ltc3525esc6-5-trpbf Linear Technology Corporation, ltc3525esc6-5-trpbf Datasheet - Page 8

• Manufacturer Part Number: ltc3525esc6-5-trpbf
• Description: 400ma Micropower Synchronous Step-up Dc/dc Converter With Output Disconnect
• Manufacturer: Linear Technology Corporation

OPERATIO
LTC3525-3/
LTC3525-3.3/LTC3525-5
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 effi 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 1V, or from two or three
cells (or a Li-ion battery), with voltages as high as 4.5V.
Once started, V
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 rectifi cation provides high effi ciency opera-
tion while eliminating the need for an external Schottky
diode. True output disconnect eliminates inrush current
at start-up, and allows V
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
rectifi er is not enabled in this mode resulting in lower ef-
fi ciency and reduced output current capability.
The operating quiescent current is only 7µA typical, allow-
ing the converter to maintain high effi ciency at extremely
light loads.
Shutdown
The LTC3525 is shut down by pulling ⎯ S ⎯ H ⎯ D ⎯ N below 0.4V,
and made active by raising it above 1V. Note that ⎯ S ⎯ H ⎯ D ⎯ N
can be driven as high as 6V, however, if it is more than
0.9V above the higher of V
current will increase from zero to 1.5µA.
After the ⎯ S ⎯ H ⎯ D ⎯ 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).
8
OUT
IN
U
. Note, however, that the synchronous
can be as low as 0.5V (depending on
OUT
to be disconnected from V
IN
or V
OUT
, the ⎯ S ⎯ H ⎯ D ⎯ N input
IN
,
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
0.2V typical and either V
1.8V typical.
During startup, the synchronous rectifi er is not enabled,
and the internal P-channel synchronous rectifi er acts as
a follower, causing the peak voltage on SW to reach (V
+ 1V) typical. This limits inrush current by maintaining
control of the inductor current when V
To reduce power dissipation in the P-channel synchronous
rectifi er when the output is shorted, a foldback feature is
incorporated that reduces the peak inductor current when
V
Normal Operation
Once V
V
begins, with synchronous rectifi cation 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 rectifi er 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 regulation 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
IN
IN
, and either voltage is above 1.8V, normal operation
is more than 1.7V greater than V
OUT
has increased more than 0.2V typical above
IN
OUT
or V
must exceed V
OUT
must be greater than
OUT
OUT
.
is less than V
IN
by at least
3525fa
IN
IN
.