VN(mVRMS)+8.5ǒmVRMS
VǓ VOUT(V)
TPS731xx
SBVS034M –SEPTEMBER 2003–REVISED AUGUST 2009..........................................................................................................................................
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An internal 27kΩresistor 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 VEN below 0.5V
capacitor, CNR, is connected from NR to ground. For (max) turns the regulator off and drops the GND pin
CNR = 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 VOUT (see Figure 23).
(3) When shutdown capability is not required, EN can be
connected to VIN. However, the pass gate may not be
for CNR = 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 VIN has been removed. This scenario can
Voltage vs CNR 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 VIN ramp times slower than a few
capacitor, CFB, 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 VOUT. 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 IOUT and COUT. achieve extremely low dropout. When (VIN – VOUT) is
less than the dropout voltage (VDO), the NMOS pass
device is in its linear region of operation and the
BOARD LAYOUT RECOMMENDATION TO input-to-output resistance is the RDS-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 VIN to VOUT to
the PCB be designed with separate ground planes for avoid degraded transient response. The boundary of
VIN and VOUT, 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 VIN – VOUT 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 (VIN to VOUT voltage drop). Under
during output short-circuit conditions by reducing worst-case conditions [full-scale instantaneous load
current limit when VOUT drops below 0.5V. See change with (VIN – VOUT) 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 VOUT 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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