Imagine a circuit connected to a power source (VDD) and ground through two wires. These wires have specific dimensions and resistance. Now, when the circuit starts drawing a direct current (DC) of 50 Amperes, something interesting happens. Instead of the expected VDD of 1 Volt, the actual voltage drops to 667 millivolts (mV). Similarly, the ground voltage, which is usually zero, increases to 333 mV. The voltage across the circuit is now just 333 mV, which is much less than the ideal 1 Volt.
To fix this issue and get closer to the ideal values, you can increase the widths of the conductors (wires) that supply and return currents to the circuit. This effectively reduces the series resistance. The key takeaway here is that VDD (power supply) and ground are not fixed; they can vary depending on how the circuit is set up.
In CMOS circuit design, circuits often consume almost no current when in a static state (not actively doing anything). This characteristic allows devices like CMOS-based calculators to run on very low-power sources, like solar cells.
However, problems arise when the circuit suddenly needs to draw a high current, like 50 Amperes, even for a short time. As explained earlier, this can cause the ground voltage to rise and the VDD voltage to drop temporarily. Although the average current supplied by VDD might be very low, the occasional need for a high current creates issues.
To solve this problem, you can add an on-chip decoupling capacitor physically close to the circuit (shown as dotted lines in the figure). This capacitor provides the necessary charge during these short bursts of high current, ensuring that the voltage across the circuit remains close to VDD. It stabilizes the power supply.
It’s also common to use an external decoupling capacitor placed across the VDD and ground pins of the chip for further stability.
In summary, these issues highlight the importance of proper circuit layout, including wire dimensions and the use of decoupling capacitors, to maintain stable power supplies and prevent voltage fluctuations when CMOS circuits switch between high and low current states.
How can you mitigate ground bounce issues in a circuit?
Answer: To mitigate ground bounce, you can increase the widths of the conductors (wires) that supply and return currents to the circuit. This action effectively reduces the series resistance in the circuit, helping to maintain closer-to-ideal voltage levels.
Why do CMOS circuits often consume very little current when in a static state, and how does this characteristic benefit low-power devices like calculators?
CMOS circuits consume minimal current in a static state because they have high input impedance and very low leakage currents. This characteristic allows devices like CMOS-based calculators to operate efficiently on low-power sources, such as solar cells, extending their battery life or reducing power requirements.
How can you address the issue of voltage fluctuations caused by occasional high current demands in CMOS circuits?
To address voltage fluctuations due to occasional high current demands, you can add an on-chip decoupling capacitor physically close to the circuit. This capacitor provides the necessary charge during short bursts of high current, ensuring that the voltage across the circuit remains close to VDD and stabilizing the power supply. Additionally, external decoupling capacitors across the VDD and ground pins of the chip can further enhance stability.
|Analog and Memory Layout Design Forum|
|Physical Layout Design Forum|
|RTL & Verilog Design Forum|
|Analog Layout Design Interview Questions||Memory Design Interview Questions|
|Physical Design Interview Questions||Verilog Interview Questions|
|Digital Design Interview Questions||STA Interview Questions|