Datasheet

V
N
ǒ
mV
RMS
Ǔ
+ 25
ǒ
mV
RMS
V
Ǔ
V
OUT
(
V
)
TPS742xx
www.ti.com SBVS064L –DECEMBER 2005–REVISED NOVEMBER 2010
OUTPUT NOISE If not used, EN can be connected to either IN or
BIAS. If EN is connected to IN, it should be
The TPS742xx provides low output noise when a
connected as close as possible to the largest
soft-start capacitor is used. When the device reaches
capacitance on the input to prevent voltage droops on
the end of the soft-start cycle, the soft-start capacitor
that line from triggering the enable circuit.
serves as a filter for the internal reference. By using a
0.001mF soft-start capacitor, the output noise is
POWER-GOOD (QFN Package Only)
reduced by half and is typically 30mV
RMS
for a 1.2V
output (10Hz to 100kHz). Because most of the output
The power-good (PG) pin is an open-drain output and
noise is generated by the internal reference, the
can be connected to any 5.5V or lower rail through an
noise is a function of the set output voltage. The RMS
external pull-up resistor. This pin requires at least
noise with a 0.001mF soft-start capacitor is given in
1.1V on V
BIAS
in order to have a valid output. The PG
Equation 3.
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
(3) the PG output low. The PG pin also asserts when the
device is disabled. The recommended operating
The low output noise of the TPS742xx makes it a
condition of PG pin sink current is up to 1mA, so the
good choice for powering transceivers, PLLs, or other
pull-up resistor for PG should be in the range of 10kΩ
noise-sensitive circuitry.
to 1MΩ. PG is only provided on the QFN package. If
output voltage monitoring is not needed, the PG pin
ENABLE/SHUTDOWN
can be left floating.
The enable (EN) pin is active high and is compatible
with standard digital signaling levels. V
EN
below 0.4V
INTERNAL CURRENT LIMIT
turns the regulator off, while V
EN
above 1.1V turns the
regulator on. Unlike many regulators, the enable The TPS742xx features a factory-trimmed, accurate
circuitry has hysteresis and deglitching for use with current limit that is flat over temperature and supply
relatively slow-ramping analog signals. This voltage. The current limit allows the device to supply
configuration allows the TPS742xx to be enabled by surges of up to 1.8A and maintain regulation. The
connecting the output of another supply to the EN current limit responds in about 10ms to reduce the
pin. The enable circuitry typically has 50mV of current during a short-circuit fault. Recovery from a
hysteresis and a deglitch circuit to help avoid on-off short-circuit condition is well-controlled and results in
cycling because of small glitches in the V
EN
signal. very little output overshoot when the load is removed.
See Figure 27 in the Typical Characteristics section
The enable threshold is typically 0.8V and varies with
for a graph of I
OUT
versus V
OUT
performance.
temperature and process variations. Temperature
variation is approximately –1mV/°C; therefore, The internal current limit protection circuitry of the
process variation accounts for most of the variation in TPS742xx is designed to protect against overload
the enable threshold. If precise turn-on timing is conditions. It is not intended to allow operation above
required, a fast rise-time signal should be used to the rated current of the device. Continuously running
enable the TPS742xx. the TPS742xx above the rated current degrades
device reliability.
Copyright © 2005–2010, Texas Instruments Incorporated Submit Documentation Feedback 15