Datasheet

LTC3729
24
3729fb
The diagram in Figure 9 illustrates all branch currents in
a 2‑phase switching regulator. It becomes very clear after
studying the current waveforms why it is critical to keep
the high‑switching‑current paths to a small physical size.
High electric and magnetic fields will radiate from these
“loops” just as radio stations transmit signals. The output
capacitor ground should return to the negative terminal of
the input capacitor and not share a common ground path
with any switched current paths. The left half of the circuit
gives rise to the “noise” generated by a switching regulator.
The ground terminations of the sychronous MOSFETs and
Schottky diodes should return to the bottom plate(s) of the
APPLICATIONS INFORMATION
input capacitor(s) with a short isolated PC trace since very
high switched currents are present. A separate isolated
path from the bottom plate(s) of the input capacitor(s)
should be used to tie in the IC power ground pin (PGND)
and the signal ground pin (SGND). This technique keeps
inherent signals generated by high current pulses from
taking alternate current paths that have finite impedances
during the total period of the switching regulator. External
OPTI‑LOOP compensation allows overcompensation for
PC layouts which are not optimized but this is not the
recommended design procedure.
Figure 9. Instantaneous Current Path Flow in a Multiple Phase Switching Regulator
R
L
V
OUT
C
OUT
+
D1
L1
SW1
R
SENSE1
V
IN
C
IN
R
IN
+
D2
BOLD LINES INDICATE
HIGH, SWITCHING
CURRENT LINES.
KEEP LINES TO A
MINIMUM LENGTH.
L2
SW2
3729 F09
R
SENSE2