LTC3735 Datasheet, PDF (26/32 Pages) Linear Technology – 2-Phase, High Efficiency DC/DC Controller for Intel Mobile CPUs

LTC3735
APPLICATIONS INFORMATION
SW1
D1
L1
R_SENSE1
V_IN
R_IN
+
C_IN
V_OUT
+ ^COUT
R_L
BOLD LINES INDICATE
HIGH, SWITCHING
CURRENT LINES.
KEEP LINES TO A
MINIMUM LENGTH.
SW2
D2
L2
R_SENSE2
3735 F12
Figure 12. Instantaneous Current Path Flow in a Multiple Phase Switching Regulator
The ground terminations of the sychronous MOSFETs and
Schottky diodes should return to the negative 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 negative 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.
Simplified Visual Explanation of How a 2-Phase
Controller Reduces Both Input and Output RMS Ripple
Current
A multiphase power supply 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 out-
put voltage). The output ripple amplitude is also reduced
by, and the effective ripple frequency is increased by the
number of phases used. Figure 13 graphically illustrates
the principle.
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