LTC3727LX-1_15 Datasheet, PDF (12/28 Pages) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulator

LTC3727LX-1
U
OPERATIO (Refer to Functional Diagram)
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 4 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,
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.
3.0
SINGLE PHASE
2.5
DUAL CONTROLLER
2.0
1.5
2-PHASE
DUAL CONTROLLER
1.0
0.5
V_O1 = 5V/3A
V_O2 = 3.3V/3A
0
0
10
20
30
INPUT VOLTAGE (V)
40
3727LX1 F04
Figure 4. RMS Input Current Comparison
APPLICATIO S I FOR ATIO
Figure 1 on the first page is a basic LTC3727LX-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
and D1 are selected. Finally, C_IN and C_OUT are selected.
The circuit shown in Figure 1 can be configured for
operation up to an input voltage of 28V (limited by the
external MOSFETs).
R_SENSE Selection for Output Current
R_SENSE is chosen based on the required output current.
The LTC3727LX-1 current comparator has a maximum
threshold of 135mV/R_SENSE and an input common mode
range of SGND to 14V. 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, ∆I_L.
Allowing a margin for variations in the LTC3727LX-1 and
external component values yields:
R_SENSE
=
90mV
I_MAX
When using the controller in very low dropout conditions,
the maximum output current level will be reduced due to
12
the internal compensation required to meet stability crite-
rion for buck regulators operating at greater than 50%
duty factor. A curve is provided to estimate this reduction
in peak output current level depending upon the operating
duty factor.
Operating Frequency
The LTC3727LX-1 uses a constant-frequency phase-lock-
able architecture with the frequency determined by an
internal capacitor. This capacitor is charged by a fixed
current plus an additional current which is proportional to
the voltage applied to the PLLFLTR pin. Refer to Phase-
Locked Loop and Frequency Synchronization in the Appli-
cations Information section for additional information.
A graph for the voltage applied to the PLLFLTR pin vs
frequency is given in Figure 5. As the operating frequency
is increased the gate charge losses will be higher, reducing
efficiency (see Efficiency Considerations). The maximum
switching frequency is approximately 550kHz.
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. However, a
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