Datasheet

LTC3827-1
13
38271fe
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 fi rst page is a basic LTC3827-1 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.
Figure 2. RMS Input Current Comparison
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
38271 F02
SINGLE PHASE
DUAL CONTROLLER
2-PHASE
DUAL CONTROLLER
V
O1
= 5V/3A
V
O2
= 3.3V/3A
APPLICATIONS INFORMATION
OPERATION
(Refer to Functional Diagram)
R
SENSE
Selection For Output Current
R
SENSE
is chosen based on the required output current.
The current comparator has a maximum threshold of
100mV/R
SENSE
and an input common mode range of
SGND to 10V. The current comparator threshold sets the
peak of the inductor current, yielding a maximum average
output current I
MAX
equal to the peak value less half the
peak-to-peak ripple current, ΔI
L
.
Allowing a margin for variations in the IC and external
component values yields:
R
SENSE
=
80mV
I
MAX
When using the controller in very low dropout conditions,
the maximum output current level will be reduced due to
the internal compensation required to meet stability cri-
terion for buck regulators operating at greater than 50%
duty factor. A curve is provided in the Typical Performance
Characteristics section to estimate this reduction in peak
output current level depending upon the operating duty
factor.
Operating Frequency and Synchronization
The choice of operating frequency, is a trade-off between
effi ciency and component size. Low frequency operation
improves effi ciency by reducing MOSFET switching losses,
both gate charge loss and transition loss. However, lower
frequency operation requires more inductance for a given
amount of ripple current.
The internal oscillator for each of the LTC3827-1’s controllers
runs at a nominal 400kHz frequency when the PLLLPF pin
is left fl oating and the PLLIN/MODE pin is a DC low or high.
Pulling the PLLLPF to INTV
CC
selects 530kHz operation;
pulling the PLLLPF to SGND selects 250kHz operation.
Alternatively, the LTC3827-1 will phase-lock to a clock
signal applied to the PLLIN/MODE pin with a frequency
between 140kHz and 650kHz (see Phase-Locked Loop
and Frequency Synchronization).
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with