LTC3733_15 Datasheet, PDF (26/32 Pages) Linear Technology – 3-Phase, Buck Controllers for AMD CPUs

LTC3733/LTC3733-1
APPLICATIO S I FOR ATIO
Figure 10 illustrates all branch currents in a three-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 elec-
tric 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 synchronous 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 and output capacitor(s)
should be used to tie in the IC power ground pin (PGND).
This technique keeps inherent signals generated by high
current pulses taking alternate current paths that have
finite impedances during the total period of the switching
regulator. External OPTI-LOOP compensation allows over-
compensation for PC layouts which are not optimized but
this is not the recommended design procedure.
Simplified Visual Explanation of How a 3-Phase
Controller Reduces Both Input and Output RMS
Ripple Current
The effect of multiphase power supply design significantly
reduces the amount of ripple current in both the input and
output capacitors. The RMS input ripple current is divided
by, and the effective ripple frequency is multiplied up by
the number of phases used (assuming that the input
voltage is greater than the number of phases used times
the output voltage). The output ripple amplitude is also
reduced by, and the effective ripple frequency is increased
by the number of phases used. Figure 12 graphically
illustrates the principle.
SW V
SINGLE PHASE
I_CIN
I_COUT
SW1 V
SW2 V
SW3 V
I_L1
I_L2
I_L3
TRIPLE PHASE
I_CIN
I_COUT
3732 F12
Figure 12. Single and Polyphase Current Waveforms
3733f
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