Datasheet

APPLICATION INFORMATION
Input and Output Capacitor Requirements
Feedback Capacitor Requirements
GNDEN NR
IN OUT
TPS734xx
Optionalbypasscapacitor
toreduceoutputnoise
andincreasePSRR.
Optionalinputcapacitor.
Mayimprovesource
impedance,noise,orPSRR.
V
IN
V
EN
2.2 F
Ceramic
m
V
OUT
Output Noise
GNDEN
FB
IN OUT
Optionalinputcapacitor.
Mayimprovesource
impedance,noise,orPSRR.
TPS73401
2.2 F
Ceramic
m
V
IN
V
EN
R
1
R
2
C
FB
V
OUT
(R +R )
1 2
R
2
V
OUT
= ´ 1.208
TPS734xx
SBVS089F – DECEMBER 2007 – REVISED FEBRUARY 2009 ........................................................................................................................................ www.ti.com
The TPS734xx family of LDO regulators combines
the high performance required of many RF and Although an input capacitor is not required for
precision analog applications with ultra-low current stability, it is good analog design practice to connect
consumption. High PSRR is provided by a high gain, a 0.1 µ F to 1 µ F low equivalent series resistance
high bandwidth error loop with good supply rejection (ESR) capacitor across the input supply near the
at very low headroom (V
IN
– V
OUT
). Fixed voltage regulator. The ground of this capacitor should be
versions provide a noise reduction pin to bypass connected as close as the ground of output capacitor;
noise generated by the bandgap reference and to a capacitor value of 0.1 µ F is enough in this condition.
improve PSRR while a quick-start circuit fast-charges When it is difficult to place these two ground points
this capacitor at startup. The combination of high close together, a 1 µ F capacitor is recommended.
performance and low ground current also make the This capacitor counteracts reactive input sources and
TPS734xx an excellent choice for portable improves transient response, noise rejection, and
applications. All versions have thermal and ripple rejection. A higher-value capacitor may be
over-current protection and are fully specified from necessary if large, fast rise-time load transients are
– 40 ° C to +125 ° C. anticipated, or if the device is located several inches
from the power source. If source impedance is not
Figure 20 shows the basic circuit connections for
sufficiently low, a 0.1 µ F input capacitor may be
fixed voltage models. Figure 21 gives the connections
necessary to ensure stability.
for the adjustable output version (TPS73401). R
1
and
R
2
can be calculated for any output voltage using the The TPS734xx is designed to be stable with standard
formula in Figure 21 . ceramic output capacitors of values 2.2 µ F or larger.
X5R and X7R type capacitors are best because they
have minimal variation in value and ESR over
temperature. Maximum ESR of the output capacitor
should be < 1.0 Ω , so output capacitor type should be
either ceramic or conductive polymer electrolytic.
(TPS73401 only)
The feedback capacitor, C
FB
, shown in Figure 21 is
required for stability. For a parallel combination of R
1
and R
2
equal to 250k Ω , any value from 3pF to 1nF
can be used. Fixed voltage versions have an internal
30pF feedback capacitor that is quick-charged at
Figure 20. Typical Application Circuit for
start-up. The adjustable version does not have this
FIxed Voltage Versions
quick-charge circuit, so values below 5pF should be
used to ensure fast startup; values above 47pF can
be used to implement an output voltage soft-start.
Larger value capacitors also improve noise slightly.
The TPS73401 is stable in unity-gain configuration
(OUT tied to FB) without C
FB
.
In most LDOs, the bandgap is the dominant noise
source. If a noise reduction capacitor (C
NR
) is used
with the TPS734xx, the bandgap does not contribute
significantly to noise. Instead, noise is dominated by
the output resistor divider and the error amplifier
Figure 21. Typical Application Circuit for
input. To minimize noise in a given application, use a
Adjustable Voltage Versions
0.01 µ F noise reduction capacitor; for the adjustable
version, smaller value resistors in the output resistor
space divider reduce noise. A parallel combination that
gives 2 µ A of divider current has the same noise
space
performance as a fixed voltage version. To further
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