6

ML65T541

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,

resistor R8. It sources current to the output through the

resistor R7 which is bypassed by another NPN (not shown)

during fast input transients, and M1 pull-up drives the

within the rail. 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 current 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.

APPLICATIONS

There are a wide variety of needs for an 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 ML65T541 is equipped with Schottky diodes

to clean up ringing from overshoot and undershoot caused

by reflections in unterminated board traces.