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Spread S pectrum Clock Generator SSCG SSCG application for 5V system
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Revision History Rev Date Remark 1.0 2009/5/18 First Edition 2
Subject Revision History...2 Subject...3 1 Introduction...4 2 Application example to 5V system...5 3
1 Introduction The document shows the example of proposal to 5V system using 3V SSCG. (SSCG : Spread Spectrum Clock Generator). Our SSCG p roduct lineup is mainly 3V operat ion. So 5V system cannot use 3V SSC directly. So we propose to 5V system using 3V SSCG and some circuit. G 4
2 Application example to 5V system FUJITSU SSCG operates with not only a gener al 3-terminal regulator bu t also a simple regulator because its power consumption is a little. The amplitude of the clock output from SSCG becomes about 3V. The threshold voltage is about 1.4V in most 5V interfaces. So, 3V SSCG can be used in 5V interface in many cases. (Please confirm the specification of the device to which the clock output of SSCG is input) Moreover, if the amplitude of the clock out put becomes small, a unnecessary radiatio n noise becomes small. Th is is because the val ue of an unnecessary radiatio n is in inverse proportion to the second power of the voltage. Because of above 2-point, It is more advantageous for the EMI decrease to use regulat or and FUJITSU SSCG than 5V operation SSCG. 5V Regulator SSCG 3V 3V swing 5V operation LSI Vth=1.4V Figure 1. Circuit example of 5V system using 3V SSCG and regulator 5
Figure 2. shows the circuit example using a s ilicon diode as a simple regulator. A silicon diode is used for dropping 5V to 3V. R1 is in serted because the excess voltage is not impresse d to SSCG when the po wer supply is turned on. (Ple ase confirm whether the SSCG powe r supply val ue is a range of recommended operation voltage (3.0V to 3.6V) or not. C1 and C 2 are b ypass cap acitors for st able power sup ply. The value of the figure is recommended one. Please examine the best value by your system. Silicon diode VDD 5V SSCG C1 0.01µF C2 10µF R1 A range of VSS kω- 10kΩ Figure 2. Circuit example of 5V system using 3V SSCG and silicon diode 6
Figure 3. shows the circuit ex ample using a Light emitting di ode(led) as a silicon diode. A LED is used for dropping 5V to 3V. It is suitable to use the L ED that has 1.6V to 1.8V volt age drop for the sys tem.(about the color, red, orange, yellow or green is suitable.) Blue or white color LED cannot use because its drop voltage is over 3.0 V. LED VDD 5V SSCG C1 0.01µF C2 10µF R1 A range of VSS kω- 10kΩ Figure 3. Circuit example of 5V system using 3V SSCG and LED 7
Figure 4. shows the circuit example using a Zener diode as a silicon diode. Zener diode has the temperature dependence. If you use zener diode, please confirm the operation temperature range of system and zener diode. R1 5V 3V Zener VDD C1 C2 SSCG 0.01µF 10µF VSS Figure 4. Circuit example of 5V system using 3V SSCG and Zener diode - End - 8