STOD1412

Introduction

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5.7

External components

5.7.1

Inductor

The 1.3 MHz frequency allows the use of small inductors for both converters. In typical

applications, a 4.7 µH and a 6.8 µH are recommended for step-up and inverting respectively.

Larger values of inductor reduce the ripple inductor current. The inductor's current

saturation rating must exceed the peak current.

5.7.2

Capacitors

In order to reduce the ripple voltage on the outputs it is recommended to use capacitors with

low equivalent series resistance (ESR) on output filters. The interaction between the ESR

value of the capacitor and peak inductor current determines the amplitude of the ripple on

the output voltage. The suggested value for output capacitors is 4.7 µF.

In order to filter the input voltage variations, a ceramic capacitor must be connected

increased to further reduce the noise coming from input power supply. A 100 nF to

1 µF capacitor on the VREF pin is also recommended.

5.7.3

PCB Layout

Board layout is important due to high current levels and high switching frequency that could

radiate noise. It is important to connect the signal GND pin, the input and output capacitor

ground leads and power ground to a single connection point to obtain a star ground

configuration. This minimizes ground noise and improves regulation. It is useful to minimize

lead lengths in order to reduce stray capacitance, trace resistance to avoid voltage drops

and noise irradiation, especially to the feedback circuit, ground circuit and LX_ traces. Place

feedback resistors close to their respective feedback pins. Place input capacitors as close

Figure 4.

Star ground plane