LTC3525L-3 Datasheet, PDF (7/12 Pages) Linear Technology – 400mA Micropower Synchronous Step-Up DC/DC Converter with Output Disconnect

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OPERATIO
The LTC3525L-3 is a high performance Burst Mode
operation only, synchronous boost converter requiring only
three small external components. Its simplicity and small
size make it a high efficiency alternative to charge pump
designs. It is designed to start up from a single alkaline
or nickel cell, with input voltages as low as 0.7V typical
or 0.88V maximum, or from two or three cells (or a Li-ion
battery), with voltages as high as 4.5V. Once started, V_IN
can be as low as 0.5V (depending on load current) and
maintain regulation. The output voltage is preset internally
to 3V. Peak switch current is 400mA minimum, providing
regulation with load currents up to 160mA, depending on
input voltage.
Synchronous rectification provides high efficiency opera-
tion while eliminating the need for an external Schottky
diode. True output disconnect eliminates inrush current
at start-up, and allows V_OUT to be disconnected from V_IN,
for zero shutdown current.
The output disconnect feature also allows the LTC3525L-3
to maintain regulation with an input voltage equal to or
greater than V_OUT. Note, however, that the synchronous
rectifier is not enabled in this mode resulting in lower
efficiency and reduced output current capability.
The operating quiescent current is only 7µA typical, allow-
ing the converter to maintain high efficiency at extremely
light loads.
LTC3525L-3
Shutdown
The LTC3525L-3 is shut down by pulling SHDN below
0.3V, and made active by raising it above 0.88V. Note that
SHDN can be driven as high as 6V, however, if it is more
than 0.9V above the higher of V_IN or V_OUT, the SHDN input
current will increase from zero to 1.5µA.
After the SHDN pin rises, there is a short delay before
switching starts. The delay is 20µs to 120µs, depending
on input voltage (see Typical Performance Characteristics
curve).
Start-Up
A start-up oscillator allows the LTC3525L-3 to start with in-
put voltages as low as 0.7V. It remains in start-up mode until
two conditions are met. V_OUT must exceed V_IN by at least
0.2V typical and either V_IN or V_OUT must be greater than
1.8V typical.
During startup, 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.
3525lf
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