Fast PWM Dimming Control for LED Drivers
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Solution Overview
Problem
Constant on time control-based LED drivers exhibit sluggish dynamic response in PWM dimming, leading to challenges in swiftly aligning output current with the PWM dimming signal, necessitating a fast PWM dimming apparatus and control method to improve accuracy.
Innovation Solution
A fast PWM dimming apparatus and control method that includes an on timer, offset current control stage, current comparison stage, and PWM comparator, disabling the on timer during a load transient to enable the current to reach the desired load level within a single turn-on pulse of the high-side switch.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If constant on time control scheme is used in PWM dimming, then the control circuit is simple (only comparator and on timer), but the dynamic response is sluggish and cannot swiftly align output current with PWM dimming signal
Solution Approach 1:
The patent implements dynamic control by switching between two operational modes: constant on-time mode for normal operation and fast transient response mode for load transitions. The controller dynamically adjusts the control strategy based on operating conditions, enabling swift current alignment during PWM dimming transients while maintaining simplicity during steady-state operation.
Solution Approach 2:
The patent changes the control parameters dynamically by disabling the on-time limit during fast transient response mode. This parameter change allows the high-side switch to remain on longer than the normal on-time limit, enabling the inductor current to reach the target level quickly during load transients without requiring a completely complex control circuit.
2Device complexity
If constant on time control is employed, then the circuit structure is simple with only comparator and on timer, but the PWM dimming accuracy is poor during load transients
Solution Approach 1:
The patent employs feedback mechanisms where the controller monitors the inductor current and PWM dimming signal, and adjusts the switch control accordingly. During fast transient response mode, the controller uses feedback from the current sense amplifier and PWM comparator to ensure the output current accurately tracks the PWM dimming signal, improving dimming accuracy without significantly increasing circuit complexity.
Solution Approach 2:
The patent prepares for transient conditions by detecting load changes and switching to fast transient response mode before the transient is complete. This preliminary action allows the system to proactively adjust the control strategy, ensuring accurate PWM dimming response during load transitions rather than reacting after the transient has already degraded accuracy.
3Ease of operation
If on timer determines on time of high-side switch, then control is straightforward, but current cannot reach desired load level within first turn-on pulse during load transient
Solution Approach 1:
The patent uses periodic switching with variable duty cycles to achieve fast transient response. During load transients, the controller extends the on-time of the high-side switch beyond the normal periodic on-time limit, allowing the inductor current to build up faster and reach the desired load level within the first turn-on pulse, thereby reducing time loss without making control overly complex.
Data Source
AI summary
An apparatus includes an on timer configured to determine an on time of a high-side switch of a step-down power converter, an offset current control stage configured to convert a voltage difference between a compensation signal and a predetermined reference into a current difference, a current comparison stage configured to amplify a sensed current signal, and a PWM comparator having inputs coupled to two outputs of the current comparison stage and two outputs of the offset current control stage, wherein during a load transient from a first load level to a second load level and in a first turn-on pulse of the high-side switch, the on timer is disabled so that a current flowing through the step-down power converter reaches the second load level within the first turn-on pulse of the high-side switch.