> 3.5 | Switched DC/DC Converter

> A switched DC/DC converter is very often used when there are requirements for a small footprint, high efficiency and little heat dissipation. Linear regulation often leads to undesirable heat dissipation that needs to be dispersed by a heat sink. This problem can be solved by using a switched DC/DC converter that creates (almost) no heat at all.
> However, there are a number of issues to take into account when working with switched DC/DC converters: Shall the DC/DC converter be isolated, which design typology is the best to use (boost, buck, fly back, or other), which operation modes is best employed (PWM, hysteretic, other), switching frequency, required efficiency, cost/size, post filtering as well as layout.
> Not all of these issues can be covered in an application note, but some guidelines are given here.
> There are DC/DC controllers available on the market with an integrated switch (like the simple switch series from National), and they are often easier to design than DC/DC controllers with external switch.
> When working with switched regulation the switching frequency is very important as it decides how fast the switcher reacts on voltage drops (together with the filter in the error amplifier) at the load (in this case, the wireless module). The switching frequency is also important for the choice of components around the switcher circuit. Higher frequency means smaller (and cheaper) components.
> Integrated switch circuits typically work with frequencies from 50kHz to 1MHz. There are disadvantages operating with high frequencies. A switch creates ripple on the supply voltage. This ripple must be minimized on the power supply line to the wireless module. Otherwise, there may be the risk of violating the transmitter modulation spectrum and therefore application approval, especially at higher DC/DC switch frequencies.
> Normally there are no EMC tests for frequencies below 150kHz. This means that with a switcher frequency of 48kHz, even the third overtone is below the limit. But this third overtone can cause a violation of the modulation of the signal (see also Chapter 4), if the ripple value is too high. This point should be considered, even if the risk of a spectrum violation is much smaller than at high switching frequencies.
> If a small, inexpensive design is required, it should be realized with higher frequencies, but the mentioned issues need to be taken into account.
> NOTE: Keep in mind that around a DC/DC switch (specially the coil) there are magnetic fields, which can create disturbances to other electronic circuits. This might be the case if the coil is placed close or under / over the RF part of the wireless module.