Unlock Layout and Design: Bandgap Reference (BGR)

Introduction to BGR

• Topic: Bandgap Reference (BGR) found in most integrated circuits (ICs).
• Key aspects to be covered:
  • Need for reference voltage
  • BGR block diagram
  • Generation of Complementary to Absolute Temperature (CTAT) and Proportional to Absolute Temperature (PTAT)
  • Actual circuit of BGR
  • Trimming and startup circuits
  • Sub 1 Volt architecture
  • Matching concepts in layout
  • Explanation of the name "Bandgap Reference"

Need for Reference Voltage

• Integrated circuits must operate effectively in all temperature ranges (e.g., extreme heat and cold).
• Importance of a reference voltage independent of temperature for stability.
• Example: 1 Volt reference at various temperatures (27°C, 120°C, -40°C) to compare with temperature-dependent voltages.
• Reference voltage can represent various quantities, not just temperature (e.g., vehicle speed).

Characteristics of Reference Voltage on Chip

• Should be independent of:
  • P: Process variation (different fabrication technologies)
  • V: Supply voltage variations (e.g., battery voltage changes)
  • T: Temperature changes (operational range of -40°C to 125°C)

Generation of Reference Voltage

• CTAT: Complementary to Absolute Temperature (decreases with temperature).
• PTAT: Proportional to Absolute Temperature (increases with temperature).
• Method to achieve a stable reference:
  • Identify CTAT and PTAT quantities.
  • Combine both to create a stable voltage (constant despite temperature changes).

BGR Circuit Components

• VBE: Voltage between base and emitter of a BJT, behaves as a CTAT.
• VT: Thermal voltage, behaves as a PTAT.
• The BGR circuit configuration involves:
  • Addition of CTAT and PTAT using adjustments to maintain constant output voltage.
  • Use of current sources and adjusting slopes to balance voltage changes over temperature.

P-TAT Generation Circuit

• Example setup with 1 transistor vs. N transistors to illustrate PTAT generation.
• Equation derived for voltage between two configurations to establish PTAT characteristics.

Trimming in BGR

• Trimming compensates for process variations affecting VBE and VBG outputs.
• Typically adjusted by changing resistor values in the circuit to achieve a target voltage (usually around 1.2V).

Startup Circuit

• Provides an initial current to stabilize the operational amplifier (op-amp) during startup (from 0 to VDD).
• Ensures that the reference voltage reaches the intended level (1.2V).

Sub 1 Volt Architecture

• Addresses scenarios where supply voltage might be lower than the standard bandgap reference voltage (1.2V).
• Introduces alternative configurations to meet design requirements.

Layout Matching in BGR

• Importance of matching transistors and resistors to ensure accurate reference voltage output.
• Matching strategies:
  • Central placement of critical components and surrounding dummy components for thermal stability.
  • Ensuring consistency in resistor values to maintain gain accuracy.

Explanation of Bandgap Reference Name

• "Bandgap" refers to the energy bandgap in semiconductors (silicon has a ~1.2 eV bandgap).
• The bandgap reference voltage generated (~1.2V) aligns conceptually with this semiconductor property.

Conclusion

• Recap of BGR functionality and its critical role in IC design.
• Encouragement to understand and appreciate the complexity of bandgap reference voltage generation and its importance in real-world applications.