AN-5031 Datasheet, PDF (1/5 Pages) Fairchild Semiconductor – GTLP Power Configuration

AN-5031
Fairchild Semiconductor
Application Note
January 2002
Revised January 2002
GTLP Power Configuration
Introduction
In order to drive high performance backplanes, GTLP
devices require power. This power comes from three dis-
crete levels: device power V_CC, termination voltage V_T, and
threshold voltage V_REF. Each of these power sources have
different needs both in terms of energy used and more
importantly, noise characteristics.
This application note provides an analysis of a GTLP back-
plane from a power perspective. This application note will
provide insight into the appropriate power requirements
and configurations for a GTLP backplane at each of the
discrete levels.
Device Bias V_CC
Datasheet specifications for GTLP devices list V_CC as
ranging from 3.15V to 3.45V, a variance of almost ±5%. In
order to meet all datasheet performance specifications, this
voltage range must be conformed to. Maintaining a level in
this range is relatively simple to do and is often generated
by a master supply, possibly a switching or linear regulator
in the backplane cage. The common problem is the dis-
tance between the cleanly filtered master supply output
and the GTLP device that it is powering. It is over this dis-
tance that a problem is most often introduced.
When troubleshooting a device, it is imperative that the
V_CC oscilloscope probe and ground be placed as close to
the device as possible. It is here that disturbances are
seen. Putting the device through the various configurations
that the device might encounter during operation is also
important. Clocking data in the AB direction while under
Multiple Outputs Switching (MOS) is perhaps a worse case
condition, but this can vary between applications. It can
also be beneficial to monitor the ground plane for noise.
There is often more than one V_CC pin on GTLP devices to
better distribute power inside the device. Each of these
pins will be wired to the V_CC plane of the PCB, but each pin
should also have its own bypass and or decoupling capaci-
tors as close as possible to the device. Depending on the
size and frequency of the disturbance, more than one size
and type of capacitor may be required.
How much noise is acceptable? Of the three voltage levels
that GTLP devices require, V_CC has the greatest amount of
noise tolerance. However, a V_CC spike of relative magni-
tude can be seen on the edge rate integrity of the device
outputs. It is at this point that noise reduction measures
should be taken such as including a local voltage regulator.
Termination Voltage V_T
Due to the open drain technology used in GTLP output
buffers, a pull-up voltage level known as V_T, nominally
1.5V, is required. When a GTLP device receives a HIGH on
the TTL input, it simply turns the output buffer OFF allowing
the voltage level of the trace, labeled as V_OH, to be equal
to the termination voltage.
Since the level of V_T directly affects the V_OH level there are
advantages to adjusting V_T. Observations to be made dur-
ing V_T adjustment include:
1. Noise Margin Associated with V_REF
When V_REF adjustment is not available similar results
can be accomplished by adjusting V_T.
2. Power Consumption
As V_T is increased, the current through the device in a
LOW state, I_OL, will increase. This may be a concern if
minimizing power is a priority. The obvious trade-off to
lowering V_T, thus lowering power consumption, is
potential noise margin violations.
It is not until a LOW bit is received that the connection is
made between the termination voltage and ground. The
effective LOW on the bus, labeled as V_OL, is then the factor
of a voltage division between the termination resistance
R_T, and the On Resistance of the GTLP device R_DSON.
Figure 1 illustrates this.
FIGURE 1. GTLP Output Buffer Showing Device R_DSON
Devices with higher drive, 100mA versus 50mA, have a
lower R_DSON, V_OL calculation can vary between devices.
The V_OL calculation can also vary considerably with tem-
perature. Each GTLP device has its own V_OL vs. I_OL plot
across temperature that can be used to calculate R_DSON. It
is important to note that two devices capable of driving
100mA do not necessarily have the same R_DSON due to
slight design differences. Always consult the datasheet or
extended characterization for specific values.
A common question regarding open drain technologies is
“what is required of the voltage source V_T and what are
possible configurations for it?” It is evident that large
amounts of current can be demanded from V_T based on
the number of bits switching on the backplane and the
value of R_T. But keep in mind that since the level of V_T sets
V_OH, any drops in the voltage level of V_T will result in a
degradation of the upper noise margin, the distance
between V_OH and V_REF.
© 2002 Fairchild Semiconductor Corporation AN500725
www.fairchildsemi.com