LTC3729_15 Datasheet, PDF (24/30 Pages) Linear Technology – 550kHz, PolyPhase, High Efficiency, Synchronous Step-Down Switching Regulator

LTC3729
APPLICATIONS INFORMATION
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.
SW1
D1
L1
R_SENSE1
V_IN
R_IN
+
C_IN
BOLD LINES INDICATE
HIGH, SWITCHING
CURRENT LINES.
KEEP LINES TO A
MINIMUM LENGTH.
SW2
D2
L2
R_SENSE2
V_OUT
+ ^COUT
Figure 9. Instantaneous Current Path Flow in a Multiple Phase Switching Regulator
R_L
3729 F09
3729fb
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