Intel Pentium 4 Processor CK00 Clock Synthesizer/Driver Design Guidelines

CK00 Clock Synthesizer/Driver Design Guidelines

Page 9

2. Example Circuits

The differential Host clock signals are to be established by a current mode current steering

buffer conceptually similar to that shown in Figure 2.1. I

Out

is established by a mirrored and

scaled copy of a reference current, I

Ref

. The method of establishing I

Ref

is explained in Section

2.1 in the manner believed to be the best known method.

Figure 2.1 – Conceptual Output Circuit

2.1 Current Reference and Mirror Circuit

The details of the current reference circuit are shown in Figure 2.2 in the fashion believed to be

the best known method using an external reference resistor. A simple current mirror connected

directly to the external resistor, as shown in Figure 2.3, is not recommended due to the difficulty

of establishing an accurate reference current. This inaccuracy in the reference current is due to

the uncertainty of the diode-connected device voltage drop.

The operational amplifier in the current reference circuit drives the gate of M

Iref

with feedback to

establish V

Ref

=1.1V at both inputs of the amplifier. Thus the reference current is established

according to the following formula:

Ref

= 1.1 / R

Ref

Iout

R=Zo

Internal External

3.3V