Datasheet

LTC3425
13
3425f
SWITCH A
VOLTAGE
SWITCH B
VOLTAGE
SWITCH C
VOLTAGE
SWITCH D
VOLTAGE
INDUCTOR A
CURRENT
INDUCTOR B
CURRENT
INDUCTOR C
CURRENT
INDUCTOR D
CURRENT
RECTIFIER A
CURRENT
RECTIFIER B
CURRENT
RECTIFIER C
CURRENT
RECTIFIER D
CURRENT
OUTPUT RIPPLE
CURRENT
INPUT CURRENT
3432 F03
OPERATIO
U
Figure 3. Simplified Voltage and Current Waveforms
for 4-Phase Operation at 75% Duty Cycle
inter nally controlled. The device can start up under some
load (see the graph Start-Up Current vs Input Voltage).
Soft-start and inrush current limiting is provided during
start-up as well as normal mode. The same soft-start
capacitor is used for each operating mode.
During start-up, all four phases switch in unison. When
either V
IN
or V
OUT
exceeds 2.3V, the IC enters normal
operating mode. Once the output voltage exceeds the
input by 0.3V, the IC powers itself from V
OUT
instead of
V
IN
. At this point the internal circuitry has no dependency
on the V
IN
input voltage, eliminating the requirement for a
large input capacitor. The input voltage can drop as low as
0.5V without affecting circuit operation. The limiting factor
for the application becomes the ability of the power source
to supply sufficient energy to the output at the low volt-
ages, and the maximum duty cycle that is clamped at 90%.