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LTC3525D-3.3 Datasheet, PDF (6/12 Pages) Linear Technology – 400mA Micropower Synchronous Step-Up DC/DC Converter with Pass Through Mode | |||
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LTC3525D-3.3
U
OPERATIO
The LTC3525D 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 efï¬ciency alternative to charge pump designs.
It is designed to start-up from a single alkaline or nickel
cell, with input voltages as low as 0.85V, or from two or
three cells (or a Li-ion battery), with voltages as high as
4.5V. Once started, VIN can be as low as 0.5V (depend-
ing on load current) and maintain regulation. The output
voltage is preset internally to 3.3V. Peak switch current is
400mA minimum, providing regulation with load currents
up to 150mA, depending on input voltage.
Synchronous rectiï¬cation provides high efï¬ciency opera-
tion while eliminating the need for an external Schottky
diode.
The LTC3525D can maintain regulation with an input
voltage equal to or greater than VOUT. Note, however, that
the synchronous rectiï¬er is not enabled in this mode,
resulting in lower efï¬ciency and reduced output current
capability.
The operating quiescent current is only 7µA typical, allow-
ing the converter to maintain high efï¬ciency at extremely
light loads.
Shutdown
The LTC3525D is shut down by pulling â¯Sâ¯Hâ¯Dâ¯N below 0.4V,
and made active by raising it above 1V. Note that â¯Sâ¯Hâ¯Dâ¯N can
be driven as high as 6V, however, if it is more than 0.9V
above the higher of VIN or VOUT, the â¯Sâ¯Hâ¯Dâ¯N input current
will increase from zero to 1.5µA.
After the â¯Sâ¯Hâ¯Dâ¯N 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).
Pass-Through Mode
When the LTC3525D is in shutdown, the internal P channel
MOSFET switch is turned on. This allows VIN to be con-
nected to VOUT through the inductor in shutdown, creating
a pass-through mode.
Start-up
A start-up oscillator allows the LTC3525D to start with input
voltages as low as 1V. It remains in start-up mode until
two conditions are met. VOUT must exceed VIN by at least
0.2V typical and either VIN or VOUT must be greater than
1.8V typical.
During startup, the synchronous rectiï¬er is not enabled,
and the internal P-channel synchronous rectiï¬er acts as
a follower, causing the peak voltage on SW to reach (VIN
+ 1V) typical. This limits inrush current by maintaining
control of the inductor current when VOUT is less than VIN.
To reduce power dissipation in the P-channel synchronous
rectiï¬er when the output is shorted, a foldback feature is
incorporated that reduces the peak inductor current when
VIN is more than 1.7V greater than VOUT.
3525d33f
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