LTC3525D-3.3_15 Datasheet, PDF (9/14 Pages) Linear Technology – 400mA Micropower Synchronous Step-Up DC/DC Converter with Pass Through Mode

LTC3525D-3.3
Operation
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 be-
tween 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 LTC3525D goes to sleep, during
which time the output capacitor supplies current to the
load. Once the output voltage drops approximately 9mV
below the regulation value the IC leaves sleep mode and
switching is resumed.
The LTC3525D 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 LTC3525D 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.
The maximum average load current that can be supported
is given by:
I_O(MAX)
=
0.3
• V_IN
V_O
•
η
Amps
where η is the efficiency (see Typical Performance
Characteristics).
The “burst” frequency (how often the LTC3525D delivers
a burst of current pulses to the load) is determined by
the internal hysteresis (output voltage ripple), the load
current and the amount of output capacitance. All Burst
Mode operation or hysteretic converters will enter the
audible frequency range when the load is light enough.
However, due to the low peak inductor current at light load,
circuits using the LTC3525D does not typically generate
any audible noise.
INDUCTOR
CURRENT
100mA/DIV
LOAD
CURRENT
50mA/DIV
10µs/DIV
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Figure 2. Inductor Current Changing as a Function of Load
For more information www.linear.com/LTC3525D3.3
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