Datasheet

V
N
(mV
RMS
) + 8.5
ǒ
mV
RMS
V
Ǔ
V
OUT
(V)
TPS731xx
SBVS034M –SEPTEMBER 2003–REVISED AUGUST 2009.......................................................................................................................................... www.ti.com
An internal 27kΩ resistor in series with the noise ENABLE PIN AND SHUTDOWN
reduction pin (NR) forms a low-pass filter for the
The enable pin (EN) is active high and is compatible
voltage reference when an external noise reduction
with standard TTL-CMOS levels. A V
EN
below 0.5V
capacitor, C
NR
, is connected from NR to ground. For
(max) turns the regulator off and drops the GND pin
C
NR
= 10nF, the total noise in the 10Hz to 100kHz
current to approximately 10nA. When EN is used to
bandwidth is reduced by a factor of ~3.2, giving the
shutdown the regulator, all charge is removed from
approximate relationship:
the pass transistor gate, and the output ramps back
up to a regulated V
OUT
(see Figure 23).
(3) When shutdown capability is not required, EN can be
connected to V
IN
. However, the pass gate may not be
for C
NR
= 10nF.
discharged using this configuration, and the pass
transistor may be left on (enhanced) for a significant
This noise reduction effect is shown as RMS Noise
time after V
IN
has been removed. This scenario can
Voltage vs C
NR
in the Typical Characteristics section.
result in reverse current flow (if the IN pin is low
The TPS73101 adjustable version does not have the
impedance) and faster ramp times upon power-up. In
NR pin available. However, connecting a feedback
addition, for V
IN
ramp times slower than a few
capacitor, C
FB
, from the output to the feedback pin
milliseconds, the output may overshoot upon
(FB) reduces output noise and improves load
power-up.
transient performance.
Note that current limit foldback can prevent device
The TPS731xx uses an internal charge pump to
start-up under some conditions. See the Internal
develop an internal supply voltage sufficient to drive
Current Limit section.
the gate of the NMOS pass element above V
OUT
. The
charge pump generates ~250μV of switching noise at
DROPOUT VOLTAGE
~4MHz; however, charge-pump noise contribution is
negligible at the output of the regulator for most The TPS731xx uses an NMOS pass transistor to
values of I
OUT
and C
OUT
. achieve extremely low dropout. When (V
IN
– V
OUT
) is
less than the dropout voltage (V
DO
), the NMOS pass
device is in its linear region of operation and the
BOARD LAYOUT RECOMMENDATION TO
input-to-output resistance is the R
DS-ON
of the NMOS
IMPROVE PSRR AND NOISE PERFORMANCE
pass element.
To improve ac performance such as PSRR, output
For large step changes in load current, the TPS731xx
noise, and transient response, it is recommended that
requires a larger voltage drop from V
IN
to V
OUT
to
the PCB be designed with separate ground planes for
avoid degraded transient response. The boundary of
V
IN
and V
OUT
, with each ground plane connected only
this transient dropout region is approximately twice
at the ground pin (GND) of the device. In addition, the
the dc dropout. Values of V
IN
– V
OUT
above this line
ground connection for the bypass capacitor should
insure normal transient response.
connect directly to the GND pin of the device.
Operating in the transient dropout region can cause
INTERNAL CURRENT LIMIT
an increase in recovery time. The time required to
recover from a load transient is a function of the
The TPS731xx internal current limit helps protect the
magnitude of the change in load current rate, the rate
regulator during fault conditions. Foldback current
of change in load current, and the available
limit helps to protect the regulator from damage
headroom (V
IN
to V
OUT
voltage drop). Under
during output short-circuit conditions by reducing
worst-case conditions [full-scale instantaneous load
current limit when V
OUT
drops below 0.5V. See
change with (V
IN
– V
OUT
) close to dc dropout levels],
Figure 12 in the Typical Characteristics section.
the TPS731xx can take a couple of hundred
Note from Figure 12 that approximately –0.2V of V
OUT
microseconds to return to the specified regulation
results in a current limit of 0mA. Therefore, if OUT is
accuracy.
forced below –0.2V before EN goes high, the device
may not start up. In applications that work with both a
positive and negative voltage supply, the TPS731xx
should be enabled first.
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