*Linear regulators are still widely used in all kind of applications. If the current needs are in the lower range and efficiency is not important, they are still first choice. Also from a cost perspective, linear regulators can be a good choice, if the total solution cost including a heat sink can compete against a switcher solution. But if, for example, the power demands of an existing board increases, even a large linear regulator in a TO220 package can come to its limit. The widely used uA78xx and LM317 for example, are rated up to 1.5A output current. But this current rating is more of a theoretical value, as we struggle due to the power dissipation even for this kind of package, what we will see later on. *

*By Matthias Ulmann, Texas Instruments*

This article shows, how a synchronous buck converter with the same size as a TO220 package as well as the same pinning can solve the problem, if the power dissipation of a linear regulator solution causes trouble or if the power demands rise and no redesign of the PCB is possible or wanted.

Limits of a linear regulator based solution In a typical small industrial application, a uA7805 generates 5.0V from a 12.0V or 24.0V input. It supplies a microcontroller, some signal LEDs, digital and analog interfaces. The current demand is mostly in the range of a few tenths up to some hundred milliampere. With this specification, the power losses on the linear regulator for a maximum output current of 200mA can be calculated easily:

Figure 1 shows the linear regulator without being mounted to a heat sink on the left side.

The losses of 1.4W cause a heating up to more than 100ºC at 21ºC ambient temperature. Although the output power of 1 W is quite low, the linear regulator in its TO-220 package needs to be attached to a heat sink to dissipate the significant losses. On the right side, a LM25017 based buck converter with the same size like a TO-220 package shows only a maximum temperature of around 44ºC under the same test conditions.

For the next comparison, the linear regulator was mounted properly on a type of standard heat sink for this kind of application (Aavid Thermalloy, 531102B02500G, 38.1x34.9x12.7mm) with a thermal resistance of 10.4k/W with natural convection. At 400mA load which equals an output power of 2W and power losses of 4.9W, the maximum temperature using a proper heat sink with natural convection is around 60ºC. This is an acceptable value but care has to be taken, that the large amount of heat can be dissipated. Using this solution inside a small closed housing will not work in most cases, since the 4.9W of power losses will heating-up the interior significant.

Due to the high efficiency of around 83% at this operation point, only 410mW need to be dissipated by the LM25017 buck-based solution shown on the right side of Figure 2. With the same size like a TO-220 package, the maximum temperature is also around 60 ºC.

So using a linear regulator in a large package like TO-220 is mostly not done due to high output current demands, but rather to allow dissipation of the losses.

### Switcher replacement in TO-220 size

For a direct replacement of this kind of linear regulators, three main prerequisites need to be fulfilled:

- Same size and pin out
- Regulation independent of additional input and output capacitors
- Wide input voltage range

The available space is 10x15mm, if the components are mounted only single sided. This needs a switching frequency in the higher range at the expense of higher switching losses, to keep the inductor small. For a pinto- pin compatible replacement it is necessary, that the control scheme is not influenced by additional capacitance on the output. Buck converters based on voltage or current mode change their transient response, if the output capacitance is increased or decreased. Therefore a control scheme which is not influenced by additional output capacitance like Constant On-Time is most suitable for this application. The LM25017 with its large input voltage range up to 48V fulfills perfectly all these requirements.

The circuit is small and needs only a few external components, which is important to get the solution on the required small size. For regulation of the output voltage, the converter needs a voltage ripple of at least 25mV on the feedback. This can be generated either by using an output capacitor with high ESR or by generating a triangular ramp using the switching node and an R-C combination (R2, C5). With the second solution, the output ripple is much lower, as a ceramic output capacitor can be used.

Resistor R1 sets the switching frequency, which is not constant due to the regulation scheme and changes between 500 and 800 kHz dependent on the input and output voltage. For 5.0V output voltage, R1 needs to be set to 80.6kΩ and R4 to 30.1kΩ. The same circuit can also be used for lower output voltages by changing only these two resistors. For an output voltage of 3.3V for example, R1 needs to be 52.3Ωk and R4 16.9kΩ.

The synchronous buck converter is designed for an input voltage range of 10 to 30V DC and for an output voltage of 5.0V or 3.3V at 600mA load. 600mA is the peak output current; the continuous output current is around 400mA if the board temperature should stay at a decent value as shown in Figure 2.

The efficiency curve (Figure 4) shows good results for this circuit. At higher input voltage, the efficiency drops due to the increasing switching losses.

As already mentioned, the Constant On-Time control scheme needs a voltage ripple on the feedback. This voltage ripple is derived from the current ripple of the inductor and therefore dependent on the ratio of the input and output voltage. This causes an influence on the load regulation as it can be seen in Figure 5. Anyhow, the output voltage is at all operation points within a tolerance band of ±3% of the nominal output voltage of 5.0V.

Measurements of the output voltage ripple showed a maximum of 40mV (0.8%) and also the output voltage variation on a load step from 200mA to 400mA and vice versa is remarkable low with maximum 90mV (1.8%).

**Conclusion **

This article showed the practical limits of linear regulators in a TO-220 package a how they can be replaced by a pin-to-pin compatible synchronous buck converter based on a LM25017 with the same size. It offers a wide input voltage range from 10V up to 30V and is ideal for industrial applications, where supply voltages of 5.0V and 3.3V with a few hundred milliampere are needed. All information for this reference design (schematic, layout, bill of material) is available on ti.com under the keyword "PMP8581".