STOD1412 Datasheet, PDF (11/21 Pages) STMicroelectronics – Step-up and inverting DC-DC converter

STOD1412
Introduction
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
between V_CC and PGND. A minimum value of 2.2 µF is recommended. This value may be
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
Figure 4.
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
as possible to V_CC and PGND.
Star ground plane
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