LTC3525-3_15 Datasheet, PDF (10/16 Pages) Linear Technology – 400mA Micropower Synchronous Step-Up DC/DC Converter with Output Disconnect

LTC3525-3/
LTC3525-3.3/LTC3525-5
Operation
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 3V, 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. Although SHDN can
be driven above V_IN or V_OUT (up to the absolute maximum
rating) without damage, the LTC3525 has a proprietary test
mode that may be engaged if SHDN is held in the range
of 0.5V to 1V higher than the greater of V_IN or V_OUT. If
the test mode is engaged, normal PWM switching action
is interrupted, which can cause undesirable operation
in some applications. Therefore, in applications where
SHDN may be driven above V_IN, a resistor divider or other
means must be employed to keep the SHDN voltage below
(V_IN + 0.4V) to prevent the possibility of the test mode
being engaged. Please refer to Figure 1 for two possible
implementations.
V_CNTRL
LTC3525
SHDN
R
1M
R > (V_CNTRL/(V_IN + 0.4) – 1) MΩ
V_IN
LTC3525
SHDN
3525 F01
ZETEX ZC2811E
1M
V_CNTRL
Figure 1
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 start-up, 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.
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
3525fc
10
For more information www.linear.com/LTC3525