LTC3859_15 Datasheet, PDF (19/42 Pages) Linear Technology – Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller

OPERATION
LTC3859
5V SWITCH
20V/DIV
3.3V SWITCH
20V/DIV
INPUT CURRENT
5A/DIV
INPUT VOLTAGE
500mV/DIV
I_IN(MEAS) = 2.53A_RMS
3859 F01a
I_IN(MEAS) = 1.55A_RMS
3859 F01b
(a)
(b)
Figure 1. Input Waveforms Comparing Single-Phase (a) and 2-Phase (b) Operation for Dual Switching
Regulators Converting 12V to 5V and 3.3V at 3A Each. The Reduced Input Ripple with the 2-Phase Regulator
Allows Less Expensive Input Capacitors, Reduces Shielding Requirements for EMI and Improves Efficiency
Figure 1 compares the input waveforms for a representa-
tive single-phase dual switching regulator to the 2-phase
dual buck controllers of the LTC3859. An actual measure-
ment of the RMS input current under these conditions
shows that 2-phase operation dropped the input current
from 2.53A_RMS to 1.55A_RMS. While this is an impressive
reduction in itself, remember that the power losses are
proportional to I_RMS2, meaning that the actual power wasted
is reduced by a factor of 2.66. The reduced input ripple
voltage also means less power is lost in the input power
path, which could include batteries, switches, trace/con-
nector resistances and protection circuitry. Improvements
in both conducted and radiated EMI also directly accrue as
a result of the reduced RMS input current and voltage.
Of course, the improvement afforded by 2-phase opera-
tion is a function of the dual switching regulator’s relative
duty cycles which, in turn, are dependent upon the input
voltage V_IN (Duty Cycle = V_OUT/V_IN). Figure 2 shows how
the RMS input current varies for single-phase and 2-phase
operation for 3.3V and 5V regulators over a wide input
voltage range.
It can readily be seen that the advantages of 2-phase op-
eration are not just limited to a narrow operating range,
for most applications is that 2-phase operation will reduce
the input capacitor requirement to that for just one channel
operating at maximum current and 50% duty cycle.
The schematic on the first page is a basic LTC3859 ap-
plication circuit. External component selection is driven
by the load requirement, and begins with the selection of
R_SENSE and the inductor value. Next, the power MOSFETs
are selected. Finally, C_IN and C_OUT are selected.
3.0
SINGLE PHASE
2.5
DUAL CONTROLLER
2.0
1.5
2-PHASE
DUAL CONTROLLER
1.0
0.5 _{VO1 = 5V/3A}
V_O2 = 3.3V/3A
0
0
10
20
30
INPUT VOLTAGE (V)
40
3859 F02
Figure 2. RMS Input Current Comparison
3859fa
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