ltc1876eg-trpbf Linear Technology Corporation, ltc1876eg-trpbf Datasheet - Page 13

• Manufacturer Part Number: ltc1876eg-trpbf
• Description: High Efficiency, 2-phase, Dual Synchronous Step-down Switching Controller And Step-up Regulator
• Manufacturer: Linear Technology Corporation

OPERATIO
controller has been started and been given adequate time
to charge up the output capacitors and provide full-load
current, the RUN/SS capacitor is used as a short-circuit
time-out circuit. If the output voltage falls to less than 70%
of its nominal output voltage, the RUN/SS capacitor be-
gins discharging on the assumption that the output is in an
overcurrent and/or short-circuit condition. If the condition
lasts for a long enough period as determined by the size of
the RUN/SS capacitor, both controllers will be shut down
until the RUN/SS pin(s) voltage(s) are recycled. This built-
in latchoff can be overridden by providing a >5 A pull-up
at a compliance of 5V to the RUN/SS pin(s). This current
shortens the soft start period but also prevents net dis-
charge of the RUN/SS capacitor(s) during an overcurrent
and/or short-circuit condition. Foldback current limiting is
also activated when the output voltage falls below 70% of
its nominal level whether or not the short-circuit latchoff
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
wasted is low due to the efficient nature of the current
mode switching regulator.
Theory and Benefits of 2-Phase Operation
The LTC1876 dual high efficiency DC/DC controller brings
the considerable benefits of 2-phase operation to portable
applications for the first time. Notebook computers, PDAs,
handheld terminals and automotive electronics will all
benefit from the lower input filtering requirement, reduced
electromagnetic interference (EMI) and increased effi-
ciency associated with 2-phase operation.
INPUT CURRENT
INPUT VOLTAGE
3.3V SWITCH
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 LTC1876 2-Phase Regulator Allows
Less Expensive Input Capacitors, Reduces Shielding Requirements for EMI and Improves Efficiency
5V SWITCH
500mV/DIV
20V/DIV
20V/DIV
5A/DIV
U
(Refer to Functional Diagram)
I
(a) Single-Phase
IN(MEAS)
= 2.53A
RMS
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Why the need for 2-phase operation? In most dual con-
stant-frequency switching regulators, both regulators are
operated 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 capaci-
tor 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
coming from 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 shield-
ing requirements for EMI and improves real world operat-
ing efficiency.
Figure 3 compares the input waveforms for a representa-
tive single-phase dual switching regulator to the LTC1876
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
INPUT CURRENT
INPUT VOLTAGE
RMS
3.3V SWITCH
5V SWITCH
500mV/DIV
RMS
to 1.55A
20V/DIV
20V/DIV
5A/DIV
2
, meaning that the actual power wasted is
RMS
. While this is an impressive reduc-
I
IN(MEAS)
(b) 2-Phase
= 1.55A
RMS
LTC1876
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