LTC3707 Datasheet, PDF (13/32 Pages) Linear Technology – High Effi ciency, 2-Phase Synchronous Step-Down Switching Regulator

LTC3707
OPERATION (Refer to Functional Diagram)
It can readily be seen that the advantages of 2-phase opera-
tion 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.
A final question: If 2-phase operation offers such an
advantage over single-phase operation for dual switching
regulators, why hasn’t it been done before? The answer
is that, while simple in concept, it is hard to implement.
Constant-frequency current mode switching regulators
require an oscillator derived “slope compensation”
signal to allow stable operation of each regulator at over
50% duty cycle. This signal is relatively easy to derive in
single-phase dual switching regulators, but required the
development of a new and proprietary technique to allow
2-phase operation. In addition, isolation between the two
channels becomes more critical with 2-phase operation
because switch transitions in one channel could potentially
disrupt the operation of the other channel.
The LTC1628 and the LTC3707 are proof that these hurdles
have been surmounted. The new device offers unique ad-
vantages for the ever-expanding number of high efficiency
power supplies required in portable electronics.
3.0
SINGLE PHASE
2.5
DUAL CONTROLLER
2.0
1.5
2-PHASE
DUAL CONTROLLER
1.0
0.5 _{VO1 = 5V/3A}
V_O2 = 3.3V/3A
0
0
10
20
30
40
INPUT VOLTAGE (V)
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Figure 4. RMS Input Current Comparison
APPLICATIONS INFORMATION
Figure 1 on the first page is a basic LTC3707 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
LTC3707 current comparator has a maximum threshold
of 75mV/R_SENSE and an input common mode range of
SGND to 1.1(INTV_CC). 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 LTC3707 and external
component values yields:
R_SENSE
=
50mV
I_MAX
Because of possible PCB noise in the current sensing loop,
the AC current sensing ripple of ΔV_SENSE = ΔI • R_SENSE
also needs to be checked in the design to get good
signal-to-noise ratio. In general, for a reasonable good
PCB layout, a 15mV ΔV_SENSE voltage is recommended
as a conservative number to start with.
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 to estimate this reducton
in peak output current level depending upon the operating
duty factor.
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