ML65244/ML65L244
The basic architecture of the ML65244 is shown in Figure
5. It is implemented on a 1.5µm BiCMOS process.
However, in this particular circuit, all of the active devices
are NPNs — the fastest devices available in the process.
In this circuit, there are two paths to the output. One path
sources current to the load capacitance when the signal is
asserted, and the other path sinks current from the output
when the signal is negated.
The assertion path is the emitter follower path consisting
of the level shift transistor Q1, the output transistor Q2,
and the bias resistor R8. It sources current to the output
through the 75Ω resistor R7 which is bypassed by another
NPN (not shown) during fast input transients. The
negation path is a current differencing op amp connected
in a follower configuration. The active components in this
amplifier are transistors Q3–Q7. R3–R6 are bias resistors,
and R1 and R2 are the feedback resistors. The key to
understanding the operation of the current differencing op
amp is to know that the currents in transistors Q3 and Q5
are the same at all times and that the voltages at the bases
of Q4 and Q6 are roughly the same. If the output is higher
than the input, then an error current will flow through R2.
This error current will flow into the base of Q6 and be
multiplied by
β
squared to the collector of Q7, closing the
loop. The larger the discrepancy between the output and
input, the larger the feedback current, and the harder Q7
sinks current from the load capacitor.
A number of MOSFETs are not shown in Figure 5. These
MOSFETs are used to three-state dormant buffers. For
instance, the feedback resistors R1 and R2 were
implemented as resistive transmission gates to ensure that
disabled buffers do not load the lines they are connected
to. Similarly, there is a PMOS in series with R8 that is
normally on but shuts off for disable. Other MOSFETs
have been included to ensure that disabled buffers
consume no power.
TERMINATION
R7 in Figure 5 also acts as a termination resistor. This 75Ω
resistor is in series with the output and therefore helps
suppress noise caused by transmission line effects such as
reflections from mismatched impedances. System
designers using CMOS transceivers commonly have to use
external resistors in series with each transceiver output to
suppress this noise. Systems using the ML65244 or
ML65L244 may not have to use these external resistors.
APPLICATIONS
There are a wide variety of needs for extremely fast buffers
in high speed processor system designs like Pentium,
PowerPC, Mips, Sparc, Alpha and other RISC processors.
These applications are either in the cache memory area or
the main memory (DRAM) area. In addition, fast buffers
find applications in high speed graphics and multimedia
applications. The high capacitive loading due to
multiplexed address lines on the system bus demand
external buffers to take up the excess drive current. The
needed current to skew the transitions between rise and
fall times must be done without adding excessive
propagation delay. The ML65244 and ML65L244 are
equipped with Schottky diodes to clean up ringing from
overshoot and undershoot caused by reflections in
unterminated board traces.
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MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
