Datasheet

TPS79901
GNDEN
FB
IN OUT
V
IN
V
OUT
R
1
C
FB
R
2
Optionalinputcapacitor.
Mayimprovesource
impedance,noise,orPSRR.
V =
OUT
´ 1.193
(R
1 2
+R )
R
2
V
EN
2.2 Fm
Ceramic
TPS799xx
GNDEN NR
IN OUT
V
IN
V
OUT
Optional input capacitor.
May improve source
impedance, noise, or PSRR.
Optional bypass capacitor
to reduce output noise
and increase PSRR.
2.2
m
F
Ceramic
V
EN
TPS799xx
SBVS056J –JANUARY 2005–REVISED AUGUST 2010 www.ti.com
APPLICATION INFORMATION
The TPS799xx family of LDO regulators combines
the high performance required of many RF and
Feedback Capacitor Requirements
precision analog applications with ultra-low current
(TPS79901 only)
consumption. High PSRR is provided by a high gain,
The feedback capacitor, C
FB
, shown in Figure 29 is
high bandwidth error loop with good supply rejection
required for stability. For a parallel combination of R
1
at very low headroom (V
IN
– V
OUT
). Fixed voltage
and R
2
equal to 250kΩ, any value from 3pF to 1nF
versions provide a noise reduction pin to bypass
can be used. Fixed voltage versions have an internal
noise generated by the bandgap reference and to
30pF feedback capacitor which is quick-charged at
improve PSRR while a quick-start circuit fast-charges
start-up. The adjustable version does not have this
this capacitor at startup. The combination of high
quick-charge circuit, so values below 5pF should be
performance and low ground current also make the
used to ensure fast startup; values above 47pF can
TPS799xx an excellent choice for portable
be used to implement an output voltage soft-start.
applications. All versions have thermal and
Larger value capacitors also improve noise slightly.
over-current protection and are fully specified from
The TPS79901 is stable in unity-gain configuration
–40°C to +125°C.
(OUT tied to FB) without C
FB
.
Figure 28 shows the basic circuit connections for
fixed voltage models. Figure 29 gives the connections
Output Noise
for the adjustable output version (TPS79901). R
1
and
R
2
can be calculated for any output voltage using the In most LDOs, the bandgap is the dominant noise
formula in Figure 29. Sample resistor values for source. If a noise reduction capacitor (C
NR
) is used
common output voltages are shown in Figure 29. with the TPS799xx, the bandgap does not contribute
significantly to noise. Instead, noise is dominated by
the output resistor divider and the error amplifier
Input and Output Capacitor Requirements
input. To minimize noise in a given application, use a
Although an input capacitor is not required for
0.01mF noise reduction capacitor; for the adjustable
stability, it is good analog design practice to connect
version, smaller value resistors in the output resistor
a 0.1mF to 1mF low ESR capacitor across the input
divider reduce noise. A parallel combination that
supply near the regulator. This will counteract
gives 2mA of divider current will have the same noise
reactive input sources and improve transient
performance as a fixed voltage version. To further
response, noise rejection, and ripple rejection. A
optimize noise, equivalent series resistance of the
higher-value capacitor may be necessary if large, fast
output capacitor can be set to approximately 0.2Ω.
rise-time load transients are anticipated or the device
This configuration maximizes phase margin in the
is located several inches from the power source. If
control loop, reducing total output noise by up to
source impedance is not sufficiently low, a 0.1mF
10%.
input capacitor may be necessary to ensure stability.
space
The TPS799xx is designed to be stable with standard
space
ceramic capacitors of values 2.2mF or larger. X5R
and X7R type capacitors are best as they have
space
minimal variation in value and ESR over temperature.
Maximum ESR should be < 1.0Ω. space
Figure 29. Typical Application Circuit for
Adjustable Voltage Version
Figure 28. Typical Application Circuit for
Fixed Voltage Versions
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