LTC3707 Linear Technology, LTC3707 Datasheet - Page 12

• Manufacturer Part Number: LTC3707
• Description: High Efficiency / 2-Phase Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

OPERATIO
LTC3707
circuit is enabled. Even if a short is present and the short-
circuit latchoff is not enabled, a safe, low output current is
provided due to internal current foldback and actual power
dissipated is low due to the efficient nature of the current
mode switching regulator.
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 interfer-
ence (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 regu-
lators 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.
12
U
(Refer to Functional Diagram)
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
I
IN(MEAS)
(a)
= 2.53A
RMS
3707 F03a/TIFF
INPUT CURRENT
INPUT VOLTAGE
3.3V SWITCH
5V SWITCH
500mV/DIV
20V/DIV
20V/DIV
5A/DIV
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 expen-
sive input capacitors to be used, reduces shielding re-
quirements 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
2.53A
tion in itself, remember that the power losses are propor-
tional to I
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 V
RMS
IN
RMS
to 1.55A
(Duty Cycle = V
2
, meaning that the actual power wasted is
I
IN(MEAS)
RMS
(b)
= 1.55A
. While this is an impressive reduc-
RMS
OUT
/V
IN
3707 F03b/TIFF
). Figure 4 shows how
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