Datasheet

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
µ
F
Ceramic
V
EN
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
TPS79901-Q1, TPS79912-Q1, TPS79915-Q1
TPS79918-Q1, TPS79925-Q1, TPS79927-Q1, TPS79933-Q1
www.ti.com SBVS097E –MARCH 2008–REVISED JANUARY 2012
APPLICATION INFORMATION
The TPS799xx family of LDO regulators combines the high performance required of many RF and precision
analog applications with ultra-low current consumption. High PSRR is provided by a high gain, high bandwidth
error loop with good supply rejection at very low headroom (V
IN
– V
OUT
). Fixed voltage versions provide a noise
reduction pin to bypass noise generated by the bandgap reference and to improve PSRR while a quick-start
circuit fast-charges this capacitor at startup. The combination of high performance and low ground current also
make the TPS799xx an excellent choice for portable applications. All versions have thermal and over-current
protection and are fully specified from –40°C to 125°C.
Figure 12 shows the basic circuit connections for fixed-voltage model. Figure 13 gives the connections for the
adjustable output version (TPS79901). R
1
and R
2
can be calculated for any output voltage using the formula in
Figure 13. Sample resistor values for common output voltages are shown in Figure 13.
Input and Output Capacitor Requirements
Although an input capacitor is not required for stability, it is good analog design practice to connect a 0.1-μF to 1-
μF low ESR capacitor across the input supply near the regulator. This counteracts reactive input sources and
improve transient response, noise rejection, and ripple rejection. A higher-value capacitor may be necessary if
large, fast rise-time load transients are anticipated or the device is located several inches from the power source.
If source impedance is not sufficiently low, a 0.1-μF input capacitor may be necessary to ensure stability.
The TPS799xx is designed to be stable with standard ceramic capacitors of values 2.2 μF or larger. X5R and
X7R type capacitors are best as they have minimal variation in value and ESR over temperature. Maximum ESR
should be <1.0 Ω.
Feedback Capacitor Requirements (TPS79901 only)
The feedback capacitor, C
FB
, shown in Figure 13 is required for stability. For a parallel combination of R
1
and R
2
equal to 250 kΩ, any value from 3 pF to 1 nF can be used. Fixed voltage versions have an internal 30-pF
feedback capacitor which is quick-charged at start-up. The adjustable version does not have this quick-charge
circuit, so values below 5 pF should be used to ensure fast startup; values above 47 pF can be used to
implement an output voltage soft-start. Larger value capacitors also improve noise slightly. The TPS79901 is
stable in unity-gain configuration (OUT tied to FB) without C
FB
.
Output Noise
In most LDOs, the bandgap is the dominant noise source. If a noise reduction capacitor (C
NR
) is used 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. To minimize noise in a given application, use a 0.01-μF noise
reduction capacitor; for the adjustable version, smaller value resistors in the output resistor divider reduce noise.
A parallel combination that gives 2 μA of divider current has the same noise performance as a fixed-voltage
version. To further optimize noise, equivalent series resistance of the output capacitor can be set to
approximately 0.2 Ω. This configuration maximizes phase margin in the control loop, reducing total output noise
by up to 10%.
Figure 12. Typical Application Circuit for Figure 13. Typical Application Circuit for
Fixed Voltage Version Adjustable Voltage Version
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Product Folder Links: TPS79901-Q1 TPS79912-Q1 TPS79915-Q1 TPS79918-Q1 TPS79925-Q1 TPS79927-Q1
TPS79933-Q1