LTC1628 datasheet(13/32 Pages) LINER

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LTC1628/LTC1628-PG

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

Figure 4. RMS Input Current Comparison

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 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 LTC1628 current comparator has a maximum thresh-

old 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,

∆IL.

Allowing a margin for variations in the LTC1628 and

external component values yields:

SENSE

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

INPUT VOLTAGE (V)

3.0

2.5

2.0

1.5

1.0

0.5

1628 F04

SINGLE PHASE

DUAL CONTROLLER

2-PHASE

DUAL CONTROLLER

VO1 = 5V/3A

VO2 = 3.3V/3A

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.

(Refer to Functional Diagram)

OPERATIO

APPLICATIO S I FOR ATIO

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