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# Need of Decoupling Capacitor

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http://vlsisystemdesign.com/need_for_decap.php

A decoupling capacitor is a capacitor, which is used decouple the critical cells from main power supply, in order to protect the cells from the disturbance occuring in the power distribution lines and source. The purpose of using decoupling capacitors is to deliver current to the gates during switching. Herein, we would peep inside the reasons for the distrubance occuring in the power distribution lines.

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### Need of Decoupling Capacitor

1. 1. So what can we conclude!!!A capacitor needs atleast Ipeak amount of currentIpeak IR To get charged upto Vdd voltage VCLAnd, the output of (single) inverter, is recognised as logic ‘1’
2. 2. Consider the amount of the switching current required For a complex block something like below
3. 3. Consider the amount of the switching current required For a complex block something like below
4. 4. Why to do?1. If the wires were ideal, i.e. zero resistance, zero inductance and infinitely short, thus no issue of power distribution 3/2/2013 4
5. 5. Why to do?1. Consider capacitance to be zero for the discussion. Rdd, Rss, Ldd and Lss are well defined values.2. During switching operation, the circuit demands switching current i.e. peak current (Ipeak).3. Now, due to the presence of Rdd and Ldd, there will be a voltage drop across them and the voltage at Node A would be Vdd instead of Vdd. 3/2/2013 5
6. 6. Why to do?1. When input of the inverter switches from logic 1 to logic 0, output of inverter should switch from logic 0 to logic 1.2. This essentially means that the output capacitance of inverter should charge till the supply voltage Vdd.3. But if Vdd goes below the noise margin, due to Rdd and Ldd, the logic 1 at the output of inverter wont be detected as logic 1 at the input of the Vdd - Vdd = Ipeak*Rdd + Ldd * (dI/dt) circuit following the inverter. 3/2/2013 6
7. 7. Why to do?Solution1. Keep Rdd and Rss minimum by increasing width of wire.2. Keep peak current Ipeak and change in current dI/dt as small as possible. Ipeak = CL * Vdd / tr dI/dt = CL * Vdd / tr23. Limit the rise time (tr). If a circuit could run at 500 ps, its unnecessary to run the circuit at 300 ps. The largest possible value of tr should be selected. Vdd - Vdd = Ipeak*Rdd + Ldd * (dI/dt) 3/2/2013 7
8. 8. Why to do?1. Addition of Decoupling Capacitor in parallel with the circuit2. Everytime the critical cell (in above daigram,an inverter) switches, it draws current from Cd , whereas, the RL network is used to replenish the charge into Cd 3/2/2013 8
9. 9. We have taken care of local communication
10. 10. We have taken care of local communication Now, Consider the below scenario
11. 11. Driver
21. 21. Driver LoadLet us trace the path of current when Driver is ‘Switching’
22. 22. Driver LoadLet us trace the path of current when Driver is ‘Switching’
23. 23. Driver LoadLet us trace the path of current when Driver is ‘Switching’
24. 24. Driver LoadLet us trace the path of current when Driver is ‘Switching’
25. 25. Driver LoadLet us trace the path of current when Driver is ‘Switching’
26. 26. Driver LoadLet us trace the path of current when Driver is ‘Switching’
27. 27. Driver LoadNo Decoupling Capacitors are involved in the path through ‘Blue’ line
28. 28. Driver LoadHence, switching through ‘Blue’ line will need peak current, which has to supplied by Vdd
29. 29. Driver Load And not through Decaps.
30. 30. Driver LoadThis will lead to ‘voltage droop’ and ‘ground bounce’, i.e. disturbance in power supply network
31. 31. Driver LoadAssume, the ‘Blue’ path is a 16-bit bus
32. 32. Also assume, the 16-bit bus is having present data as below 1110010111000110
33. 33. Also assume, the 16-bit bus is having present data as below 1110010111000110
34. 34. Also assume, the 16-bit bus is having present data as below 1110010111000110
35. 35. Also assume, the 16-bit bus is having present data as below 1110010111000110V V V V V V V V V
36. 36. Also assume, the 16-bit bus is having present data as below 1110010111000110V V V 0 0 V 0 V V V 0 0 0 V V 0
37. 37. V V V 0 0 V 0 V V V 0 0 0 V V 0
38. 38. V V V 0 0 V 0 V V V 0 0 0 V V 0
39. 39. V V V 0 0 V 0 V V V 0 0 0 V V 0Now, Lets the output of 16 – bit bus, is connected to an inverter
40. 40. V V V 0 0 V 0 V V V 0 0 0 V V 0Now, Lets the output of 16 – bit bus, is connected to an inverter 1110010111000110 16-bit bus 16-bit bus
41. 41. V V V 0 0 V 0 V V V 0 0 0 V V 0Now, Lets the output of 16 – bit bus, is connected to an inverter 1110010111000110 16-bit bus 16-bit bus 0001101000111001
42. 42. V V V 0 0 V 0 V V V 0 0 0 V V 0 0001101000111001
43. 43. V V V 0 0 V 0 V V V 0 0 0 V V 0 0001101000111001 What does this mean?
44. 44. V V V 0 0 V 0 V V V 0 0 0 V V 0 0001101000111001 What does this mean?This means, all capacitors which were charged to ‘V’ volts will have to discharge to ‘0’ volts through single ‘Ground’ tap point. This will cause a bump in ‘Ground’ tap point.
45. 45. V V V 0 0 V 0 V V V 0 0 0 V V 0 0001101000111001 What does this mean?This means, all capacitors which were charged to ‘V’ volts will have to discharge to ‘0’ volts through single ‘Ground’ tap point. This will cause a bump in ‘Ground’ tap point. V V V 0 0 V 0 V V V 0 0 0 V V 0
46. 46. V V V 0 0 V 0 V V V 0 0 0 V V 0 0001101000111001 What does this mean?This means, all capacitors which were charged to ‘V’ volts will have to discharge to ‘0’ volts through single ‘Ground’ tap point. This will cause a bump in ‘Ground’ tap point. V V V 0 0 V 0 V V V 0 0 0 V V 0
47. 47. V V V 0 0 V 0 V V V 0 0 0 V V 0 0001101000111001 What does this mean?This means, all capacitors which were charged to ‘V’ volts will have to discharge to ‘0’ volts through single ‘Ground’ tap point. This will cause a bump in ‘Ground’ tap point. V V V 0 0 V 0 V V V 0 0 0 V V 0 Ground Bounce
48. 48. V V V 0 0 V 0 V V V 0 0 0 V V 0 0001101000111001 What does this mean?
49. 49. V V V 0 0 V 0 V V V 0 0 0 V V 0 0001101000111001 What does this mean? Also, all capacitors which were ‘0’ volts will have to charge to ‘V’ voltsthrough single ‘Vdd’ tap point. This will cause lowering of voltage at ‘Vdd’ tap point.
50. 50. V V V 0 0 V 0 V V V 0 0 0 V V 0 0001101000111001 What does this mean? Also, all capacitors which were ‘0’ volts will have to charge to ‘V’ voltsthrough single ‘Vdd’ tap point. This will cause lowering of voltage at ‘Vdd’ tap point. V V V 0 0 V 0 V V V 0 0 0 V V 0
51. 51. V V V 0 0 V 0 V V V 0 0 0 V V 0 0001101000111001 What does this mean? Also, all capacitors which were ‘0’ volts will have to charge to ‘V’ voltsthrough single ‘Vdd’ tap point. This will cause lowering of voltage at ‘Vdd’ tap point. V V V 0 0 V 0 V V V 0 0 0 V V 0
52. 52. V V V 0 0 V 0 V V V 0 0 0 V V 0 0001101000111001 What does this mean? Also, all capacitors which were ‘0’ volts will have to charge to ‘V’ voltsthrough single ‘Vdd’ tap point. This will cause lowering of voltage at ‘Vdd’ tap point. Voltage Droop V V V 0 0 V 0 V V V 0 0 0 V V 0
53. 53. So what could be the solution ?
54. 54. So what could be the solution ? Driver Load
55. 55. So what could be the solution ? Driver LoadHow can ‘Driver’ and ‘Load’ can be brought close to each other in ‘L’ sense ?
56. 56. If ‘Driver’ and ‘Load’ have a lot of communication between them, thenthe power supply network must be designed in such a way, that they are Inductively close to each other
57. 57. If ‘Driver’ and ‘Load’ have a lot of communication between them, thenthe power supply network must be designed in such a way, that they are Inductively close to each other So, how do we design power supply distribution network?
58. 58. If ‘Driver’ and ‘Load’ have a lot of communication between them, thenthe power supply network must be designed in such a way, that they are Inductively close to each other So, how do we design power supply distribution network? MESH!!!!
59. 59. Vdd Vss Vdd Vss Vdd VssVss VssVdd VddVss VssVdd VddVss VssVdd Vdd Vdd Vss Vdd Vss Vdd Vss
60. 60. Vdd Vss Vdd Vss Vdd VssVss VssVdd VddVss VssVdd VddVss VssVdd Vdd Vdd Vss Vdd Vss Vdd Vss
61. 61. Vdd Vss Vdd Vss Vdd VssVss VssVdd VddVss VssVdd VddVss VssVdd Vdd Vdd Vss Vdd Vss Vdd Vss
62. 62. Power should not be coming only from one place
63. 63. Power should not be coming only from one place But from many places
64. 64. Power should not be coming only from one place But from many placesAssume that, the boundary power is available, now we have to get it inside the chip
65. 65. Power should not be coming only from one place But from many placesAssume that, the boundary power is available, now we have to get it inside the chip Thus, local communication is taken care by de-coupling capacitors
66. 66. Power should not be coming only from one place But from many placesAssume that, the boundary power is available, now we have to get it inside the chip Thus, local communication is taken care by de-coupling capacitors And, common rail inductance coupling issue is taken care by power mesh
67. 67. DECAP1 D Block a Block b 4 DECAP2 Block c DECAP3 DieVss CoreVdd