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| 型號(hào)： | TPS74815DRCT |
| 廠商： | Texas Instruments, Inc. |
| 英文描述： | 1.5A LDO Linear Regulator with Programmable Soft-Start |
| 中文描述： | 1.5A的LDO線性穩(wěn)壓器，具有可編程軟啟動(dòng) |
| 文件頁(yè)數(shù)： | 12/19頁(yè) |
| 文件大小： | 351K |
| 代理商： | TPS74815DRCT |
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V ( V
) = 25
m
RMS
x V
(V)
OUT
m
V
RMS
V
(
)
(3)
SEQUENCING REQUIREMENTS
V
, V
, and V
can be sequenced in any order
without causing damage to the device. However, for
the soft-start function to work as intended, certain
sequencing rules must be applied. Connecting EN to
IN is acceptable for most applications, as long as V
IN
is greater than 1.1V and the ramp rate of V
IN
and
V
is faster than the set soft-start ramp rate. If the
ramp rate of the input sources is slower than the set
soft-start time, the output tracks the slower supply
minus the dropout voltage until it reaches the set
output voltage. If EN is connected to BIAS, the
device will soft-start as programmed, provided that
V
is present before V
. If V
and V
are
present before V
IN
is applied and the set soft-start
time has expired, then V
OUT
tracks V
IN
. If the
soft-start time has not expired, the output tracks V
until V
OUT
reaches the value set by the charging
soft-start capacitor.
Figure 28
shows the use of an
RC-delay circuit to hold off V
until V
has
ramped. This technique can also be used to drive EN
from V
IN
. An external control signal can also be used
to enable the device after V
IN
and V
BIAS
are present.
ENABLE/SHUTDOWN
The enable (EN) pin is active high and is compatible
with standard digital signaling levels. V
EN
below 0.4V
turns the regulator off, while V
EN
above 1.1V turns
the regulator on. Unlike many regulators, the enable
circuitry has hysteresis and deglitching for use with
relatively
slowly
ramping
configuration allows the TPS74801 to be enabled by
connecting the output of another supply to the EN
pin. The enable circuitry typically has 50mV of
hysteresis and a deglitch circuit to help avoid on-off
cycling as a result of small glitches in the V
EN
signal.
The enable threshold is typically 0.8V and varies with
temperature and process variations. Temperature
variation
is
approximately
variation accounts for most of the rest of the variation
to the 0.4V and 1.1V limits. If precise turn-on timing
is required, a fast rise-time signal must be used to
enable the TPS74801.
POWER-GOOD
The power-good (PG) pin is an open-drain output
and can be connected to any 5.5V or lower rail
through an external pull-up resistor. This pin requires
at least 1.1V on V
BIAS
in order to have a valid output.
The PG output is high-impedance when V
OUT
is
greater than V
IT
+ V
HYS
. If V
OUT
drops below V
IT
or if
V
BIAS
drops below 1.9V, the open-drain output turns
on and pulls the PG output low. The PG pin also
asserts when the device is disabled. The pull-up
resistor for PG should be in the range of 10k
to
1M
.
TPS74801
GND
SS
OUT
FB
EN
IN
BIAS
V
IN
V
OUT
R
2
R
1
C
SS
C
IN
C
V
BIAS
C
BIAS
R
C
OUT
OUTPUT NOISE
The TPS74801 provides low output noise when a
soft-start capacitor is used. When the device reaches
the end of the soft-start cycle, the soft-start capacitor
serves as a filter for the internal reference. By using
a 0.001
μ
F soft-start capacitor, the output noise is
TPS74801
SBVS074–JANUARY 2007
The maximum recommended soft-start capacitor is
0.015
μ
F. Larger soft-start capacitors can be used
and will not damage the device; however, the
soft-start capacitor discharge circuit may not be able
to fully discharge the soft-start capacitor when
enabled. Soft-start capacitors larger than 0.015
μ
F
could be a problem in applications where it is
necessary to rapidly pulse the enable pin and still
require the device to soft-start from ground. C
SS
must
be
low-leakage;
X7R,
X5R,
materials
are
preferred.
suggested soft-start capacitor values.
reduced by half and is typically 30
μ
V
RMS
for a 1.2V
output (10Hz to 100kHz). Further increasing C
has
little effect on noise. Because most of the output
noise is generated by the internal reference, the
noise is a function of the set output voltage. The
RMS noise with a 0.001
μ
F soft-start capacitor is
given in
Equation 3
:
or
C0G
Table
dielectric
2
Refer
to
for
The low output noise of the TPS74801 makes it a
good choice for powering transceivers, PLLs, or
other noise-sensitive circuitry.
analog
signals.
This
–1mV/
°
C;
process
If not used, EN can be connected to either IN or
BIAS. If EN is connected to IN, it should be
connected as close as possible to the largest
capacitance on the input to prevent voltage droops
on that line from triggering the enable circuit.
Figure 28. Soft-Start Delay Using an RC Circuit to
Enable the Device
12
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