Emulated ripple technique advances hysteretic switchmode supplies

How hysteretic control works
The typical hysteretic mode controller (Fig. 1, such an example is National Semiconductor's LM3485) offers a very simple way to regulate a fixed output voltage. The necessary building blocks are a reference, hysteretic comparator, and a power stage that creates a pulse width modulation (PWM) duty-cycle based on the output of the comparator.

When the output voltage is below the comparator's low-voltage threshold, the power stage turns on; the duration of this on-time pulse is a function of how long the output voltage stays below the threshold. As a result, the pulse frequency is not constant, but depends on how the output voltage changes during both the on-time and off-time. Hysteretic control does not require an internal oscillator. Switching frequency depends on the external components and operating conditions such as load current and line voltage, since these parameters influence when the output voltage crosses the hysteretic comparator thresholds.

As mentioned previously, this technique is fast, simple, and low-cost. Its disadvantage is its varying switching frequency, which depends on such power stage components as the inductor and output capacitor, as well as the changing input voltage. A second disadvantage is the aforementioned ripple voltage requirement at the input (feedback pin) of the comparator. If the ripple generated by the inductor current ripple and the output capacitor is phase shifted, the output voltage will not cross the hysteretic comparator thresholds in phase, and the control scheme will not work smoothly. The result is burst-mode operation; the waveform will not be continuous or clean.

When a ceramic capacitor, which has a very low equivalent series resistance (ESR), is used as an output capacitor, the wave shape of the main voltage ripple will be mostly determined by the capacitance and not by the ESR of the component. The resulting output contains a sinewave-like voltage ripple, shifted by 90 degrees. A capacitor with higher ESR (such as an electrolytic) creates a triangle-like voltage ripple in sync with the switch waveform. Figure 2 depicts the differences in the waveforms.

The benefit of constant on-time (COT)
Constant on-time (COT) control minimizes the change in switching frequency with line variations which, depending on the particular hysteretic design, would otherwise vary the switching frequency significantly and create many problems. Traditional designs, for example, require a system's EMI filter (if implemented) to work effectively over a wide range of frequencies. Consequently, the power stage components and the input capacitor must also be appropriately selected to operate well over the wide range of switching frequency.

Constant on-time control sets a fixed on-time, which is influenced by only the input voltage. The on-time becomes inversely proportional to the input voltage. With this feed-forward correction of line variations, the switching frequency will not have to change and so the simple relationship of duty cycle to input and output voltage (i.e., D = V_out/V_in), is retained.

This technique also improves line regulation in some voltage-mode regulators. In a pure voltage-mode regulator, the duty-cycle is corrected only after the effect of a line transient is seen on the output voltage. In constant on-time systems (such as National's LM5010), line changes are taken into consideration before they even influence the output voltage. See Fig. 3. The timer determines an on-time that is a function of V_in as well as a frequency setting resistor between V_in and the on-timer function block.

Adding ERM
The emulated ripple mode (ERM) for pure hysteretic control allows the use of ceramic output capacitors with very low ESR values. The circuitry senses the supply's inductor current during the off-time and injects the necessary ripple into the hysteretic comparator. In typical constant on-time regulators, the on-time is fixed and changes only with the input voltage. The critical threshold that defines the timing for stable operation is the low-voltage threshold of the hysteretic comparator. This threshold needs to be crossed with a signal that is in sync with the switching action. If an output capacitor with higher ESR is selected, its ripple on the feedback node can be used to drive the hysteretic comparator.

Instead of requiring the output voltage to drop, the circuit senses a portion of the off-time low-side current and injects it into the error comparator. Thus the comparator has a correctly timed signal to start a new cycle.

The general block diagram in Figure 4 shows how an ERM loop is added on to a constant on-time hysteretic mode power converter in order to eliminate the greatest drawbacks of traditional circuits. National's LM3100 family of buck regulators provides one example of the ERM implementation in a complete one-chip synchronous design. The circuitry senses current during the off-time at node A. This is very easily done in synchronous regulators where the low side switch is integrated into the IC. As a result, the off-time current information can easily be collected without the need for an external sense resistor or additional IC input. The off-time current ramp information is in sync with the switch node and is AC coupled to node B by a small capacitance. The positive input of the hysteretic error comparator now sees the required ripple, which is in phase with the switch node voltage waveform. The negative input of the comparator will not need to provide this ripple information and by design the output capacitor may be a ceramic, organic, or other very low ESR capacitor.