The bases of the output NPNs are still tied to the pull-up
resistors, so both outputs will be pulled high in shutdown,
and it is the user’s responsibility to disconnect the external
loading if power reduction is to be realized.
The simplest way to terminate and bias the LTC6957-1
outputs is to route the differential output to the differen-
tial receiver and terminate the lines at that point with the
three resistor network shown in Figure 9. The differential
termination will be 100Ω, while the common mode termina-
tion will be 75Ω which could result in additional common
mode susceptibility. A bypass capacitor on the midpoint
of the Y can be used to improve this.
If the common mode termination impedance is not an
issue, the three resistor Y configuration can be changed
to a three resistor delta configuration, which is a simpler
layout in most cases.
applicaTions inForMaTion
Figure 9. LTC6957-1 LVPECL Outputs
6957 F09
50Ω
50Ω
LTC6957-1
PCB ROUTING TRACES
Z0 = 50Ω
–
+
50Ω
V
+
V
+
V
+
V
+
24Ω
5Ω
V
+
24Ω
5Ω
This data is shown for the LTC6957-2, but the effect is
due to the input stage that is common to all versions, so
any other version will have the same general behavior.
The LTC6957-3 and LTC6957-4 CMOS outputs may have
additional t
PD
+ vs t
PD
– discrepancies due to differences
between the NMOS and PMOS output devices, particularly
when driving heavy loads. These are independent of input
overdrive, but can change with supply voltage and tem-
perature, and can vary part to part. The complementary
outputs of the LTC6957-4 will therefore be higher skew
than the like edges of the LTC6957-3. Both the LTC6957-3
and LTC6957-4 will have large (120ps typ) t
PD
+
to t
PD
–
discrepancies compared to LVPECL or LVDS outputs.
LVPECL Outputs of the LTC6957-1
Figure 9 shows a simplified schematic of the LTC6957-1
LVPECL output stage. As with most ECL outputs, there are
no internal pull-down devices so the user must provide
both termination and biasing external to the device. Note