Buck Regulator Architectures 4.3 Hysteretic Buck Regulators Author's Original Notes:

Hysteretic Buck-Regulator Architecture Ripple is needed to properly switch the comparator! Advantages: Wide transient Bandwidth, fast transient response. No frequency compensation to deal with. VIN feed-forward is inherent. Simple system design. Disadvantages: tON and tOFF , and therefore the frequency, are functions of VIN, VOUT, L, ESR, ESL, VHYS*(RF1+RF2)/RF2, and tD.. The operating frequency is variable giving rise to a potential EMI issue! Requires ripple at feedback comparator, in order to function as a regulator. Sensitive to noise on the output – interpreted as ripple to manage Simplest and fastest topology but variable frequency!

Hysteretic Regulator Waveforms IN SW -0.6V I OUT L t ON OFF (DC) SW Pin Inductor Current Output Voltage Buck Switch stays on for an On-time Determined by VIN and RON Inductor's current ripple determined by VIN, VOUT, On-time and L Output voltage ripple determined by inductor's and COUT ESR Reference Threshold DI = DV Dt L The hysteretic regulator’s fast transient response is suitable for microprocessors and DSPs or other high-slew-rate transition loads. The control reacts to the load-current transient in the same switching cycle that the transient occurs. The simple solution of hysteretic-mode control requires no compensation components. It is a self- oscillating circuit that regulates the output voltage by keeping it within a hysteresis window set by a reference and comparator. The top P-MOSFET does not need the required capacitor and diode- charge pump as required with N-MOSFET drivers. The switching frequency is derived from equations that describe the output-ripple voltage (ΔVOUT). In most cases, the dominant cause of ΔVOUT is the output capacitor’s ESR. From the equation we can see that it depends on the ESR, L, input and output voltage, hysteresis window, and internal delays. Thus in order to operate at >500 kHz for example, this can be a problem because the switching frequency is proportional to ESR and using a very low ESR by connecting many ceramic capacitors in parallel causes the operating frequency to become relatively low.

Output Ripple Voltage Detail The following diagram shows how the ripple elements caused by the output capacitor's ESR and ESL components combine into a composite ripple waveform.

LM3485 Architecture This is the block diagram of the LM3485 from the datasheet. This is shown as an example of a hysteretic controller. The LM3485 is designed to drive a PFET in order to provide simplicity in the drive circuit. PFETs are a little more expensive than NFETs for the same on resistance because hole mobility is half electron mobility. The current sense circuit allows for a very simple current limit function. If the current limit is reached, the PFET is turned off for a period of 9us. This action protects the FET, but the power may need to be recycled or the load significantly reduced to begin normal operation again.

LM3485 Hysteretic Controller In this example schematic, RF1 is 62kohm and speed up capacitor CFF is 1nF. CFF allows the high frequency ripple to appear at the FB pin without being attenuated by RF1. This means the total output ripple can be reduced to the minimum required by the LM3485. However, it also means that the operating frequency will be increased. So lets look at how we determine operating frequency.

Calculating Switching Frequency In most cases, switching frequency is determined by the output ripple voltage (ΔVOUT) resulting from the output capacitor’s ESR. The amplitude of ΔVOUT is described by the following two equations:  Combining these two equations yields an expression for the switching frequency. Note: If a speed up capacitor is used the circled term in the denominator of this equation becomes 1 which means the switching frequency value will increase. Author's Original Notes:

Pros & Cons of Hysteretic Control Hysteretic controllers have excellent load current transient-response characteristics compared to the other types of controllers (such as PWM voltage and current mode) with slow feedback loops The controllers react to transients within the same cycle in which the transient occurs and keep the corresponding FET in an on-state until the output voltage returns to the required dc level Thus a minimum number of bulk output capacitors are required, saving total system cost The hysteretic regulator does not have compensation circuitry that requires an accurate design in the whole input-voltage, output-voltage, temperature, and load-current range This compensation can be further complicated if additional capacitors are added to the output of the voltage regulator around the microprocessor package The main problem associated with the hysteretic type regulator relates to the ability to predict its switching frequency due to the dependence of this parameter on the output filter characteristics and circuit operation.

LM3475/85 Using Electrolytic COUT The graphic shows the output ripple and switch node voltage The operating frequency is 1.43MHz The feedback network does not use a CFF speed up capacitor The output ripple is 250mV because the propagation delay is such a large percentage of the period. The output capacitor is an electrolytic with a significant amount of ESR. However, as ESR falls to zero, the equation is no longer valid. Why? The equation is only looking at ripple in phase with the switching action. It does not take into account any ripple caused by charge and discharge of the capacitance.

LM3475/85 Using Ceramic COUT An example of using a ceramic output capacitor with almost no ESR Operating frequency has dropped and can not be calculated using the equation mentioned previously Reason: The output ripple is 90°phase shifted from the switching action Essentially there is no feedback ripple in phase with the switching action which results in poor operation. This can not be fixed by using CFF since the problem is due to the ripple at the output. However, many designers prefer to use ceramic capacitors. The following pages show some easy fixes that can be used to allow the use of ceramic capacitors with hysteretic buck regulators.

LM3475/85 – Working with Ceramic Capacitors Desire Use ceramic capacitors Challenge Ceramic capacitors have very low ESR Results in 90°phase shift of output voltage ripple, resulting in low operating frequency and increased output ripple Solution Add a low value resistor in series with the ceramic output capacitor to provide an ESR value Although counter intuitive, this combination provides highly accurate control over output voltage ripple The use of ceramic capacitors has become a common desire of many power supply designers. However, ceramic capacitors have a very low ESR resulting in a 90°phase shift of the output voltage ripple. This results in low operating frequency and increased output ripple. To fix this problem a low value resistor can be added in series with the ceramic output capacitor to provide an ESR value. Although counter intuitive, this combination of a ceramic capacitor and external series resistance provides highly accurate control over the output voltage ripple.

Working with Ceramic Capacitors Another Technique By adding the three components circled in the diagram, we provide AC feedback in phase with the switching action. The 100pF capacitor provides bypassing of any high frequency edge noise which may cause improper triggering of the FB comparator. This method has a number of advantages. It is a reliable way of using low ESR output capacitors while insuring very low output ripple. It allows the use of high ESL output capacitors which generate spikes which may trigger the comparator early. Lastly, it makes the operating frequency independent of output capacitor ESR and results in a more predictable operating frequency. This method is highly recommended. Adjustment of R3 will adjust the operating frequency

Thank you! Author's Original Notes: