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Cypress Semiconductor Corporation • 3901 North First Street • San Jose • CA 95134 • 408-943-2600
Ma
y
1996 – Revised June 26
,
1998
Using CY7B991 (RoboClock™), CY7B9911 (RoboClock+),
and CY7B9910 (Robo Jr.) in 3.3-Volt Environments
Introduction
The RoboClock™ family of low skew clock buffers includes
six products listed in
Table 1
.
F or 3.3V appl icat ions , the CY7B991 V (Low Voltage Pr ogram -
mable Sk ew Clock Buf fer) is ideal . It is a true 3.3V de vice with
all the same functionalit y of the CY7B991/2. The rest of the
RoboClo c k f ami ly i s a 5V pr oduct l ine requ iring al l of the p ow-
er supply pins to be connected to a si ngle +5V supply. How-
ever, the 5V RoboClock family can still operate in mixed
5V/3V appl ications .
Alt hough the 5 V RoboClock is not as i deal as the CY7B991V
in 3.3V applications, it is still possible to make the outputs
3.3V compliant. the voltage levels on RoboClock’s 5V TTL
outpu ts ma y not b e toler able b y t he inputs of strict 3.3V LVTTL
products. However, by using t he termination network recom-
mended in the dat a sheet (see
Figure 4
) it is possib le to mak e
the TTL level RoboClock (CY7B991, CY7B9911 and
CY7B9910) outputs 3.3V-compliant. The CMOS level Rob-
oClocks (CY7B992 and CY7B9920) cannot be made
3.3V-compliant due to their design w hich achieves rail-to-rail
output swings. All the following sections pertain to the
CY7B991, CY7B9910, and CY7B9911.
RoboClock’s Power Pins
All RoboClock products have si x pow er supply pins sep arated
into t wo groups. The first group consists of two pins labeled
VCCQ. These pins supply power to the logic and Phase
Locked Loop (PLL) circuitry. The second group, labeled VC-
CN, consists of four pins. Each VCCN is a dedicated power
supply pin for a par ticular clock output pair (1Qx, 2Qx, 3Qx,
and 4Qx). Both the VCCQ and VCCN pins
must
be connected
to a +5V power supply (except for the CY7B991V). It is
not
possible to operate RoboClock with the VCCN pins at 3.3V in
the hopes of li miting the output buffers to 3.3V tolerant oper -
ation. Tying the VCCN pins to a voltage other than +5V may
damage Robo Clock.
RoboClock’s Output Buffers
The output buff ers on RoboCloc k can driv e transmi ssion li nes
down to 50Ω. The ability to drive low-impeda nce transmissi on
lines is a result of RoboCloc k’ s high current drive output buff -
ers. The voltage-current relationships (or V-I curves) of
RoboClock’s outputs are shown in
Figures 1
and
2.
Note in
Figures 1
and
2
current polarity is defined as positive when
sinking (i.e., current is flowing into the buffer), and negative
when sourcing (i.e., curr ent is flowing out of the buffer ).
Tabl e 1. RoboClock Family
Cypress
Part No. Features
CY7B991 15–80 M H z outputs with /2, /4, i nvert and
programmabl e skew. TTL output levels .
CY7B992 15–80 M H z outputs with /2, /4, i nvert and
progr ammabl e skew . CMOS output lev els.
CY7B9910 15–80 MHz out puts. TTL output levels.
CY7B9920 15–80 MHz ou tputs . CMOS output level s.
CY7B9911 15–100 MHz outputs with /2, /4, invert and
programmabl e skew. TTL output levels .
CY7B991V 15-80 MHz output with /2, /4, invert and
programmable skew . L VTTL output levels.
3.3V VCC.
Figure 1. RoboClock Ou tput Bu ff er V- I Curve,
Output = High
Using RoboClock in 3.3-Volt Environm e nts
2
The output buffers are designed to operate in systems with
terminated transmission lines. By modifying the termination
netw ork, the output buff ers can be load ed down (i.e ., required
to s upply mo re current) re sulting in a redu ction i n their voltag e
swing. In other words, RoboClock’s outputs can be modified
for 3.3V tolerant operation by choosing the correct ter mina-
tion network.
3.3V-Compliant RoboClock Outputs
The JEDEC standard JESD8-A “Interface Standard for Nom-
inal 3V/3.3V Supply Digital Integrated Circuits” defines the
voltage levels for 3V- and 3.3V-compliant signaling. For a
3.3V-compliant digital input, the allowable voltage levels, as
indi cated in J ESD8-A, are shown in
Table 2.
The outputs of the CY7B991V naturally comply t o this stan-
dard. To achieve 3.3V compliant output levels, RoboClock’s
output buffers must be limited to swing no higher than 3.6V
(3. 3V+0.3V). F rom the curve trace i n
Figur e 1
, the output buff-
ers can source 6.63 mA of current at 3.6V. Therefore, the
appropri ate termination networ k needed t o achieve 3.3V op-
eration is 3.6V/6.63mA≈560Ω. The simplest 3.3V-compliant
RoboClock design could use a 560Ω pull-down resistor on the
RoboClock outputs.
However, since transmission lines should be terminated to
their characteristic impedance, a 560Ω termination resistor
on the output of RoboClock would require use of a 560Ω
tr ansmission line (un common among pri nte d circuit board de-
signs) . Most PCB tr ansmission l ines are 50Ω, requi ring a 50Ω
termination.
50Ω Load for 3.3V Compliance
A 50Ω ch aracteristic impedance transmission li ne requires a
50Ω termination in order to pre vent voltage reflections. Ho w-
ever, the actual termination is not as simple as usi ng a 50Ω
pull- down res istor. RoboClock’s data sh eet swi tchi ng charac -
teristics (tSKEWPR, tSKEW1–4, tDEV, tODCV, tPWH, tPWL, tORISE,
and tOFALL) are optimized when terminating to a voltage of
2.06V. Theref ore, the best RoboClock outp ut terminati on pro-
vides for a 50Ω equivalent load, but also sets the termination
voltage to 2.06V.
To verify that a RoboClock output terminated to a specific
Thevenin resistance and voltage actually meets the JEDEC
3.3V req uirements ,
Equat ion 1
must be solv ed iterativ ely, with
the result compared against the V-I curve of
Figure 1
.
Eq. 1
Eq. 2
With a 50Ω to 2.06V termination,
Equation 2
can be solved
using a n itera tive process (i .e ., choos in g a VOutput and solving
for I Output, until the VOutput and IOutput results agree with the
V-I curve shown in
Figure 1
since we are concerned with lim-
iting the maximum output HIGH voltage) gi vi ng a solution of
VOutput= 3.25V and IOutput=23.8 mA.
With the termination chosen to meet 3.3V voltage require-
ments, the actual resistor values can be found using the cir-
cuit and equations shown in
Figure 3
.
Solving
Equatio ns 3
and
4
for VDD val ues of 5V and 3.3V, and
choosing standard resistor values, gives the two termination
netw o rks sh own i n
Figure 5
.
Figure 2. RoboCloc k Output Buffer V-I Curve,
Output = Low
Table 2. JEDEC 3.3V Input Specif ications, VDD=3.3V
Parameter Min. Max Units
VIH High-Level
Input Voltage 2.0 VDD+0.3 V
VIL Low-Level
Input Voltage –0.3 0.8 V
Eq. 3
Eq. 4
Figure 3. Choos ing Term inati on Resistor Values
V
Output
V
Termination
–
Z
Thevenin
-----------------------------------------------------
I
Output
=
V
Output
2.06
V
–
50Ω
----------------------------------------
I
Output
=
R2
R1
VDD
1
R
1
-------1
R
2
-------+ 1
50Ω
-----------=
R
2
R
1
R
2
+
--------------------
V
DD
×2.06
V
=
Using RoboClock in 3.3-Volt Environm e nts
© Cypress Semiconductor Corporation, 1998. The information contained herein is subject to change without notice. Cypress Semiconductor Corporation assumes no responsibility for the use
of any circui try other than circuitry embodied in a Cypress Semic onductor product. Nor does it conv ey or imply any license under patent or other rights . Cypress Semi conductor does not authorize
its products for use as critical components in life-support systems where a malfunction or failure may reasonably be expected to result in significant injury to the user. The inclusion of Cypress
Semiconductor products in life-support systems application implies that the manufacturer assumes all risk of such use and in doing so indemnifies Cypress Semiconductor against all charges.
Note, t he termination network for VDD=5V is actuall y the rec-
ommended output load as indicated in the RoboClock data
sheet. With these t er mination networ ks, the RoboClock out-
put waveform conforms to 3.3V specifications as shown in
Figure 5
.
Conclusion
The CY7B991V is a true 3.3V device and should be used in
3.3V applications whenever possible. The rest of the Rob-
oClock family of clock drivers were designed for 5V environ-
ments. With the advent of 3.3V systems, many designs re-
quire “3.3V tolerant” waveforms. This application note has
shown how to use the V-I relationships of the CY7B991,
CY7B9910 and CY7B9911 o utputs, and the characteristic i m-
pedance of a transm issi on line , to choo se an appropri ate ter -
mination network which guarantees that the outputs remain
within 3.3V voltage swing specifications. Using these tech-
niques, the test l oad f rom the RoboClock data sheet proves
to be an app rop riat e terminat ion to achie v e 3.3V c ompl iance .
RoboClock is a trad em ark of Cypr ess Semiconductor Corporat ion.
Figure 4. Typical 50Ω Terminati ons to 5V and 3.3V for
3.3V-Compli ant RoboClock Outp uts
Figure 5. RoboClock Output with 50Ω Load, VDD=5V
91
130
+5V
130
82
+3.3V
3V
