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Texas Instruments LDO Noise Demystified (Rev. A) Application notes
Application Report
SLAA412A – June 2009 – Revised August 2017
LDO Noise Demystified
Sanjay Pithadia and Ankur Verma
.................................................................. PMP - LP Linear Regulators
ABSTRACT
This application report explains the difference between noise and PSRR of an LDO. It also explains the
different ways noise is specified in LDO datasheets and which specification should be used in the
application. Finally it explains how LDO noise is reduced.
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Contents
LDO Noise and PSRR ......................................................................................................
LDO Noise Types ............................................................................................................
Noise Specifications in LDO Data Sheets ................................................................................
Which Specification is for Your Application? .............................................................................
How to Reduce the LDO Noise?...........................................................................................
Implications of the LDO Noise .............................................................................................
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3
5
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List of Figures
...................................................................................................
1
PSRR and Noise in LDO
2
LDO Noise (types) ........................................................................................................... 2
2
3
Noise Specified in Two Ways (for TPS717xx LDOs) ................................................................... 3
4
Transmit Mask and Sidebands Due to Noise ............................................................................ 4
5
Increase in VCO Noise Floor Due to LDO Noise ........................................................................ 4
6
LDO Noise Aliasing .......................................................................................................... 5
List of Tables
Trademarks
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1
LDO Noise and PSRR
Low dropout linear regulators (LDOs) are a simple way to regulate an output voltage that is powered from
a higher voltage input. Though it is simple to operate, its self generated noise is most of the times
confused with its Power Supply Rejection Ratio (PSRR). Many times the two are combined together and
loosely called just “noise”. This is not correct. Noise is generated by the transistors and resistors in the
LDO’s internal circuitry and by the external components. The type of noise may include thermal, flicker
and shot noise. PSRR is a measure of circuit’s power supply rejection expressed as a ratio of output noise
to noise at the power supply input. It provides a measure of how well a circuit rejects ripple at various
frequencies injected from its input power supply. In the case of a LDO, it is a measure of the output ripple
compared to the input ripple over a wide frequency range and is expressed in decibels (dB). The basic
equation for PSRR is given in Equation 1:
RippleInput
PSRR = 20 log
RippleOutput
(1)
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1
LDO Noise Types
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Figure 1 explains how noise and PSRR are different from each other. The noise is something which may
be internal and external to the LDO whereas PSRR is an internal parameter of the LDO. LDO users
generally concentrate on PSRR and not on the self-generated output noise. PSRR rejects noise coming
from outside of LDO but there is always noise generated inside the LDO. So an LDO with high PSRR may
not be better for noise rejection. The user should always think of both parameters.
Figure 1. PSRR and Noise in LDO
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LDO Noise Types
Noise is purely physical phenomenon that occurs with transistors and resistors. Transistors generate shot
noise and flicker noise. The resistive element of MOSFETs also generates thermal noise like resistors.
Thermal and shot noise is truly random in nature and its power is flat over frequency. It remains flat up to
the bandwidth of the amplifier. Flicker noise is the noise due to trapped charges at the gate of the
MOSFET. It follows Poisson’s Distribution with 1/f roll-off in power versus frequency hence it is higher at
low frequencies. This noise dominates until it becomes smaller than thermal noise. (See Figure 2)
Figure 2. LDO Noise (types)
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Noise Specifications in LDO Data Sheets
Typically noise in an LDO is specified by datasheets in two fashions. One is “Total (Integrated) output
noise – in μVrms”, which is RMS value of the spectral noise density integrated over a finite frequency
range. The second method is to show a “Spectral Noise density curve – in μV/√Hz”, which is a plot of
Noise density vs Frequency. Figure 3 shows both the specifications for TPS717xx series LDOs.
2
LDO Noise Demystified
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OUTPUT SPECTRAL NOISE DENSITY vs
OUTPUT CURRENT
• Ultra-High PSRR
– 70dB at 1kHz, 67dB at 100kHz and 45dB at
1MHz
• Low Noise: 30μA typical (100Hz to 100 kHz)
Output Noise Density (μV/√Hz)
16
COUT = 1mF
CNR = 10nF
IOUT = 150mA
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12
IOUT = 10mA
10
8
6
4
2
0
100
10k
1k
100k
Frequency (Hz)
Figure 3. Noise Specified in Two Ways (for TPS717xx LDOs)
Since the output noise voltage is specified by a single number, it is very useful for comparison purposes.
When noise specifications of different LDOs are compared, it is imperative that the two regulators’ noise
measurements be taken over the same frequency range and at the same output voltage and current
values.
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Which Specification is for Your Application?
The user should know which noise specification of the LDO is to be used in the application, because there
are some application where the Spectral noise density is pertinent and some applications where total
(integrated) noise can be used. The following examples explain this.
1. Consider an RF system, where an LDO powers a Voltage Controlled Oscillator (VCO). VCO takes two
input signals and mixes them together. If the two signals are sin(ω1t) and sin(ω2)t, then the result is
two outputs sin((ω1–ω2)t), sin((ω1+ω2)t) and harmonics. The RF signal chain following the VCO is
typically a band-pass system tuned for only one frequency, that means, only higher frequency remains
after mixing. Most broadband applications have very tight regulation on the frequency spectrum and
the power in each band. For any band, the spurious noise has to be controlled to meet what is called
the “transmit mask”. This mask is very important for agency certification of the final product. Any
humps in the noise floor at higher frequencies may cause the transmitted signal to be outside of the
transmit mask and thus fail the certification testing.
Now if the noise is present on the conductors that are supplying power or in the LDO output then that
noise at a frequency, FR, gets mixed with the carrier frequency and produces two sidebands as shown
in the Figure 4. If the noise is so high that the sideband produced due to noise is out of transmit mask
then it results in failure of the system.
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Which Specification is for Your Application?
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Figure 4. Transmit Mask and Sidebands Due to Noise
Also, if we assume that the RF system is working at a frequency of 2.4GHz, then LDO noise
contributes to the VCO noise spectrum both above and below 2.4GHz with up to bandwidth of LDO.
The LDO noise shown in Figure 2 adds to the original VCO noise plot which increases the VCO noise
floor level around the center frequency.
Figure 5. Increase in VCO Noise Floor Due to LDO Noise
4
LDO Noise Demystified
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So in this RF application the user should use Spectral Noise Density curve since the single noise
number loses the frequency dependence and would not be an accurate representation of the final
output.
2. Consider a system in which LDO is powering an ADC or DAC. Any sampled system causes the high
frequency noise to fold to lower frequencies due to aliasing. For example, if the sampling frequency is
100kHz and the noise due to LDO is at 90kHz and 110kHz, 190kHz and 210kHz, etc., then all the
noise will fold back to 10kHz which is the beat frequency. This will occur for any frequency of the
output noise so that all of the LDO noise folds back to within the bandwidth of the sampling system.
This is same as integrating all the noise from DC to bandwidth of the system and computing the total
noise. This way, the performance of ADC/DAC suffers if the LDO total (integrated) noise is high.
Figure 6 below shows how LDO noise aliasing takes place. First graph is for a system powered by
ideal LDO, second is for a system powered by LDO with thermal noise that increases noise floor and
third is for a system powered by LDO with noise at high frequency that aliases to lower frequency.
–80
–80
–80
–90
–90
–90
–100
–100
Level (dB)
Level (dB)
–110
–120
–130
–110
Level (dB)
Signal
–100
–120
–130
–110
–120
–130
–140
–140
–140
–150
–150
–150
–160
0
–160
0
2
4
6
8
10
12
14
16
Frequency (kHz)
2
4
6
8
10
12
14
16
Frequency (kHz)
–160
0
2
4
6
8
10
12
14
16
Frequency (kHz)
Figure 6. LDO Noise Aliasing
So in this application, the user can use Total (Integrated) output noise since all of the noise will be folded
in frequency and integrated by the system.
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How to Reduce the LDO Noise?
The primary noise source in the LDO is the band gap. The noise in LDO can be reduced using two
methods. The following discussion explains both the methods.
One method to reduce the noise is by reducing the bandwidth of the LDO. This can be done by lowering
the bandwidth of error amplifier inside the LDO. But if we reduce the bandwidth of error amplifier then it
reduces the transient response of the LDO.
Another method is by using a low-pass filter (LPF). As we know, the most dominant source of noise in an
LDO is the internal band gap. A LPF can be inserted between the band gap output and the input of the
error amplifier. This reduced the band gap noise before it is gained up by the error amplifier. Typically this
LPF is formed with a large internal resistor and an external capacitor. The cutoff frequency of this filter is
set as low in frequency as possible to filter out nearly all of the noise coming from the band gap.
There is always the question “Why the huge power pass element (mostly FET), which takes up most of
the total die area, is not a primary noise contributor?” the answer is the lack of gain. The primary noise
source, the band gap, is connected to the input of the error amplifier and thus amplified by the gain of the
error amplifier. As we know, the procedure to find the output noise is first refer each noise contributor to
the op-amp input; so to find the noise from the pass FET you would first divide the noise contribution by
the open-loop gain that exists between it and the error amplifier input. This gain is very large; therefore,
the noise contribution from the pass FET is usually negligible.
To summarize, the LDO noise and PSRR both are important specifications to be taken into account when
selecting the LDO. There are two ways the LDO noise is specified, and the user should look for the
appropriate specification for their application.
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Implications of the LDO Noise
6
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Implications of the LDO Noise
Let us take an example of a DC-DC Converter with integrated LDOs (such as TPS57140-Q1).
The bandgap noise that is internal to the LDO regulator becomes a limiting factor in the rejection of high
frequency components. One of the example where its bad affect can be seen is when a a fast-falling input
transient is applied at the input of the DC/DC converter. During the fast input falling edge, if the slew rate
of the input is higher than a particular value, the internal LDO regulator of the device resets because of the
power-supply rejection-ratio (PSRR) limitation. The fast transition corresponds to higher frequencies. The
bandgap noise that is internal to the LDO regulator becomes a limiting factor in the rejection of high
frequency components. For example, using the TPS57140- Q1 design simulation and bench test
measurement, the resulting slew-rate value measured is 1.2 V/μs. If the slew rate is higher than this value,
the device gets disabled and regenerates a soft start. Higher ESR of the input capacitor negatively affects
the slew rate of the input voltage and the duration of this rate because of high current transient across the
ESR according to ESR × C × dV/dt. Therefore, the use of a low-ESR ceramic capacitor is recommended.
Refer to Design Considerations for DC-DC Converters in Fast-Input Slew Rate Applications (SLVA693) for
more information
6
LDO Noise Demystified
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Revision History
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Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Original (July 2009) to A Revision ........................................................................................................... Page
•
Added Implications of the LDO Noise section ......................................................................................... 6
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Revision History
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