LTC1628_15 Datasheet, PDF (13/32 Pages) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulators

U
OPERATIO (Refer to Functional Diagram)
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)
1628 F04
Figure 4. RMS Input Current Comparison
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
LTC1628/LTC1628-PG
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 develop-
ment 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 is proof that these hurdles have been sur-
mounted. The new device offers unique advantages for the
ever-expanding number of high efficiency power supplies
required in portable electronics.
APPLICATIO S I FOR ATIO
Figure 1 on the first page is a basic LTC1628 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 LTC1628 current comparator has a maximum thresh-
old 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, ∆I_L.
Allowing a margin for variations in the LTC1628 and
external component values yields:
R_SENSE
=
50mV
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 crite-
rion 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.
Selection of Operating Frequency
The LTC1628 uses a constant frequency architecture with
the frequency determined by an internal oscillator capaci-
tor. This internal capacitor is charged by a fixed current
plus an additional current that is proportional to the
voltage applied to the FREQSET pin.
A graph for the voltage applied to the FREQSET pin vs
frequency is given in Figure 5. As the operating frequency
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