Considerations during the Switching Regulator Design

Some of the useful tips for switcher design which will help the beginners. When you are deep into the power supply design come back here and have a look at these.

The main things to look at while designing a circuit around switching regulator are:

1. Study the requirments of the design
2. Select a approriate switching regulator IC from different vendors available in the market (Vendors like Texas Instruments,Linear Technologies,ROHM, etc.,)based on the below factors:

Efficiency
Internal or External inductor
Fixed or Adjustable regulator
Switching frequency
Load current supported
Operating ans storage Temperature
Domain it is intended to be used (Commercial, Military, automotive, Industrial, etc.,)

3. The input capacitors are to be selected based on the maximum input voltage, input ripple voltage, variation with temperature. The voltage rating of the capacitor must be greater than the input voltage applied. The capacitors must be able to handle the ripples that are coming on the input rail. Capacitors like ceramic and especially X5R/X7R must be used.

4. If you look at the datasheets of the switcing regulators, you will get to know that simple passive components connected at the output of the pins mentioned helps to select the switching frequency of that regulator. Take for example LM5165-Q1 regulator from TExas Instruments where the resistor connected to RT pin determines the switching frequency. Below is the snapshot from the datasheet of LM5165-Q1.

5. To eliminate any inrush current, the output should ramp up slowly. For controlling the output rise time there is a specific pin and connecting a specified capacitor to that pin controls the rise time of output. The calculation of resistor can be done using the equations mentioned in the datasheet.

6. The bandwidth of the regulator can be controlled by adding the compensatory circuit mentioned in the datasheet. As we have studied in the academics where the addition of pole and zero changes the transient response of the regulator.

7. The output capacitors are selected based on the output voltage and current ratings. Here also Ceramic capacitors are preferred.

8. Modern processor boards require power sequencing. Some of the pins of the regulator can be used to achieve power sequencing. The power good output of the switching regulator can be connected to enable pin of next regulator to control the sequencing. This helps reduce the board size as there are no external components required for sequencing.

9. Most of the designers use power good pin to connect to a LED. The power good pin is generally open-drain in regulators and designers connect LED circuit to this pin which glows when power good goes high. This visual representation is very useful while debugging.

10. Switching regulators use catch diode which comes into action when the internal MOSFET is OFF. The inductor current passes through the catch diode. This is also known as flyback diode, snubber diode, commutating diode, freewheeling diode, suppressor diode, suppression diode. Clamp diode generally is a schottky diode as it is desired that the forward voltage is low. Lower the forward drop of the catch diode, higher the efficiency of the regulator. Also, clamp diode must have the capability to carry the desired load current. So, when selecting a catch diode, designers take the inductor current into consideration. Catch diode should always have current rating more than inductor current. Also, the reverse voltage rating of catch diode must always be higher than input voltage applied to swithcing regulator.

11. The output inductor must be selected based on the load current and the switching frequency of the regulator.

These are few considerations and as we dig deep into the respective datasheets of switching regulator we tend to realize the depth of this subject.