LTC3707IGN datasheet(12/32 Pages) LINER

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LTC3707

3707fb

OPERATION

THEORY AND BENEFITS OF 2-PHASE OPERATION

The LTC1628 and the LTC3707 are the ﬁrst dual high

efﬁciency DC/DC controllers to bring the considerable

beneﬁts of 2-phase operation to portable applications.

Notebook computers, PDAs, handheld terminals and au-

tomotive electronics will all beneﬁt from the lower input

ﬁltering requirement, reduced electromagnetic interference

(EMI) and increased efﬁciency associated with 2-phase

operation.

Why the need for 2-phase operation? Up until the LTC1628

was introduced, constant-frequency dual switching regula-

tors operated both channels in phase (i.e., single-phase

operation). This means that both switches turned on at

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

amplitude of those for one regulator to be drawn from the

input capacitor and battery. These large amplitude current

pulses increased the total RMS current ﬂowing from the

input capacitor, requiring the use of more expensive input

capacitors and increasing both EMI and losses in the input

capacitor and battery.

With 2-phase operation, the two channels of the dual-

switching regulator are operated 180 degrees out of phase.

This effectively interleaves the current pulses drawn by the

switches, greatly reducing the overlap time where they add

together.

The result is a signiﬁcant reduction in total RMS

input current, which in turn allows less expensive input

capacitors to be used, reduces shielding requirements for

EMI and improves real world operating efﬁciency.

Figure 3 compares the input waveforms for a representa-

tive single-phase dual switching regulator to the LTC3707

2-phase dual switching regulator. An actual measurement 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,

meaning that the actual power wasted is reduced by a fac-

tor 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/connector 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.

(Refer to Functional Diagram)

IIN(MEAS) = 2.53ARMS

IIN(MEAS) = 1.55ARMS

3707 F03a

3707 F03b

5V SWITCH

20V/DIV

3.3V SWITCH

20V/DIV

INPUT CURRENT

5A/DIV

INPUT VOLTAGE

500mV/DIV

Figure 3. 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 LTC1628 2-Phase Regulator Allows Less Expensive Input Capacitors, Reduces

Shielding Requirements for EMI and Improves Efﬁciency

(a)

(b)

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