Application information

ST2S08B

12/18

Doc ID 18290 Rev 2

Equation 4

In addition to the inductance value, in order to avoid saturation, the maximum saturation

8.4

Input and output capacitor selection

It is recommended to use ceramic capacitors with X5R or X7R dielectric and low ESR as

input and output capacitors, in order to filter any disturbance present in the input line and to

obtain stable operation. The output capacitor is very important for satisfying the output

voltage ripple requirements.

be calculated as:

Equation 5

where

The use of ceramic capacitors with voltage ratings in the range higher than 1.5 times the

maximum input or output voltage is recommended.

8.5

Layout considerations

Due to the high switching frequency and peak current, the layout is an important design step

for all switching power supplies. Important parameters (efficiency, output voltage ripple,

switching noise immunity, etc.) can be affected if the PCB layout is not designed paying

close attention to the following DC-DC general layout rules:

●

Short, wide traces must be implemented for mains current and for power ground paths.

The input capacitor must be placed as close as possible to the IC pins as well as the

inductor and output capacitor.

●

The FB pin connection to the external resistor divider is a high impedance node, so

interference can be minimized by placing the routing of the feedback node as far as

possible from the high current paths. To reduce pick-up noise, the resistor divider must

be placed very close to the device.

●

A common ground node minimizes ground noise.

●

The exposed pad of the package must be connected to the common ground node.

Moreover, the exposed pad ground connection must be properly designed in order to

facilitate heat dissipation from the exposed pad to the ground layer using PCB vias.

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