Datasheet
DocID022607 Rev 2 15/22
STOD1317B Application information
Figure 12. Ground schematic
Such isolation is necessary to prevent high-level switching currents from returning to the
battery, or other power supply, through the same ground-return path as the analog signals.
If that happens, the ground path of those sensitive signals is disturbed; the high-level
switching currents flowing through the ground's resistance and inductance cause the
voltage along the return path to vary.
In addition to the grounding scheme, proper placement of the regulator's components is
important.
Beginning a new layout, for the reasons above, it is necessary to firstly place the capacitors
C
IN
, C
OUT
and C
MID
as close as possible to the related device pins.
After that, it is possible to place the inductors and the Power GND routing. Next, we can
trace the GND connected through vias to the PGND near to one of the main filter capacitors.
The LDO needs a quiet ground signal in order to operate properly.
It is important to pay close attention to the routing of traces from capacitor terminals in a
DC-DC converter circuit.
Large-valued low-ESR capacitors are expensive, and bad routings can cancel their
performance.
A good routing, on the other hand, can lower the output noise.
Ripple is directly related to the inductor value, the capacitor ESR, the switching frequency,
and so forth, but HF noise (spikes) depends on parasitic elements and the switching action.
In a bad routing, parasitic inductance associated with trace lengths causes problems:
In Figure 12, L1 brings about an increase in noise, and L2 limits the attenuation of an added
HF capacitor. The solution is to bring the input trace in on one side of the capacitor pad, and
the output trace out on the other side of the pad.
This track can be longer.
In fact we add here an inductor
that creates a second order filter
with the CoHF
CoHF
Via wich dives into
the ground plane
Via wich dives into
the power supply plane
L2
L1
We add here
an impedance
that lowers the
resonating
frequency of
CoHF
Do not !!
Vout
COUT
CoHF
COUT
GND
1/2 L3
1/2 L3
Vout
Do !!
Vout
Start from the
component pad and not
the incoming track
# V routing #:
the incoming and the outgoing
track are not connected to each
other but only to the capacitor pad
CO HF L3 f
100 nF
100nF
100nF
30 nH
(1via)
10 nH
1 nH
3 MHz
5 MHz
16 MHz
Co HF resonating frequency