Datasheet

LTC3859A
19
3859af
operaTion
Figure 1 compares the input waveforms for a representative
single-phase dual switching regulator to the 2-phase dual
buck controllers of the LTC3859A. 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.
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 2. RMS Input Current Comparison
(a) (b)
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 LTC3859A 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.
INPUT VOLTAGE (V)
0
INPUT RMS CURRENT (A)
3.0
2.5
2.0
1.5
1.0
0.5
0
10 20 30 40
3859A F02
SINGLE PHASE
DUAL CONTROLLER
2-PHASE
DUAL CONTROLLER
V
O1
= 5V/3A
V
O2
= 3.3V/3A
I
IN(MEAS)
= 2.53A
RMS
I
IN(MEAS)
= 1.55A
RMS
3859A F01b3859A F01a
5V SWITCH
20V/DIV
3.3V SWITCH
20V/DIV
INPUT CURRENT
5A/DIV
INPUT VOLTAGE
500mV/DIV