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

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