LTC3525L-3
7
3525lfa
During start-up, the synchronous rectifier is not enabled,
and the internal P-channel synchronous rectifier acts
as a follower, causing the peak voltage on SW to reach
(V
IN
+ 1V) typical. This limits inrush current by maintaining
control of the inductor current when V
OUT
is less than V
IN
.
To reduce power dissipation in the P-channel synchronous
rectifier when the output is shorted, a foldback feature is
incorporated that reduces the peak inductor current when
V
IN
is more than 1.7V greater than V
OUT
.
Normal Operation
Once V
OUT
has increased more than 0.2V typical above
V
IN
, and either voltage is above 1.8V, normal operation
begins, with synchronous rectification enabled. In this
mode, the internal N-channel MOSFET connected between
SW and GND stays on until the inductor current reaches a
maximum peak value, after which it is turned off and the
P-channel synchronous rectifier is turned on. It stays on,
delivering current to the output, until the inductor current
has dropped below a minimum value at which point it
turns off and the cycle repeats. When the output voltage
reaches its regulated value both switches are turned off
and the LTC3525L-3 goes to sleep, during which time the
output capacitor supplies current to the load. Once the
output voltage drops below the regulation value the IC
leaves sleep mode and switching is resumed.
The LTC3525L-3 has been designed for low output voltage
ripple. The output voltage ripple is typically only 20mV
peak-to-peak at light load and 60mV peak-to-peak at
full load using the minimum recommended 10µF output
capacitor. An anti-ring circuit damps any oscillation at the
switch node when the inductor current falls to zero.
Power Adjust Feature
The LTC3525L-3 incorporates a feature that maximizes
efficiency at light load while providing increased power
capability at heavy load by adjusting the peak and valley
of the inductor current as a function of load. Lowering the
peak inductor current to 150mA at light load optimizes
efficiency by reducing conduction losses in the internal
MOSFET switches. As the load increases, the peak inductor
current is automatically increased to a maximum of 400mA.
At intermediate loads, the peak inductor current may vary
from 150mA to 400mA. Figure 2 shows an example of
how the inductor current changes as the load increases.
Please note that output capacitor values greater than 47µF
will result in higher peak currents than necessary at light
load. This will lower the light load efficiency.
The valley of the inductor current is automatically adjusted
as well, to maintain a relatively constant inductor ripple
current. This keeps the switching frequency relatively
constant.
Figure 2. Inductor Current Changing as a Function of Load
OPERATION
3525l F02
INDUCTOR
CURRENT
100mA/DIV
LOAD
CURRENT
50mA/DIV
10µs/DIV
