LTC3525D-3.3
8
3525d33fb
For more information www.linear.com/LTC3525D3.3
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 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 0.85V, 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 (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 rectification
provides high efficiency opera
-
tion while eliminating the need for an external Schottky
diode.
The
L
TC3525D can 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 L
TC3525D 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 LTC3525D 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 ac-
tion 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.
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).
Pass-Through Mode
When the LTC3525D is in shutdown, the internal P-channel
MOSFET switch is turned on. This allows V
IN
to be con-
nected to V
OUT
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. 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
.
operaTion
LTC3525D-3.3
1M
V
CNTRL
R
V
IN
V
CNTRL
SHDN
LTC3525D-3.3
1M
3525d33 F01
ZETEX ZC2811E
R > (V
CNTRL
/(V
IN
+ 0.4) – 1) MΩ
SHDN
Figure 1
