LTC3826-1 Datasheet, PDF (13/32 Pages) Linear Technology – 30μA IQ, Dual, 2-Phase Synchronous Step-Down Controller

OPERATION (Refer to Functional Diagram)
Of course, the improvement afforded by 2-phase operation
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
operation 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 LTC3826-1
application 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.
LTC3826-1
3.0
SINGLE PHASE
2.5
DUAL CONTROLLER
2.0
1.5
2-PHASE
DUAL CONTROLLER
1.0
0.5
V₀₁ = 5V/3A
V₀₂ = 3.3V/3A
0
0
10
20
30
INPUT VOLTAGE (V)
40
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Figure 2. RMS Input Current Comparison
APPLICATIONS INFORMATION
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 criterion 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
efficiency and component size. Low frequency operation
improves efficiency 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 LTC3826-1’s con-
trollers runs at a nominal 390kHz frequency when the
PLLLPF pin is left floating 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 LTC3826-1 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
interrelated in that higher operating frequencies allow the
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