Hysteretic Buck Regulators

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• It is one of the simplest switching regulators to implement, and it is also one of the fastest.

• Hysteretic control architecture is used on the LM3475 and LM3485, and by some new products that are being developed.

• The modulator is simply a comparator with a fixed amount of hysteresis that compares the feedback voltage to a reference voltage.

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• When the feedback voltage exceeds the reference by the hysteresis voltage amount, the comparator output goes low, turning off the switch.

• The switch will remain off until the feedback voltage falls below the reference, at which point, the comparator output goes high, turning on the switch.

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• This topology is extremely fast at reacting to load transients, has a very simple, wide bandwidth control loop that doesn't use an error amplifier, and it doesn't require frequency compensation.

• However, unlike a PWM regulator, it's switching frequency isn't set by an oscillator, but is dependent on several variables

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Switching Waveforms

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• When operating in steady-state continuous mode, the inductor current must follow the same charge-balance requirement of a PWM regulator.

• As a result, the ripple current equations for this regulator are the same as those for a PWM buck regulator.

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• This ripple current generates a AC voltage across the output capacitor's ESR and ESL. This results in a triangle-shaped waveform at ΔVOUT that determines the on and off time of the output switch.

• The output switch alternates between on and off when the waveform ramps beyond the upper and lower thresholds set by VHYS and the feedback resistors.

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Output Ripple Voltage Detail