Power Distribution

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_2 Course outline Contents Digital Signal Model Non Ideal Behavior of Components High Speed Properties of Digital Gate Ground Planes Crosstalk Power DistributionPower Distribution Terminations Power system Single-ended logic systems Common-path noise voltage Impedance between power pins Power system design rules Single plane power system Power and ground plane system Differential transmission Multilayered power distribution Power distribution wiring Wiring inductance Board-level filtering Example Power and ground planes

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_3 Power system Two essential purposes –Provide stable voltage references –Distribute power to all devices

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_4 Single-ended logic systems Differential input –Any noise which induces a voltage difference between the ground terminals of gate A and C shows up directly at the differential amplifier, reducing noise margin for gate C + Gate A Gate C Wire B Ground connection V1V1 N Internal reference generator V REF Hypothetical noise source in series with ground connection + - V IN

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_5 Common-path noise voltage The most common cause for noise voltage generation involves returning signal currents  common-path noise voltage Signal current flowing between gates E and F Gate A Gate C Ground inductance N Gate E Gate F

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_6 Impedance between power pins Common-path inductance in the power wiring can also cause problems –When A switches HI, current comes from the battery to charge up the capacitor –When B switches HI, it connects its output to the positive supply terminal; any noise there is transferred to its output Signal current path + - N Gate A Gate BGate C +

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_7 Power system design rules Use low-impedance ground connections between gates –Solid ground planes present a remarkably low inductance to return signal currents. The impedance between power pins on any two gates should be as low as the impedance between ground pins –Any changes in the power voltage, caused by return signal currents flowing in the power wiring, directly affect the output voltage There must be a low-impedance path between power and ground

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_8 Single plane power system Single ground plane that carries all returning currents Bypass capacitors are added at each gate between power and ground Power wiring can be arbitrary Power wiring is arbitrary Capacitors provide low impedance between power and ground at every gate Low-impedance ground-plane

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_9 Power and ground plane system The impedance of single-plane’s bypass capacitors may not be low enough A better approach consists of separate copper planes for power and ground Low-impedance ground plane Low-impedance power plane

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_10 Power and ground plane system When the planes lie very close to each other, they share a lot of mutual capacitance, that is a very low impedance at high frequencies Low impedance path between power and ground is provided by bypass capacitors and natural capacitance of power and ground planes

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_11 Differential transmission Differential voltage received is This configuration provides a built-in return current path for every signal wire Every signal carries its own reference voltage + Gate A Gate C Wire B Ground connection V 0 -V 1 N No internal reference generator needed Hypothetical noise source in series with ground connection + - Wire B’ V 0 +V 1 V 0 is a fixed offset voltage V IN

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_12 Multilayered power distribution The relatively large inductance of power distributing wiring raises the naturally low output impedance of most power supplies for use in digital electronics Designers generally place a single large bypass capacitor on each printed circuit card, connected in parallel to the power supply At high frequencies bypass capacitor loses its effectiveness as a result of the inductance of its mounting leads. Designers generally include an array of other smaller bypass capacitors that pick up where the big bypass capacitor left off

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_13 Multilayered power distribution Power supply Wiring Big bypass capacitor Small bypass capacitor array Together provide a low-impedance power source across the entire operating frequency range

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_14 Power distribution wiring Wiring resistance –Proportional to the inverse square of wire diameter (if possible use big wires) –Remote sense wires inform the supply of the output voltage as measured at the far end of its distribution wiring Wiring inductance –It is almost impossible to reduce wiring inductance by simply using a bigger wire –Use if possible wide, flat parallel structures –Differential transmission is practically immune to power supply fluctuations

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_15 Wiring inductance Inductance of two parallel power distribution wires (power and ground): X = length of wire H = average separation between wires D = wire diameter L = inductance

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_16 Wiring inductance Wide, flat parallel structures work much better. The inductance of a stack of parallel flat ribbons is: X = length of ribbon H = separation between ribbon plates W = width of ribbon N = number of plates L = inductance

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_17 Board-level filtering Power supply current requirement at the power pin of the logic gate consists of a big spike every 100 ns Ground Power +5 V C = 50 pF Current path when charging HI Output charges capacitor C to +5 V in 5 ns 10 MHz clock input + N

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_18 Board-level filtering If the power supply inductance is large, when logic gate tries to drive HI, the power supply input at the gate will droop near zero, slowly rising as capacitor C is charged The solution is to install a bypass capacitor Ground Power +5 V C = 50 pF Output charges capacitor C to +5 V in 5 ns 10 MHz clock input + N C Bypass Abbreviated path for charging current

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_19 Board-level filtering If the impedance of bypass capacitor is lower than the impedance of the power wiring, changing currents will flow through it instead of through the wiring Bypass capacitor reduces the rate of change of currents flowing in the power wiring. The power supply provides low impedance at low frequencies. The local bypass capacitor provides low impedance at higher frequencies

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_20 Example CMOS circuit having 200 gates each switching 20 pF loads in 7 ns. The power supply inductance is 150 nH. Let’s find the right value of bypass capacitor Worst case peak while charging all loads Maximum tolerable common-path impedance (  V = noise margin) Frequency below which the power supply wiring is adequate

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_21 Example Below frequency f W the power supply wiring is fine. Above frequency f W we need a bypass capacitor Above frequency f w, local bypass capacitor prevent power supply noise At some higher frequency (f Bypass ) the bypass capacitor will stop working, because of the series lead inductance

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_22 Power and ground planes Parallel power and ground planes provide a third level of bypass capacitance The power-to-ground-plane capacitance has zero lead inductance and no ESR A = area of shared power-ground planes d = separation between planes

@ Copyright F. Canavero, R. Fantino 7/6/2017Licensed to HDT - High Design TechnologyPD_23 Next topic Terminations