LTC3525L-3_15 Datasheet, PDF (6/12 Pages) Linear Technology – 400mA Micropower Synchronous Step-Up DC/DC Converter with Output Disconnect

LTC3525L-3
OPERATION
The LTC3525L-3 is a high performance Burst Mode op-
eration 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.
Shutdown
The LTC3525L-3 is shut down by pulling SHDN below
0.3V, and made active by raising it above 0.88V. Although
SHDN can be driven above V_IN or V_OUT (up to the absolute
maximum rating) without damage, the LTC3525L-3 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 opera-
tion 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).
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.
V_CNTRL
LTC3525L-3
SHDN
R
1M
R > (V_CNTRL/(V_IN + 0.4) – 1) MΩ
V_IN
LTC3525L-3
SHDN
3525L F01
ZETEX ZC2811E
1M
V_CNTRL
Figure 1. Recommended Shutdown Circuits when Driving SHDN Above V_IN
3525lfa
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