LTC3525-3.3 Datasheet, PDF (9/12 Pages) Linear Technology – 400mA Micropower Synchronous Step-Up DC/DC Converter with Output Disconnect

U
OPERATIO
LTC3525-3.3/LTC3525-5
INDUCTOR
CURRENT
100mA/DIV
LOAD
CURRENT
50mA/DIV
10µs/DIV
3525 F01
Figure 1. Inductor Current Changing as a Function of Load
load using the minimum recommended 10µF output ca-
pacitor for the LTC3525-3.3 and a 22µF capacitor for the
LTC3525-5 (due to the capacitor’s DC bias effect). An
antiring circuit damps any oscillation at the switch node
when the inductor current falls to zero.
Power Adjust Feature
The LTC3525 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 induc-
tor current is automatically increased to a maximum of
400mA. At intermediate loads, the peak inductor current
may vary from 150mA to 400mA. Figure 1 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 neces-
sary 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 Char-
acteristics).
The “burst” frequency (how often the LTC3525 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 LTC3525 do not typically generate any audible
noise.
3525f
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