LTC3828 Datasheet, PDF (12/32 Pages) Linear Technology – Dual 2-Phase Step-Down Controller with Tracking

LTC3828
U
OPERATIO ^{(Refer to Functional Diagram)}
THEORY AND BENEFITS OF 2-PHASE OPERATION
The LTC3728 and the LTC3828 family of dual high effi-
ciency DC/DC controllers brings the considerable benefits
of 2-phase operation to portable applications for the
first time. Notebook computers, PDAs, handheld termi-
nals and automotive electronics will all benefit from the
lower input filtering requirement, reduced electromag-
netic interference (EMI) and increased efficiency associ-
ated with 2-phase operation.
Why the need for 2-phase operation? Up until the 2-phase
family, constant-frequency dual switching regulators op-
erated both channels in phase (i.e., single-phase opera-
tion). 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 reduc-
tion 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 2 compares the input waveforms for a representa-
tive single-phase dual switching regulator to the LTC3828
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
2.6A_RMS to 1.9A_RMS. While this is an impressive reduction
in itself, remember that the power losses are proportional
to I_RMS2, meaning that the actual power wasted is reduced
by a factor of 1.86. The reduced input ripple voltage also
means less power is lost in the input power path, which
could include batteries, switches, trace/connector resis-
tances 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 V_IN (Duty Cycle = V_OUT/V_IN). Figure 3 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
the input capacitor requirement to that for just one channel
operating at maximum current and 50% duty cycle.
I_IN(MEAS) = 2.6A_RMS
5V SWITCH
20V/DIV
3.3V SWITCH
20V/DIV
DC236 F02a
INPUT CURRENT
5A/DIV
INPUT VOLTAGE
100mV/DIV
I_IN(MEAS) = 1.9A_RMS
DC236 F02b
I_N(MEAS) = 2.6A_RMS
(a)
I_N(MEAS) = 1.9A_RMS
(b)
Figure 2. 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 LTC3828 2-Phase Regulator Allows
Less Expensive Input Capacitors, Reduces Shielding Requirements for EMI and Improves Efficiency
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