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SBVS037I – AUGUST 2003 – REVISED MAY 2006
V
N
(mV
RMS
)
+
8.5
mV
RMS
V
V
OUT
(V)
(3)
DROPOUT VOLTAGE
The TPS732xx uses an NMOS pass transistor to
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
input-to-output resistance is the R
DS-ON
of the NMOS
pass element.
For large step changes in load current, the
TPS732xx requires a larger voltage drop from V
IN
to
V
OUT
to avoid degraded transient response. The
boundary of this transient dropout region is
approximately twice the dc dropout. Values of V
IN
-
V
OUT
above this line insure normal transient
response.
Operating in the transient dropout region can cause
an increase in recovery time. The time required to
recover from a load transient is a function of the
magnitude of the change in load current rate, the
rate of change in load current, and the available
headroom (V
IN
to V
OUT
voltage drop). Under
worst-case conditions [full-scale instantaneous load
change with (V
IN
- V
OUT
) close to dc dropout levels],
the TPS732xx can take a couple of hundred
microseconds to return to the specified regulation
accuracy.
for C
NR
= 10nF.
This noise reduction effect is shown as
RMS Noise
Voltage vs C
NR
in the Typical Characteristics section.
The TPS73201 adjustable version does not have the
noise-reduction pin available. However, connecting a
feedback capacitor, C
FB
, from the output to the FB
pin will reduce output noise and improve load
transient performance.
The TPS732xx uses an internal charge pump to
develop an internal supply voltage sufficient to drive
the gate of the NMOS pass element above V
OUT
.
The charge pump generates ~250µV of switching
noise at ~2MHz; however, charge-pump noise
contribution is negligible at the output of the regulator
for most values of I
OUT
and C
OUT
.
BOARD LAYOUT RECOMMENDATION TO
IMPROVE PSRR AND NOISE
PERFORMANCE
To improve ac performance such as PSRR, output
noise, and transient response, it is recommended
that the PCB be designed with separate ground
planes for V
IN
and V
OUT
, with each ground plane
connected only at the GND pin of the device. In
addition, the ground connection for the bypass
capacitor should connect directly to the GND pin of
the device.
TRANSIENT RESPONSE
The low open-loop output impedance provided by the
NMOS pass element in a voltage follower
configuration allows operation without an output
capacitor for many applications. As with any
regulator, the addition of a capacitor (nominal value
1µF) from the output pin to ground will reduce
undershoot magnitude but increase duration. In the
adjustable version, the addition of a capacitor, C
FB
,
from the output to the adjust pin will also improve the
transient response.
The TPS732xx does not have active pull-down when
the output is over-voltage. This allows applications
that connect higher voltage sources, such as
alternate power supplies, to the output. This also
results in an output overshoot of several percent if
the load current quickly drops to zero when a
capacitor is connected to the output. The duration of
overshoot can be reduced by adding a load resistor.
The overshoot decays at a rate determined by output
capacitor C
OUT
and the internal/external load
resistance. The rate of decay is given by:
(Fixed voltage version)
V
OUT
dV dt
+
C
OUT
80kW
ø
R
LOAD
INTERNAL CURRENT LIMIT
The TPS732xx internal current limit helps protect the
regulator during fault conditions. Foldback helps to
protect the regulator from damage during output
short-circuit conditions by reducing current limit when
V
OUT
drops below 0.5V. See
in the Typical
Characteristics section for a graph of I
OUT
vs V
OUT
.
SHUTDOWN
The Enable pin is active high and is compatible with
standard TTL-CMOS levels. V
EN
below 0.5V (max)
turns the regulator off and drops the ground pin
current to approximately 10nA. When shutdown
capability is not required, the Enable pin can be
connected to V
IN
. When a pull-up resistor is used,
and operation down to 1.8V is required, use pull-up
resistor values below 50 kΩ.
(4)
12
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