PL135-37
Low Power, 1.62V to 3.63V, 10MHz to 40MHz, 1:3 Oscillator Fanout Buffer
Micrel Inc. • 2180 Fort une Drive • San Jose, CA 95131 • USA • tel +1(408) 944 -0800 • fax +1 (408) 474-10 00 • www.micrel.com Rev 03/18/11 Page 2
PIN DESCRIPTION
Output enable input. This pin has internal pull-up resistor. All outputs will be
tri-stated when pulled low.
* Note: This pin includes an internal 60KΩ pull up.
LAYOUT RECOMMENDATIONS
The following guidelines are to assist you with a performance optimized PCB design:
Signal Integrity and Termination Considerations
- Keep traces short!
- Trace = Inductor. With a capacitive load this equals
ringing!
- Long trace = Transmission Line. Without proper ter-
mination this will cause reflections (looks like ringing).
- Design long traces as “striplines” or “microstrips” with
defined impedance.
- Match trace at one side to avoid reflections bouncing
back and forth.
Decoupling and Power Supply Considerations
- Place decoupling capacitors as close as possible to
the V
DD
pin(s) to limit noise from the power supply
- Multiple V
DD
pins should be decoupled separately
for best performance.
- Addition of a ferrite bead in series with V
DD
can
help prevent noise from other board sources
- Value of decoupling capacitor is frequency de-
pendant. Typical value to use is 0.1F.
Typical CMOS termination
Place Series Resistor as close as possible to CMOS output
CMOS Output Buffer
(Typical buffer impedance 20Ω)
To CMOS Input
Series Resistor
Use value to match output buffer impedance to
50Ω trace. Typical value 30Ω
50Ω line
Crystal Tuning Circuit
Series and parallel capacitors used to fine tune the crystal load to the circuit load.
CST – Series Capacitor, used to lower circuit load to match crystal load. Raises frequency offset.
This can be eliminated by using a crystal with a Cload of equal or greater value than the oscillator.
CPT – Parallel Capacitors, Used to raise the circuit load to match the crystal load. Lowers
frequency offset.
Crystal
XIN
1 8
XOUT
Cpt
Cpt
Cst
