LTC3707IGN datasheet(13/32 Pages) LINER

13 / 32 page

LTC3707

3707fb

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 RSENSE

and the inductor value. Next, the power MOSFETs and

D1 are selected. Finally, CIN and COUT 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).

RSENSE Selection For Output Current

RSENSE is chosen based on the required output current. The

LTC3707 current comparator has a maximum threshold

of 75mV/RSENSE and an input common mode range of

SGND to 1.1(INTVCC). The current comparator threshold

sets the peak of the inductor current, yielding a maximum

average output current IMAX equal to the peak value less

half the peak-to-peak ripple current,

ΔIL.

Allowing a margin for variations in the LTC3707 and external

component values yields:

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.

INPUT VOLTAGE (V)

3.0

2.5

2.0

1.5

1.0

0.5

3707 F04

SINGLE PHASE

DUAL CONTROLLER

2-PHASE

DUAL CONTROLLER

VO1 = 5V/3A

VO2 = 3.3V/3A

Figure 4. RMS Input Current Comparison

APPLICATIONS INFORMATION

SENSE =

50mV

MAX

Because of possible PCB noise in the current sensing loop,

the AC current sensing ripple of

ΔVSENSE = ΔI • RSENSE

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

ΔVSENSE 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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