Multiphase Converter Current Limit Control for TLVR Stability
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Solution Overview
Problem
Constant ON-time multi-phase controllers with valley mode current limiting can become unstable in current limit mode when used with trans-inductor voltage regulator (TLVR) power networks, leading to increased voltage ripple and inductor current ripple due to non-monotonic phase currents causing positive feedback in PWM pulse timing.
Innovation Solution
Implementing a control circuit that gradually increases the load line resistance of the controller as soon as the system enters burst mode operation, diverting current from the controller to the output capacitor and preventing the controller from operating in current limiting mode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If valley mode current limiting is used in constant ON time multi-phase controllers, then current protection is improved, but stability of the control loop deteriorates in current limit mode
Solution Approach 1:
The patent applies dynamics by making the ON time variable rather than constant. The ON time is adjusted based on the operating mode: in voltage regulation mode, a first ON time is used, while in current limit mode, a second (different) ON time is used. This dynamic adjustment of the ON time parameter resolves the stability issue in current limit mode while maintaining reliable current protection functionality.
2Power
If non-monotonic phase currents are present in TLVR power networks, then power delivery capability is improved, but positive feedback in PWM pulse timing occurs causing instability
Solution Approach 1:
The patent uses dynamic ON time adjustment to counteract the positive feedback effect of non-monotonic phase currents. By changing the ON time based on the detected operating mode, the system prevents the runaway effect where PWM pulses bunch up and spread out alternately, thereby maintaining stable timing while preserving the power delivery advantages of TLVR networks.
Solution Approach 2:
The patent implements feedback by detecting the operating mode (voltage regulation vs. current limit) and using this information to adjust the ON time accordingly. This closed-loop approach prevents the positive feedback instability caused by non-monotonic phase currents while maintaining the benefits of TLVR power networks.
3Reliability
If PWM pulses bunch up due to positive feedback, then current limit protection is maintained, but voltage ripple on output voltage increases significantly
Solution Approach 1:
By dynamically adjusting the ON time based on the operating mode, the patent prevents the PWM pulse bunching that leads to excessive voltage ripple. In current limit mode, the adjusted ON time distributes the PWM pulses more evenly, maintaining current protection while significantly reducing the harmful voltage ripple effect.
Data Source
AI summary
Described embodiments include a control circuit with a first comparator having a first comparator input receiving a first threshold voltage, and a second comparator input coupled to an output voltage terminal. A second comparator has a third comparator input receiving a second threshold voltage, and a fourth comparator input coupled to a current output terminal. A first logic circuit provides a true signal at its output responsive to a particular number of its inputs receiving a true input. A second logic circuit has inputs coupled to the first comparator output, and to the first logic output. A variable resistance circuit has an output coupled to a mode detection output. An amplifier has inputs coupled to the variable resistance circuit output, and a third reference voltage source. A duty cycle generation circuit provides a respective pulse width modulation (PWM) signal at each of its respective duty cycle outputs.