Decoupling for TTL totem-pole outputs — mitigating Vcc current spikes Whenever a totem-pole TTL output switches from LOW to HIGH, a large current spike is drawn from Vcc. What is the standard corrective measure at the board level to protect digital circuits from this effect?

Difficulty: Easy

Correct Answer: By connecting a radio-frequency capacitor from Vcc to ground.

Explanation:


Introduction / Context:
Totem-pole outputs momentarily source significant current during transitions, causing supply transients that can introduce noise, jitter, or logic errors. Proper decoupling at each IC suppresses these spikes and maintains local supply integrity.


Given Data / Assumptions:

  • TTL totem-pole outputs generate transient current on rising edges.
  • Goal: reduce supply-line impedance at high frequencies.
  • Board-level mitigation is expected, not redesign of the logic family.


Concept / Approach:
Place a small RF bypass capacitor (e.g., 0.01–0.1 µF ceramic) as close as possible between Vcc and ground at each IC. This provides a low-impedance path for high-frequency currents, preventing them from propagating along the power rails. Larger bulk capacitors support lower-frequency load changes; both are often used together.


Step-by-Step Solution:

Identify the noise source: switching current spikes from totem-pole outputs.Provide a local shunt path with a small-value ceramic capacitor.Place it physically close to the IC pins to minimize ESL/ESR.Optionally add bulk decoupling per rail for aggregate transients.


Verification / Alternative check:
Oscilloscope measurements with and without local decoupling clearly show reduced supply ringing and cleaner edges.


Why Other Options Are Wrong:

  • B: A switching supply does not inherently fix on-board high-frequency transients.
  • C: A cap from Vout to ground loads the output and distorts logic levels.
  • D: A large resistor from Vcc to Vout is ineffective and wastes power.


Common Pitfalls:
Using long traces to decouplers; parasitic inductance defeats high-frequency bypass action.


Final Answer:
By connecting a radio-frequency capacitor from Vcc to ground.

Discussion & Comments

No comments yet. Be the first to comment!
Join Discussion