Hysteretic converters are widely used to drive LEDs in emerging lighting applications. This converter is very easy to use and its topology is quite stable, making it the first choice for high efficiency inductive switching regulator solutions. This simple topology can be used in many different configurations, sometimes even beyond the general scope of use. However, there are still many problems to be solved, and understanding the limitations of this converter will also help improve system performance. This article will detail the topology of this converter through different circuit configuration examples, and discuss some of the inherent problems and the impact of these problems on some special applications.
Topological structure
The hysteresis converter actually uses an on-off topology. It can be used in buck, boost or buck-boost configurations, and its superior stability makes it ideal for buck LED driver applications because hysteretic converters can be stable during one oscillation period. Down, like a PWM controller usually takes dozens of cycles to stabilize. The characteristics of hysteretic converters are reflected in control mechanisms, precision, frequency, duty cycle and propagation delay.
Referring to Figure 1, the control is implemented based on a comparator that predetermines the hysteresis voltage. The current in the LED is typically measured with a resistor (Rsense), the value of which typically varies between the upper and lower thresholds set by the comparator. Threshold settings are balanced between measurement precision/anti-noise performance and efficiency. Typical hysteresis voltages are between 50mV and 250mV.
Figure 1 Hysteretic Buck Converter
The frequency of oscillation depends on many factors, and the choice of inductance is the most important. One of the key features of the hysteresis converter is its self-oscillating characteristics. This means that the frequency will vary with the input voltage, the LED current, and the number of LEDs that must be driven. However, such converters are often implemented in continuous mode, which means that the inductor will never saturate and will not completely drain the current. This inherent stability means that the hysteretic converter can operate over a wide voltage range without the need for external components to compensate. Like many PWM topologies, this converter has no limits on the duty cycle range.
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