Datasheet

V ( V )=25m
N RMS
xV (V)
OUT
mV
RMS
V
( )
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
TPS74801
www.ti.com SBVS074J –JANUARY 2007– REVISED JANUARY 2012
rapidly pulse the enable pin and still require the 0.001μF soft-start capacitor, the output noise is
device to soft-start from ground. C
SS
must be reduced by half and is typically 30μV
RMS
for a 1.2V
low-leakage; X7R, X5R, or C0G dielectric materials output (10Hz to 100kHz). Further increasing C
SS
has
are preferred. Refer to Table 2 for suggested little effect on noise. Because most of the output
soft-start capacitor values. noise is generated by the internal reference, the
noise is a function of the set output voltage. The RMS
noise with a 0.001μF soft-start capacitor is given in
SEQUENCING REQUIREMENTS
Equation 3:
V
IN
, V
BIAS
, and V
EN
can be sequenced in any order
without causing damage to the device. However, for
(3)
the soft-start function to work as intended, certain
sequencing rules must be applied. Connecting EN to
The low output noise of the TPS74801 makes it a
IN is acceptable for most applications, as long as V
IN
good choice for powering transceivers, PLLs, or other
is greater than 1.1V and the ramp rate of V
IN
and
noise-sensitive circuitry.
V
BIAS
is faster than the set soft-start ramp rate. If the
ramp rate of the input sources is slower than the set
ENABLE/SHUTDOWN
soft-start time, the output tracks the slower supply
minus the dropout voltage until it reaches the set
The enable (EN) pin is active high and is compatible
output voltage. If EN is connected to BIAS, the device
with standard digital signaling levels. V
EN
below 0.4V
soft-starts as programmed, provided that V
IN
is
turns the regulator off, while V
EN
above 1.1V turns the
present before V
BIAS
. If V
BIAS
and V
EN
are present
regulator on. Unlike many regulators, the enable
before V
IN
is applied and the set soft-start time has
circuitry has hysteresis and deglitching for use with
expired, then V
OUT
tracks V
IN
. If the soft-start time has
relatively slowly ramping analog signals. This
not expired, the output tracks V
IN
until V
OUT
reaches
configuration allows the TPS74801 to be enabled by
the value set by the charging soft-start capacitor.
connecting the output of another supply to the EN
Figure 28 shows the use of an RC-delay circuit to
pin. The enable circuitry typically has 50mV of
hold off V
EN
until V
BIAS
has ramped. This technique
hysteresis and a deglitch circuit to help avoid on-off
can also be used to drive EN from V
IN
. An external
cycling as a result of small glitches in the V
EN
signal.
control signal can also be used to enable the device
The enable threshold is typically 0.8V and varies with
after V
IN
and V
BIAS
are present.
temperature and process variations. Temperature
NOTE: When V
BIAS
and V
EN
are present and V
IN
is
variation is approximately –1mV/°C; process variation
not supplied, this device outputs approximately 50μA
accounts for most of the rest of the variation to the
of current from OUT. Although this condition does not
0.4V and 1.1V limits. If precise turn-on timing is
cause any damage to the device, the output current
required, a fast rise-time signal must be used to
may charge up the OUT node if total resistance
enable the TPS74801.
between OUT and GND (including external feedback
If not used, EN can be connected to either IN or
resistors) is greater than 10kΩ.
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.
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
Figure 28. Soft-Start Delay Using an RC Circuit to
V
IT
+ V
HYS
. If V
OUT
drops below V
IT
or if V
BIAS
drops
Enable the Device
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 recommended operating
OUTPUT NOISE
condition of PG pin sink current is up to 1mA, so the
The TPS74801 provides low output noise when a
pull-up resistor for PG should be in the range of 10kΩ
soft-start capacitor is used. When the device reaches
to 1MΩ. If output voltage monitoring is not needed,
the end of the soft-start cycle, the soft-start capacitor
the PG pin can be left floating.
serves as a filter for the internal reference. By using a
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