13
LTC3776
3776fa
OPERATIO
U
(Refer to Functional Diagram)
Figure 3. Example Waveforms for a Single Phase
Dual Controller vs the 2-Phase LTC3776
Single Phase
Dual Controller
2-Phase
Dual Controller
SW1 (V)
SW2 (V)
I
L1
I
L2
I
IN
3776 F03
The peak inductor current is determined by the peak sense
voltage and the on-resistance of the external P-channel
MOSFET:
I
V
R
PK
SENSE MAX
DS ON
=
Δ
()
()
Power Good (PGOOD) Pin
A window comparator monitors both feedback voltages
and the open-drain PGOOD output pin is pulled low when
either or both feedback voltages are not within ±10% of
their reference voltages. PGOOD is low when the LTC3776
is shut down or in undervoltage lockout.
2-Phase Operation
Why the need for 2-phase operation? Until recently, con-
stant frequency dual switching regulators operated both
controllers in phase (i.e., single phase operation). This
means that both topside MOSFETs (P-channel) are turned
on at the same time, causing current pulses of up to twice
the amplitude of those from a single regulator to be drawn
from the input capacitor. These large amplitude pulses
increase the total RMS current flowing in the input capaci-
tor, requiring the use of larger and more expensive input
capacitors, and increase both EMI and power losses in the
input capacitor and input power supply.
With 2-phase operation, the two controllers of the LTC3776
are operated 180 degrees out of phase. This effectively
interleaves the current pulses coming from the topside
MOSFET switches, greatly reducing the time where they
overlap and add together. The result is a significant
reduction in the total RMS current, which in turn allows the
use of smaller, less expensive input capacitors, reduces
shielding requirements for EMI and improves real world
operating efficiency.
Figure 3 shows qualitatively example waveforms for a
single phase dual controller versus a 2-phase LTC3776
system. In this case, 2.5V and 1.8V outputs, each drawing
a load current of 2A, are derived from a 7V (e.g., a 2-cell
Li-Ion battery) input supply. In this example, 2-phase
operation would reduce the RMS input capacitor current
from 1.79A
RMS
to 0.91A
RMS
. While this is an impressive
reduction by itself, remember that power losses are
proportional to I
RMS
2
, meaning that actual power wasted
is reduced by a factor of 3.86.
The reduced input ripple current also means that less
power is lost in the input power path, which could include
batteries, switches, trace/connector resistances, and pro-
tection circuitry. Improvements in both conducted and
radiated EMI also directly accrue as a result of the reduced
RMS input current and voltage. Significant cost and board
footprint savings are also realized by being able to use
smaller, less expensive, lower RMS current-rated input
capacitors.
Of course, the improvement afforded by 2-phase opera-
tion is a function of the relative duty cycles of the two
controllers, which in turn are dependent upon the input
supply voltage. Figure 4 depicts how the RMS input
current varies for single phase and 2-phase dual control-
lers with 2.5V and 1.8V outputs over a wide input voltage
range.
It can be readily seen that the advantages of 2-phase
operation are not limited to a narrow operating range, but
in fact extend over a wide region. A good rule of thumb 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.
