> 5.2 | Guidelines for DC/DC Converters

> When working with switched DC/DC converters there are some special issues to consider for PCB layouts. Guidelines can very often be found in the data sheet for the IC, but here are some general hints:

> • | Place the inductor as close as possible to the switch.

> • | Place the freewheeling diode (or second switch) on the line between the switch and the inductor.

> • | Place the output capacitor as close as possible to the inductor.

> • | Place the input capacitor as close as possible to the switch.

> • | Keep all lines to the components mentioned above as short and thick as possible.

> • | Consider the block consisting of switch, inductor, free-wheeling diode, and input and output capacitors as a source of disturbance signals, and keep it as far away from micro controller, audio blocks and other parts that can easily be interfered.

> • | the RF part should not be placed too close to the converter

> It is sometimes recommended to run the ground lines to the power section separately from other ground lines. This is to prevent the high current in the output circuit to influence the regulating circuit. However, both ground lines must be connected in one single point to ensure that the potentials for the two grounds cannot differ.
> Figure 8 shows an approved example for a DC/DC converter with fixed 150kHz switching frequency. The circuit has a low pass prefilter (C4-L2) to suppress the 150kHz switching ripple feeded to the input supply cable. The output capacitors C1 and C2 are doubled to reduce the ESR value as well as the input capacitor C3 must be a low ESR capacitor. An additional output L-C low pass post filter (L3-C7) should be added during the first design phase as it decreases the high frequency spectrum which may violate the GSM modulation spectrum. This post filter can be skipped if not needed after design validation.
> Figure 8: DC&DC converter (example with 150kHz switching frequency)

> Table 2: | Item list for DC/DC converter (example with 150kHz switching frequency)

> Item | Description | Ordering code / Value | Manufacturer
> C1, C2, C7 | Tantal capacitor | 100µF 6V3 |
> C3 | Aluminium capacitor | 220µF 35V |
> C4, C6 | Ceramic capacitor | 2µ2 35V |
> C5 | Ceramic capacitor | 1n5 |
> D1 | 3A, 40V power schottky rectifier | STPS340U | STM
> IC1 | 3A fixed 150kHz switching converter | LM2596 | National Semicunductor
> L1 | Inductor closed housing | CDRH125NP-470M / 47µH | Sumida
> L2 | Inductor | 74477310 / 10µH | Würth Elektronik
> L3 | Inductor | 744773022 / 2µ2 | Würth Elektronik