24
LTC3729
sn3729 3729fas
APPLICATIO S I FOR ATIO
W
U
U
U
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 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
