LTC1876

13

1876fa

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

µA 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.

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

tion in itself, remember that the power losses are propor-

reduced by a factor of 2.66. The reduced input ripple

voltage also means less power is lost in the input power

(Refer to Functional Diagram)

OPERATIO

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

(a) Single-Phase

(b) 2-Phase

5V SWITCH

20V/DIV

1876 F03a

1876 F03b

3.3V SWITCH

20V/DIV

INPUT CURRENT

5A/DIV

INPUT VOLTAGE

500mV/DIV

5V SWITCH

20V/DIV

3.3V SWITCH

20V/DIV

INPUT CURRENT

5A/DIV

INPUT VOLTAGE

500mV/DIV