Datasheet

V V
N RMS
(m )=8.5 (V)x V
OUT
()
mV
RMS
V
TPS737xx
www.ti.com SBVS067P –JANUARY 2006–REVISED JULY 2013
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
This noise reduction effect is shown as RMS Noise
transistor may be left on (enhanced) for a significant
Voltage vs C
NR
in the Typical Characteristics section.
time after V
IN
has been removed. This scenario can
result in reverse current flow (if the IN pin is low
The TPS73701 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 power-
(FB) reduces output noise and improve load transient
up.
performance. This capacitor should be limited to
0.1µF.
Note that current limit foldback can prevent device
start-up under some conditions. See the Internal
The TPS737xx uses an internal charge pump to
Current Limit section for more information.
develop an internal supply voltage sufficient to drive
the gate of the NMOS pass element above V
OUT
. The
DROPOUT VOLTAGE
charge pump generates ~250μV of switching noise at
The TPS737xx uses an NMOS pass transistor to
~4MHz; however, charge-pump noise contribution is
achieve extremely low dropout. When (V
IN
– V
OUT
) is
negligible at the output of the regulator for most
less than the dropout voltage (V
DO
), the NMOS pass
values of I
OUT
and C
OUT
.
device is in its linear region of operation and the
input-to-output resistance is the R
DS, ON
of the NMOS
BOARD LAYOUT RECOMMENDATION TO
pass element.
IMPROVE PSRR AND NOISE PERFORMANCE
For large step changes in load current, the TPS737xx
To improve ac performance such as PSRR, output
requires a larger voltage drop from V
IN
to V
OUT
to
noise, and transient response, it is recommended that
avoid degraded transient response. The boundary of
the printed circuit board (PCB) be designed with
this transient dropout region is approximately twice
separate ground planes for V
IN
and V
OUT
, with each
the dc dropout. Values of V
IN
– V
OUT
above this line
ground plane connected only at the GND pin of the
ensure normal transient response.
device. In addition, the ground connection for the
bypass capacitor should connect directly to the GND
Operating in the transient dropout region can cause
pin of the device.
an increase in recovery time. The time required to
recover from a load transient is a function of the
INTERNAL CURRENT LIMIT
magnitude of the change in load current rate, the rate
of change in load current, and the available
The TPS737xx internal current limit helps protect the
headroom (V
IN
to V
OUT
voltage drop). Under worst-
regulator during fault conditions. Foldback current
case conditions [full-scale instantaneous load change
limit helps to protect the regulator from damage
with (V
IN
– V
OUT
) close to dc dropout levels], the
during output short-circuit conditions by reducing
TPS737xx can take a couple of hundred
current limit when V
OUT
drops below 0.5V. See
microseconds to return to the specified regulation
Figure 12 in the Typical Characteristics section.
accuracy.
Note from Figure 12 that approximately –0.2V of V
OUT
results in a current limit of 0mA. Therefore, if OUT is
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 TPS737xx
should be enabled first.
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