LTC3425 Datasheet, PDF (12/24 Pages) Linear Technology – 5A, 8MHz, 4-Phase Synchronous Step-Up DC/DC Converter

LTC3425
U
OPERATIO
SWITCH A
VOLTAGE
SWITCH B
VOLTAGE
SWITCH C
VOLTAGE
SWITCH D
VOLTAGE
INDUCTOR A
CURRENT
INDUCTOR B
CURRENT
INDUCTOR C
CURRENT
INDUCTOR D
CURRENT
INPUT CURRENT
RECTIFIER A
CURRENT
RECTIFIER B
CURRENT
RECTIFIER C
CURRENT
RECTIFIER D
CURRENT
OUTPUT RIPPLE
CURRENT
3425 F02
Figure 2. Simplified Voltage and Current Waveforms
for 4-Phase Operation at 50% Duty Cycle
This approach can be used to reduce solution cost and
board area in applications not requiring the full power
capability of the LTC3425, or where peak efficiency may
not be as important as cost and size. In this case, phase A
should always be used, since this is the only phase active
in Burst Mode operation and phase C is recommended as
the second phase for the lowest output ripple, since it is
180° out of phase with phase A. Figure 4 illustrates the
efficiency differences with two, three and four phases in a
typical 2-cell to 3.3V boost application. In this example,
12
you can see that for maximum loads less than 1A, the
efficiency penalty for using only two or three phases is
fairly small. Keep in mind, however, that this penalty will
grow larger as the input voltage drops. Output ripple will
also increase with each phase that is eliminated.
Low Voltage Start-Up
The LTC3425 includes an independent start-up oscillator
designed to start up at input voltages as low as 0.88V. The
frequency and peak current limit during start-up are
3425f