Datasheet

V ( V )=25m
N RMS
xV (V)
OUT
mV
RMS
V
( )
TPS74701
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
TPS74701
www.ti.com SBVS099F –NOVEMBER 2007–REVISED NOVEMBER 2010
Soft-start capacitors larger than 0.015mF could be a OUTPUT NOISE
problem in applications where it is necessary to
The TPS74701 provides low output noise when a
rapidly pulse the enable pin and still require the
soft-start capacitor is used. When the device reaches
device to soft-start from ground. C
SS
must be
the end of the soft-start cycle, the soft-start capacitor
low-leakage; X7R, X5R, or C0G dielectric materials
serves as a filter for the internal reference. By using a
are preferred. Refer to Table 2 for suggested
0.001mF soft-start capacitor, the output noise is
soft-start capacitor values.
reduced by half and is typically 30mV
RMS
for a 1.2V
output (10Hz to 100kHz). Further increasing C
SS
has
SEQUENCING REQUIREMENTS
little effect on noise. Because most of the output
noise is generated by the internal reference, the
V
IN
, V
BIAS
, and V
EN
can be sequenced in any order
noise is a function of the set output voltage. The RMS
without causing damage to the device. However, for
noise with a 0.001mF soft-start capacitor is given in
the soft-start function to work as intended, certain
Equation 3:
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
(3)
V
BIAS
is faster than the set soft-start ramp rate. If the
The low output noise of the TPS74701 makes it a
ramp rate of the input sources is slower than the set
good choice for powering transceivers, PLLs, or other
soft-start time, the output tracks the slower supply
noise-sensitive circuitry.
minus the dropout voltage until it reaches the set
output voltage. If EN is connected to BIAS, the device
soft-starts as programmed, provided that V
IN
is ENABLE/SHUTDOWN
present before V
BIAS
. If V
BIAS
and V
EN
are present
The enable (EN) pin is active high and is compatible
before V
IN
is applied and the set soft-start time has
with standard digital signaling levels. V
EN
below 0.4V
expired, then V
OUT
tracks V
IN
. If the soft-start time has
turns the regulator off, while V
EN
above 1.1V turns the
not expired, the output tracks V
IN
until V
OUT
reaches
regulator on. Unlike many regulators, the enable
the value set by the charging soft-start capacitor.
circuitry has hysteresis and deglitching for use with
Figure 27 shows the use of an RC-delay circuit to
relatively slowly ramping analog signals. This
hold off V
EN
until V
BIAS
has ramped. This technique
configuration allows the TPS74701 to be enabled by
can also be used to drive EN from V
IN
. An external
connecting the output of another supply to the EN
control signal can also be used to enable the device
pin. The enable circuitry typically has 50mV of
after V
IN
and V
BIAS
are present.
hysteresis and a deglitch circuit to help avoid on-off
NOTE: When V
BIAS
and V
EN
are present and V
IN
is cycling as a result of small glitches in the V
EN
signal.
not supplied, this device outputs approximately 50mA
The enable threshold is typically 0.8V and varies with
of current from OUT. Although this condition does not
temperature and process variations. Temperature
cause any damage to the device, the output current
variation is approximately –1mV/°C; process variation
may charge up the OUT node if total resistance
accounts for most of the rest of the variation to the
between OUT and GND (including external feedback
0.4V and 1.1V limits. If precise turn-on timing is
resistors) is greater than 10kΩ.
required, a fast rise-time signal must be used to
enable the TPS74701.
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.
POWER GOOD
The power good (PG) pin is an open-drain output and
Figure 27. Soft-Start Delay Using an RC Circuit to
can be connected to any 5.5V or lower rail through an
Enable the Device
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
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