Hysteretic voltage regulators offer the potential advantages of simplicity, fast response, 100%-duty-cycle operation, high efficiency at light loading, and low cost. They need no loop-compensation components to add delays; thus, response time to a load change is less than one switching cycle. What's the catch? You must be able to accept a switching frequency that is not precisely controlled and a sensitivity to noise that requires layout skill.

Figure 1 shows a simple hysteretic switching regulator made from a comparator with a fixed hysteresis and a PFET. The comparator switches on the PFET whenever V_{OUT} falls to its low threshold and off again when V_{OUT} rises to its high threshold. The time V_{OUT} lingers between the thresholds determines the on-time and, hence, the switching frequency. The inductor's ripple current flowing through the ESR of C_{OUT} provides a triangular voltage-ripple waveform, which produces unpredictable operation.

Figure 1. |
This hysteretic regulator suffers from unpredictability ofthe switching frequency because of C _{OUT} ESR variance. |

Herein lies a potential problem with simple circuits of this type. ESR is a major factor in determining switching frequency, and ESR can vary over a wide range for any given capacitor type. This variance is seldom a good thing and can lead to inductor saturation if the frequency falls too low or FET overheating arising from switching losses if the frequency rises too high. A simple solution to the ESR-variance problem is to use a ceramic C_{OUT} capacitor in series with a resistor. Although this technique works nicely in the lab, it often poses problems in the real world, in which several ceramic capacitors bypass loads.

Another approach to predictable frequency control allows the use of low-ESR capacitors (Figure 2). It is almost identical to Figure 1's circuit except for the added resistor, R_{SERIES}, and the new connection point for C_{FF}. The inductor's ripple current induces the ac voltage present across R_{SERIES} and connects to comparator by C_{FF}. This controlled ac voltage eliminates the need for any C_{OUT }ESR. The feedback loop eliminates the dc voltage drop that R_{SERIES} creates. This new configuration produces predictable switching frequency with even zero-ESR capacitors and offers the potential of nearly zero V_{OUT} ripple at the cost of a resistor and the small added dissipation of R_{SERIES} carrying full load current.

Figure 2. |
An added series resistor makes this circuit’s switchingfrequency more predictable. |

The following equation approximates the switching frequency for either circuit, provided that C_{OUT}’s reactance at the switching frequency is lower than the ESR and C_{FF}’s reactance is much lower than R_{FB1}:

where ESR is the sum of C_{OUT}’s ESR and R_{SERIES}, V_{HYST} is the comparator's hysteresis voltage, and T_{PD} is the average propagation delay of the comparator plus the P_{FET}.

You can build the circuits of Figure 1 and Figure 2 as drawn, using a comparator, such as the LMV7219, which claims 7.5-mV built-in hysteresis, or by using a controller, such as the LM3485, which provides a current-limiting feature, wider V_{IN} range, and lower cost. You cannot overemphasize the layout sensitivity for hysteretic regulators. You cannot allow the feedback connection to pick up any stray signals. Open-core inductors are attractive for cost reasons but difficult to use, because any induced voltages from stray magnetic fields can produce unpredictable switching frequencies and ripple.

Figure 3. |
This circuit occupies an area smaller than a postage stamp. |

You can build the circuit in Figure 3 in an area smaller than a postage stamp. This circuit produces output current of at least 1 A, using small ceramic capacitors, a TSOP-6 PFET, a 6×7-mm inductor, and an SMB-package, surface-mount Schottky diode. F_{S} varies from 600 to 700 kHz over a V_{IN} range of 5 to 15 V for V_{OUT} = 1.8 V and V_{OUT} ripple less than 5 mV p-p. The 30.1-kΩ resistor and the PFET's on-resistance of 0.1 Ω set the current limit to trigger at 1.5 A. The no-load bias current is lower than 500 µA. Most impressive is the dynamic V_{OUT} change of only 10 mV for a load transient greater than 0.5 A.