LTC3859 datasheet(18/40 Pages) LINER

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LTC3859

3859f

OPERATION

Figure 1 compares the input waveforms for a representa-

tive single-phase dual switching regulator to the 2-phase

dual buck controllers of the LTC3859. An actual measure-

ment of the RMS input current under these conditions

shows that 2-phase operation dropped the input current

from 2.53ARMS to 1.55ARMS. While this is an impressive

reduction in itself, remember that the power losses are

proportional to IRMS2,meaningthattheactualpowerwasted

is reduced by a factor of 2.66. The reduced input ripple

voltage also means less power is lost in the input power

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 2 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.

Figure 1. Input Waveforms Comparing Single-Phase (a) and 2-Phase (b) Operation for Dual Switching

Regulators Converting 12V to 5V and 3.3V at 3A Each. The Reduced Input Ripple with the 2-Phase Regulator

Allows Less Expensive Input Capacitors, Reduces Shielding Requirements for EMI and Improves Efﬁciency

Figure 2. RMS Input Current Comparison

IIN(MEAS) = 2.53ARMS

IIN(MEAS) = 1.55ARMS

3859 F01b

3859 F01a

5V SWITCH

20V/DIV

3.3V SWITCH

20V/DIV

INPUT CURRENT

5A/DIV

INPUT VOLTAGE

500mV/DIV

(a)

(b)

INPUT VOLTAGE (V)

3.0

2.5

2.0

1.5

1.0

0.5

3859 F02

SINGLE PHASE

DUAL CONTROLLER

2-PHASE

DUAL CONTROLLER

VO1 = 5V/3A

VO2 = 3.3V/3A

It can readily be seen that the advantages of 2-phase op-

eration are not just limited to a narrow operating range,

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.

The schematic on the ﬁrst page is a basic LTC3859 ap-

plication 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

are selected. Finally, CIN and COUT are selected.

LTC3859 Datasheet(PDF) 18 Page - Linear Technology

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