Designing a BGR with 1.2V Supply and 0.
9V
Reference: A Deeper Dive
Jay Kim, CMO at Silicon Mitus
October 30, 2024
Understanding the Challenge:
As you've correctly pointed out, designing a BGR with a reference voltage higher than
the supply voltage presents a unique challenge. Traditional BGR designs rely on the
inherent temperature-dependent voltage drop across a diode-connected transistor to
generate a reference voltage. However, this voltage is typically lower than the supply
voltage.
Potential Solutions:
To overcome this limitation, we can explore the following approaches:
1. Voltage Boosting Techniques:
• Charge Pump: This circuit can effectively boost the voltage by storing and
releasing charge in capacitors. However, it might introduce noise and ripple,
which can degrade the reference voltage's quality.
• Switched-Capacitor Circuit: Similar to a charge pump, this circuit can boost the
voltage by transferring charge between capacitors. It can offer better noise
performance but might require more complex control circuitry.
2. Leveraging Process Variations:
• Mismatch-Based Techniques: By exploiting process variations, we can
generate additional voltage headroom. However, this approach can be sensitive
to manufacturing tolerances and temperature variations.
• Body Effect: In CMOS processes, the body effect can be utilized to modulate
the threshold voltage of transistors. This can be exploited to generate additional
voltage headroom.
3. Innovative Circuit Topologies:
• Sub-Threshold BGR: By operating transistors in the sub-threshold region, we
can reduce the voltage headroom requirement. However, this can impact the
performance and stability of the circuit.
The Solution: Sub-Threshold Operation
Sub-threshold operation involves biasing transistors at very low voltages,
significantly below their threshold voltage. In this regime, the transistor's current-
voltage characteristics exhibit an exponential relationship, which can be exploited
to generate a temperature-dependent voltage.
How it Works:
1. Transistor Biasing: Transistors are biased in the sub-threshold region to
operate at low currents.
2. Temperature-Dependent Voltage Generation: The voltage drop across
the sub-threshold transistor exhibits a temperature-dependent behavior.
3. Reference Voltage Generation: The temperature-dependent voltage is
amplified and filtered to generate a stable reference voltage.
Key Advantages:
• Low-Voltage Operation: Sub-threshold BGRs can operate at very low
supply voltages.
• Low Power Consumption: Due to the low current operation, power
consumption is significantly reduced.
• Simple Circuitry: The circuit topology is relatively simple, reducing
complexity and cost.
Challenges and Considerations:
• Sensitivity to Process Variations: Sub-threshold operation is highly
sensitive to process variations, which can impact the accuracy and
stability of the BGR.
• Noise: Noise can be a significant issue in sub-threshold circuits,
especially at low supply voltages.
• Slow Response Time: The response time of sub-threshold circuits can
be slower compared to traditional BGRs.
Design Considerations:
• Component Selection: Careful selection of transistors and resistors is
crucial to ensure accurate and stable operation.
• Bias Circuit Design: The bias circuit should be designed to provide
stable and precise bias currents to the sub-threshold transistors.
• Layout Techniques: Proper layout techniques can help minimize
parasitic effects and improve the circuit's performance.
• Simulation and Verification: Thorough simulation and verification are
essential to optimize the circuit's performance and ensure it meets the
desired specifications.
• Curvature-Compensated BGR: This technique can improve the accuracy of the
BGR by compensating for the non-linearity of the bandgap voltage.
The Solution: Curvature Compensation
A curvature-compensated BGR addresses this issue by introducing additional
circuitry to compensate for the non-linearity. This is typically achieved by adding
a current source or a resistor to the BGR circuit.
How it Works:
1. Non-linearity Detection: The circuit monitors the deviation of the BGR
output from the ideal linear behavior.
2. Compensation Signal Generation: A compensation signal is generated
based on the detected non-linearity.
3. Reference Voltage Adjustment: The compensation signal is used to
adjust the reference voltage, effectively canceling out the non-linearity.
Key Advantages:
• Improved Accuracy: By compensating for the non-linearity, the BGR can
achieve higher accuracy over a wide temperature range.
• Reduced Temperature Sensitivity: The temperature sensitivity of the
reference voltage is significantly reduced.
• Enhanced Stability: The circuit becomes more stable, reducing the
impact of process variations and noise.
Circuit Implementation:
Curvature Compensated BGR Circuit
Design Considerations:
• Component Selection: The choice of components, particularly the
transistors and resistors, is crucial for achieving optimal performance.
• Bias Current: The bias current of the transistors should be carefully
selected to balance performance and power consumption.
• Layout Techniques: Proper layout techniques can minimize parasitic
effects and improve the circuit's performance.
• Simulation and Verification: Thorough simulation and verification are
essential to ensure the circuit meets the desired specifications.
Key Design Considerations:
• Power Efficiency: The voltage boosting circuitry can consume significant power.
It's crucial to optimize the design to minimize power dissipation.
• Noise and Ripple: The voltage boosting techniques can introduce noise and
ripple, which can degrade the reference voltage's quality. Careful filtering and
noise reduction techniques are essential.
• Stability: The BGR circuit must be stable over temperature and process
variations. Feedback techniques can be used to improve stability.
• Accuracy: The reference voltage must be accurate and precise. Careful
component selection and layout techniques can help achieve high accuracy.
Simulation and Verification:
• Circuit Simulation: Use circuit simulation tools like SPICE to analyze the
circuit's performance under different conditions.
• Layout Design: Consider the layout of the circuit to minimize parasitic effects
and optimize performance.
• Testing and Characterization: Thoroughly test the BGR to verify its
performance and ensure it meets the specified requirements.
By carefully considering these factors and employing appropriate circuit design
techniques, it is possible to design a BGR that meets the specified requirements of a
1.2V supply and a 0.9V reference voltage.
