LTC1876 datasheet(14/36 Pages) LINER

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LTC1876

1876fa

path, which could include batteries, switches, trace/con-

nector resistances and protection circuitry. Improvements

in both conducted and radiated EMI also directly accrue as

a result of the reduced RMS input current and voltage.

Of course, the improvement afforded by 2-phase opera-

tion is a function of the dual switching regulator’s relative

duty cycles which, in turn, are dependent upon the input

voltage VIN (Duty Cycle = VOUT/VIN). Figure 4 shows how

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

Allowing a margin for variations in the LTC1876 and

external component values yields:

SENSE

MAX

Figure 4. RMS Input Current Comparison

(Refer to Functional Diagram)

OPERATIO

APPLICATIO S I FOR ATIO

Figure 1 on the first page is a basic LTC1876 application

circuit. For the step-down regulators, the external compo-

nent selection is driven by the load requirement, and

begins with the selection of RSENSE. Once RSENSE is

known, L can be chosen. 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).

For the step-up regulator, its component selection is much

simpler. A 4.7

µH or 10µH inductor that can handle at least

1A without saturating will work well with most design. A

Shottky diode is recommended and a MBR0520 from ON

Semiconductor is a very good choice.

RSENSE Selection For Output Current

RSENSE is chosen based on the required output current.

The LTC1876 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.

INPUT VOLTAGE (V)

3.0

2.5

2.0

1.5

1.0

0.5

1876 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.

OPERATING FREQUENCY (kHz)

120

170

220

270

320

1876 F05

2.5

2.0

1.5

1.0

0.5

Figure 5. FREQSET Pin Voltage vs Frequency

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