Datasheet

LTC3025-1/LTC3025-2/
LTC3025-3/LTC3025-4
11
30251234ff
Figure 5. Output Start-Up and Shutdown
OFF
1.2V
0V
ON
SHDN
V
OUT
200mV/DIV
T
A
= 25°C
V
IN
= 1.5V
V
BIAS
= 3.6V
C
OUT
= 1µF
R
LOAD
= 4Ω
500µs/DIV
30251234 F05
applicaTions inForMaTion
Soft-Start Operation
The LTC3025-X includes a soft-start feature to prevent
excessive current flow during start-up. When the LDO is
enabled, the soft-start circuitry gradually increases the
LDO reference voltage from 0V to 0.4V over a period of
about 600µs. There is a short 700µs delay from the time
the part is enabled until the LDO output starts to rise. Fig-
ure 5 shows the start-up and shutdown output waveform.
V
OUT
Start-Up and Supply Sequencing
During power-up, the output shutdown circuitry is not
active below V
IN
of about 0.65V DC (typical). As a result,
the output voltage can drift up during power-up due to
leakage current (<1 mA typical) from V
IN
to V
OUT
. At 0.9V
input, the shutdown circuitry is active and the output is
actively held off. This usually causes no circuit problems
and is similar to 3-terminal regulators such as the LT3080,
LT1086 and LT317 which have no ground pin and can have
the output rise under some conditions. A slowly rising
V
IN
with the part enabled may result in non-monotonic
ramping of V
OUT
due to LDO circuitry becoming active at
V
IN
of about 0.65V (typical) as well.
With fast rising inputs (>1V/ms) or with sufficient resis-
tive load on V
OUT
, output voltage rise during power-up
is reduced or eliminated. Such conditions also reduce or
eliminate non-monotonic initial power-up with the part
enabled. If V
BIAS
is sequenced up before V
IN
, the leakage
current from V
IN
to V
OUT
may increase until the shutdown
circuitry is active at a V
IN
of about 0.65V typical. Thus,
to minimize V
OUT
rise during start-up, sequence up V
IN
before V
BIAS
. At V
IN
= 0.9V, the output is actively held off
in shutdown or it is actively held on when enabled under
all conditions.