Bandgap Reference Circuit Isolation Transistor Trim
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Solution Overview
Problem
Conventional test/trim procedures for bandgap reference circuits are hindered by stray capacitance from test/trim apparatus probes, leading to increased response time and manufacturing costs due to the need for longer stabilization times of the bandgap voltage.
Innovation Solution
Incorporating isolation transistors to separate test/trim apparatus probes from critical nodes of the bandgap reference circuit, allowing for reduced stray capacitance and faster stabilization of the bandgap voltage during the test/trim procedure by using current control circuits with trim elements that adjust current flow through critical nodes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional test/trim procedures are used with direct probe connections to critical nodes, then trimming functionality is achieved, but stray capacitance from probes increases response time and reduces productivity
Solution Approach 1:
An isolation transistor is introduced as an intermediary element between the test/trim probes and the critical nodes of the bandgap reference circuit. The isolation transistor's gate is controlled by a control signal to enable or disable connectivity. During normal operation, the isolation transistor is off, preventing probe capacitance from loading the critical nodes. During test/trim operations, the isolation transistor is turned on to allow probe access while minimizing capacitance impact, thus resolving the contradiction between measurement accuracy and productivity.
2Manufacturing precision
If trim elements are directly connected to critical nodes, then trimming accuracy is improved, but stray capacitance from test probes increases stabilization time
Solution Approach 1:
The isolation transistor serves as a mediator that allows precise trimming to be performed while minimizing the time penalty. By controlling the isolation transistor's on-state resistance and timing its activation, the system achieves accurate trimming measurements without prolonged stabilization periods, effectively resolving the trade-off between manufacturing precision and time loss.
Solution Approach 2:
The isolation transistor introduces dynamic control over the connection between trim elements and critical nodes. The transistor's state can be switched between on and off based on operational requirements, allowing the system to optimize between accurate trimming measurements and rapid voltage stabilization, thus resolving the static contradiction between precision and time.
3Ease of manufacture
If test probes are connected directly to critical nodes for trimming, then trimming functionality is enabled, but stray capacitance increases response time and manufacturing costs
Solution Approach 1:
The isolation transistor enables trimming functionality while acting as a capacitor-isolating intermediary during normal operation. This allows the manufacturing process to maintain high productivity by preventing capacitance-related delays, while still enabling accurate trimming when needed, thus resolving the contradiction between ease of manufacture and production throughput.
Data Source
AI summary
A bandgap reference circuit utilizes differential transistors to generate a temperature-independent bandgap voltage. In place of conventional trim elements that are connected in parallel to and adjust the resistance values of the bandgap reference circuit, current control circuits are placed in the current paths passing through the differential transistors (i.e., connected to the critical nodes located at the terminals of the differential transistors). Each current control circuit includes a resistive “trim” element (e.g., a zener diode) and associated trim pads that are separated from the critical nodes (i.e., the terminals of the differential transistors) by isolation transistors such that, during a trim/test procedure, the stray capacitances introduced by trim/test equipment probes are prevented from altering the performance of the bandgap reference circuit. In one embodiment, a current control circuit is connected to the critical node connected to the base of at least one of the differential transistors.