LTC3707 Linear Technology, LTC3707 Datasheet - Page 13

• Manufacturer Part Number: LTC3707
• Description: High Efficiency / 2-Phase Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

OPERATIO
APPLICATIO S I FOR ATIO
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.
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
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
and the inductor value. Next, the power MOSFETs and D1
are selected. Finally, C
circuit shown in Figure 1 can be configured for operation
up to an input voltage of 28V (limited by the external
MOSFETs).
R
R
The LTC3707 current comparator has a maximum thresh-
old of 75mV/R
SGND to 1.1(INTV
sets the peak of the inductor current, yielding a maximum
average output current I
half the peak-to-peak ripple current, I
Allowing a margin for variations in the LTC3707 and
external component values yields:
SENSE
SENSE
Selection For Output Current
is chosen based on the required output current.
SENSE
U
CC
U
and an input common mode range of
). The current comparator threshold
(Refer to Functional Diagram)
IN
MAX
U
and C
equal to the peak value less
OUT
W
are selected. The
L
.
U
SENSE
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 advantages for the ever-expanding number of high
efficiency power supplies required in portable electronics.
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 LTC3707 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
R
SENSE
3.0
2.5
2.0
1.5
1.0
0.5
Figure 4. RMS Input Current Comparison
0
0
50
I
V
V
MAX
O1
O2
mV
= 5V/3A
= 3.3V/3A
10
INPUT VOLTAGE (V)
DUAL CONTROLLER
2-PHASE
SINGLE PHASE
DUAL CONTROLLER
20
30
LTC3707
3707 F04
40
13
3707f