LTC3776 datasheet(13/28 Pages) LINER

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LTC3776

3776f

OPERATIO

(Refer to Functional Diagram)

Figure 3. Example Waveforms for a Single Phase

Dual Controller vs the 2-Phase LTC3776

Single Phase

Dual Controller

2-Phase

Dual Controller

SW1 (V)

SW2 (V)

IL1

IL2

IIN

3776 F03

The peak inductor current is determined by the peak sense

voltage and the on-resistance of the external P-channel

MOSFET:

SENSE MAX

DS ON

∆

()

Power Good (PGOOD) Pin

A window comparator monitors both feedback voltages

and the open-drain PGOOD output pin is pulled low when

either or both feedback voltages are not within ±10% of

their reference voltages. PGOOD is low when the LTC3776

is shut down or in undervoltage lockout.

2-Phase Operation

Why the need for 2-phase operation? Until recently, con-

stant frequency dual switching regulators operated both

controllers in phase (i.e., single phase operation). This

means that both topside MOSFETs (P-channel) are turned

on at the same time, causing current pulses of up to twice

the amplitude of those from a single regulator to be drawn

from the input capacitor. These large amplitude pulses

increase the total RMS current flowing in the input capaci-

tor, requiring the use of larger and more expensive input

capacitors, and increase both EMI and power losses in the

input capacitor and input power supply.

With 2-phase operation, the two controllers of the LTC3776

are operated 180 degrees out of phase. This effectively

interleaves the current pulses coming from the topside

MOSFET switches, greatly reducing the time where they

overlap and add together. The result is a significant

reduction in the total RMS current, which in turn allows the

use of smaller, less expensive input capacitors, reduces

shielding requirements for EMI and improves real world

operating efficiency.

Figure 3 shows qualitatively example waveforms for a

single phase dual controller versus a 2-phase LTC3776

system. In this case, 2.5V and 1.8V outputs, each drawing

a load current of 2A, are derived from a 7V (e.g., a 2-cell

Li-Ion battery) input supply. In this example, 2-phase

operation would reduce the RMS input capacitor current

from 1.79ARMS to 0.91ARMS. While this is an impressive

reduction by itself, remember that power losses are

proportional to IRMS2, meaning that actual power wasted

is reduced by a factor of 3.86.

The reduced input ripple current also means that less

power is lost in the input power path, which could include

batteries, switches, trace/connector resistances, and pro-

tection circuitry. Improvements in both conducted and

radiated EMI also directly accrue as a result of the reduced

RMS input current and voltage. Significant cost and board

footprint savings are also realized by being able to use

smaller, less expensive, lower RMS current-rated input

capacitors.

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

tion is a function of the relative duty cycles of the two

controllers, which in turn are dependent upon the input

supply voltage. Figure 4 depicts how the RMS input

current varies for single phase and 2-phase dual control-

lers with 2.5V and 1.8V outputs over a wide input voltage

range.

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

operation are not 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.

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