LTC3525-3.3 Datasheet, PDF (8/12 Pages) Linear Technology – 400mA Micropower Synchronous Step-Up DC/DC Converter with Output Disconnect

LTC3525-3.3/LTC3525-5
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OPERATIO
The LTC3525 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 1V, 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 either 3.3V or 5V. Peak
switch current is 400mA minimum, providing regulation
with load currents up to 150mA, 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 LTC3525 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.
Shutdown
The LTC3525 is shut down by pulling SHDN below 0.4V,
and made active by raising it above 1V. 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 LTC3525 to start with input
voltages as low as 1V. 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 synchro-
nous 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 LTC3525 goes to sleep, during which time the
output capacitor supplies current to the load. Once the
output voltage drops approximately 9mV below the regu-
lation value the IC leaves sleep mode and switching is
resumed.
The LTC3525 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
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