AN-1120: Noise Sources in Low Dropout (LDO) Regulators (Rev. 0)

AN-1120: Noise Sources in Low Dropout (LDO) Regulators (Rev. 0)
AN-1120
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
Noise Sources in Low Dropout (LDO) Regulators
by Glenn Morita
WHY THE SOURCE OF NOISE MATTERS
NOISE SOURCES
The difference between insignificant noise and significant noise
is the degree to which the noise affects the operation of the circuit
in question.
The noise sources in a low dropout (LDO) regulator, or any
circuit for that matter, can be separated into two broad categories,
intrinsic and extrinsic. Intrinsic noise is like the noise in your
head, and extrinsic noise is like the noise from a jet airplane.
For example, a switching power supply has a significant amount
of output voltage ripple at 3 MHz. If the circuit it is powering
has a bandwidth of only a few hertz, such as a temperature sensor,
this ripple may be of no consequence. On the other hand, if the
same switching power supply powers an RF phase-locked loop
(PLL), the result could be quite different.
Understanding the sources of noise, their spectral characteristics,
noise reduction strategies, and the sensitivity of the circuits in
question to this noise is crucial to successfully designing a
robust system.
LDOs are easy to use; however, PSRR and internally generated
noise are often confused. In many cases, the two are simply
classified together as noise. This is a misapplication of
the specifications because the two types of noise have different
characteristics, and the methods for reducing their effect on
system performance are different.
Figure 1 is a simplified block diagram of an LDO and shows how
the intrinsic and extrinsic noise sources differ from each other. The
error amplifier determines the PSRR of the LDO, and therefore,
its ability to reject noise at its input. Intrinsic noise, however, always
appears at the output of the LDO.
DC OUTPUT
PLUS
AC NOISE
DC
SOURCE
INTERNAL
AC NOISE
EXTERNAL
AC NOISE
ERROR
AMPLIFIER
+ REFERENCE
– VOLTAGE
SIMPLIFIED LDO
Figure 1. Simplified LDO with Intrinsic and Extrinsic Noise Sources
Rev. 0 | Page 1 of 12
09924-001
This application note also attempts to clarify the difference
between power supply rejection ratio (PSRR) and internally
generated noise, and describes how to apply the data sheet
specifications for each parameter.
Applied to electronic circuits, intrinsic noise is noise that is
internally generated by any electronic device, whereas extrinsic
noise is noise that is passed on from a source outside the circuit.
AN-1120
Application Note
TABLE OF CONTENTS
Why the Source of Noise Matters................................................... 1 PSRR as a Function of Frequency ...................................................6 Noise Sources .................................................................................... 1 PSRR as a Function of Load Current..............................................7 Revision History ............................................................................... 2 PSRR as a Function of LDO Headroom.........................................8 Intrinsic Noise................................................................................... 3 Improving PSRR ................................................................................8 Extrinsic Noise .................................................................................. 3 Cascading LDOs to Increase PSRR.................................................9 Noise in LDOs................................................................................... 4 Total LDO Noise............................................................................. 10 LDO PSRR......................................................................................... 6 Summary ......................................................................................... 10 REVISION HISTORY
6/11—Revision 0: Initial Version
Rev. 0 | Page 2 of 12
Application Note
AN-1120
INTRINSIC NOISE
There are many sources of intrinsic noise, and each has unique
characteristics. Figure 2 shows how the noise of a typical device
varies with frequency and the contribution of each type of noise
to the total. The transition point from the 1/f region to the thermal
region is called the corner frequency. The major types of intrinsic
noise include the following: thermal noise, 1/f noise, shot noise,
and burst, or popcorn, noise.
1/f
fC
FREQUENCY
09924-002
THERMAL/SHOT
CORNER
FREQUENCY
In general, devices with buried junctions, such as bipolar transistors
and JFETs, tend to have lower 1/f noise than surface devices such
as MOSFETs.
Shot Noise
Shot noise occurs where there is a potential barrier as in a PN
junction. Due to the quantum nature of current flow in semiconductor devices, the current flow is not continuous. When
the charge carriers, holes, and electrons cross the barrier, shot
noise is generated. Like thermal noise, shot noise is random and
does not vary with frequency.
SLOPE ~1/f
NOISE
POWER
1/f noise is highly dependent on device geometry, device type,
and semiconductor material. Because of this, mathematical models
are extremely difficult to create, and empirical testing of individual
cases is used to characterize and predict 1/f noise.
Figure 2. Typical Noise Power vs. Frequency
Thermal Noise
Thermal, Johnson, or white noise results from the agitation of
charge carriers (electrons and holes) in any conductor or semiconductor at any temperature above absolute zero. Thermal noise
power is proportional to temperature. It is random in nature and
therefore does not vary with frequency.
Thermal noise is a physical process, and it can be calculated as
Vn = √(4kTRB)
(1)
where:
k is Boltzmann’s constant (1.38−23 joules/Kelvin).
T is the temperature in degrees Kelvin (K = 273°C).
R is the resistance in ohms.
B is the bandwidth in Hz in which the noise is observed (rms
voltage measured across the resistor is also a function of the
bandwidth in which the measurement is made).
For example, a 100 kΩ resistor over a 1 MHz bandwidth, at room
temperature, adds noise to a circuit and can be calculated as
Vn = (4 × 1.38−23 × 300 × 15 × 16)½ = 40.7 μV rms
(2)
1/f Noise
1/f noise results from surface defects in semiconductors. 1/f
noise power is proportional to the bias current of a device and
unlike thermal noise, is inversely proportional to frequency.
This inverse proportionality holds true for very low frequencies;
however, above a few kilohertz, it is nearly flat. 1/f noise is also
called pink noise because it tends to be weighted toward the low
end of the frequency spectrum.
Burst or Popcorn Noise
Burst, or popcorn, noise is a low frequency noise that seems to
be associated with ionic contamination. Burst noise manifests
itself as an abrupt shift in the bias current or output voltage of a
circuit. This shift lasts for a short time before the bias current or
output voltage suddenly returns to its original state. These shifts
are random but seem to be proportional to the bias current and
inversely proportional to the square of the frequency (1/f2).
Due to the extraordinary cleanliness of modern semiconductor
process technology, burst noise has been virtually eliminated
and is not a major factor in device noise.
EXTRINSIC NOISE
Extrinsic noise sources are even more numerous than intrinsic
sources. Extrinsic noise sources include the following:
•
•
•
Electromagnetic fields that couple into sensitive circuits.
Mechanical shock or vibration that causes piezoelectric
materials to generate unintended ac voltages.
Noise from other circuits that is conducted or radiated into
circuits via the power supply or poorly designed PCB layouts.
Electromagnetic Coupling
Electromagnetic fields can induce noise into a circuit by one or
more of the following methods: radiative coupling, capacitive
coupling, inductive coupling, and conductive coupling. The effects
of these types of coupling can be reduced through proper PCB
layout and shielding techniques that are beyond the scope of
this application note.
Rev. 0 | Page 3 of 12
AN-1120
Application Note
Piezoelectric Effects
Reference Noise
Some components, such as high value multilayer ceramic
capacitors, are sensitive to mechanical shock and vibration
(that is, they are microphonic), which is due to the use of high
dielectric constant materials in their construction. These dielectrics
are highly piezoelectric in nature and can easily transform small
mechanical vibrations into microvolt or even millivolt level signals.
This is the reasons that high value ceramic capacitors are not
recommended for use in low-level signal chain circuits.
Because the thermal noise of a resistor is defined as Vn =
√(4kTRB), it can be seen that resistors can contribute a significant
amount of noise to the reference circuit. Fortunately, the reference
voltage of an LDO does not require a bandwidth of more than a
few hertz, and on-chip passive filtering is easily achieved to
reduce this noise.
Although film capacitors are not piezoelectric, they can also be
sensitive to vibration. This is because any mechanical stress
on the film dielectric changes the thickness of the film slightly,
which causes the capacitance to increase or decrease very slightly.
Because the energy stored in a capacitor is constant, the voltage
must change slightly to accommodate the change in capacitance.
The relationship between energy, capacitance, and voltage is
described by the equation
E = ½ CV2
For example, a band gap reference with a source impedance
of 0.1 GΩ has 407 μV rms of noise from 10 Hz to 100 kHz. By
limiting the bandwidth to 10 Hz, the noise can be reduced to
4.1 μV rms. If the bandwidth is reduced to 1.6 Hz, the noise
contribution of the reference falls to 1.3 μV rms. A single-pole
RC filter with a corner frequency of 1.6 Hz can be built with a
1 GΩ resistor and a 100 pF capacitor. Figure 3 shows how such
a 1.0 V ultralow noise reference can be implemented in silicon.
VIN
0.1GΩ
(3)
~1.22VDC
Vn = ~407µV rms
10Hz TO 100kHz
When mechanical stress is removed, the voltage on the capacitor
returns to its original state. If the mechanical stress is periodic,
a small ac voltage is generated.
1GΩ
0.8×
1×
100pF
VREF = ~1.0VDC
Vn = ~1.3µV rms
0.1Hz TO 1.6Hz
In many systems, power from the ac mains or batteries is
converted to intermediate voltages for distribution throughout
the system by highly efficient switch mode power supplies. These
intermediate voltages are converted to specific voltages at the
point of use.
The noise from a switch mode power supply depends heavily on
its topology and load state. The spectral content can be from a
few hertz to several tens of megahertz. In many cases, these noisy
power distribution buses are cleaned up by LDOs in order to
power sensitive analog loads. The ability of an LDO to reject
noise from an input source depends on its PSRR and how it
varies over frequency.
Figure 3. Ultralow Noise, Ultralow Power Reference (ADP223)
Error Amplifier Noise
If a low noise reference is used, the error amplifier becomes
an important contributor to the total output noise. The noise
contributions from the reference and error amplifier are
uncorrelated and must be summed by the root mean square
method.
Figure 4 shows an example of a 2.5 V output LDO with a 500 mV
reference. The noise of the reference is given as 1 μV rms, and the
error amplifier noise is 1.5 μV rms. The total noise of 9 μV rms is
calculated as follows:
Vn = √((G × 1.5 μV rms)2+(G × 1.0 μV rms)2)
(4)
G = 1 + 400 kΩ /100 kΩ = 5
Vn = √(7.52 + 52) = 9 μV rms
NOISE IN LDOs
DC
SOURCE
The major sources of intrinsic noise in LDOs are the internal
reference voltage and the error amplifier.
2.5VDC PLUS
9µV rms
400kΩ
100kΩ
Modern LDOs operate with internal bias currents of a few tens
of nano amps in order to achieve quiescent currents of 15 μA or
less. These low bias currents require the use of large value bias
resistors of up to a GΩ in value.
1.5µV rms
ERROR
AMPLIFIER
1.0µV rms
+
= 500mV
V
– REF
09924-004
After intrinsic noise, power supply noise and ripple are generally
the most prevalent noise sources at the output of an LDO.
Depending on the spectral content of the noise source, an LDO
can greatly improve the quality of the power to downstream
circuits.
09924-003
Power Supply Noise
Figure 4. Noise Contributions from Reference and Error Amplifier (ADP223)
Rev. 0 | Page 4 of 12
Application Note
AN-1120
Reducing LDO Noise
There are two major methods for reducing the noise of an LDO:
Filtering the reference
Reducing the noise gain of the error amplifier
There is also an improvement in PSRR over the same frequency
range (see the Improving PSRR section for more information).
Some LDOs allow the use of an external capacitor to filter the
reference. In fact, many so-called ultralow noise LDOs require
the use of an external noise reduction capacitor to achieve their
low noise specifications. The drawback of using external filtering of
the reference is that the start-up time is proportional to the size
of the filter capacitor. Figure 3 shows why this is the case. The
node to which the 100 pF capacitor is connected is brought out
for connection to an external capacitor.
NOISE SPECTRAL DENSITY (µV/ Hz)
10
Reducing the noise gain of the error amplifier does not have as
dramatic an effect on the start-up time as filtering the reference,
thus making the trade-off between start-up time and output noise
easier. Unfortunately, reducing the output noise is generally not
possible for fixed output LDOs because there is no access to the
feedback node. However, the feedback node is readily accessible
in most adjustable output LDOs.
5V FIXED
5V ADJ
5V ADJ NR CIRCUIT
1
0.1
0.01
10
100
1k
FREQUENCY (Hz)
10k
100k
09924-006
•
•
which is because the closed-loop gain of the error amplifier meets
the open-loop characteristic of the amplifier, and no further
reduction in noise gain is possible.
Figure 6. Noise Spectral Density of an Adjustable LDO
If the noise contribution of the error amplifier is greater than the
contribution of the reference, reducing the noise gain of the
error amplifier can significantly reduce the overall noise of the
LDO. One way to determine if the error amplifier is the main
noise contributor is to compare the noise of the fixed vs. the
adjustable versions of a specific LDO. If the fixed LDOs have much
less noise than the adjustable LDOs, the error amplifier is the
main noise source.
Noise Specification in LDO Data Sheets
Figure 5 shows a 2.5 V output adjustable LDO where R1, R2,
R3, and C1 are external components. R3 was chosen to set the
high frequency gain of the amplifier to 1.5× to 2×. Some LDOs
have low phase margins or are not stable at unity gain. C1 was
chosen to set the low frequency zero of the noise reduction network
(C1, R1, and R3) between 10 Hz and 100 Hz to ensure that the
noise in the 1/f region is adequately reduced.
Analog Devices, Inc., data sheets specify total integrated noise
over a bandwidth of 10 Hz to 100 kHz. Figure 7 shows the
relationship between the total rms noise for the ADP223 for
various output voltages and load currents over a bandwidth
of 10 Hz to 100 kHz.
C1
10nF
DC
SOURCE
R1
400kΩ
R2
100kΩ
2.5VDC PLUS
~3.2µV rms
R3
100kΩ
Typically, the intrinsic noise of an LDO is specified in the data
sheet in two ways:
•
Total integrated noise over some bandwidth expressed
as μV rms (see Figure 7)
As a noise spectral density curve where the noise is plotted
as μV/√Hz vs. frequency (see Figure 6)
•
Typically, the rms noise is lower at light loads because the
bandwidth of the LDO is reduced along with the quiescent
current. When the load current reaches a few milliamps, the
LDO is operating at full bandwidth, and the noise remains
constant with load.
70
1.5µV rms
60
ERROR
AMPLIFIER
NOISE (µV rms)
50
Figure 5. Reducing Noise Gain in an Adjustable LDO
40
30
20
Figure 6 shows the effect of the noise reduction (NR) network on
the noise spectral density of a high voltage adjustable LDO. It can
be seen in Figure 6 that between 20 Hz and 2 kHz there is an
improvement of about a factor of three (~10 dB) in the noise
performance. Notice that the two curves converge above 20 kHz,
3.3V
2.8V
1.8V
1.2V
10
0
0.001
0.01
0.1
1
10
LOAD CURRENT (mA)
100
1k
09924-007
+
V
= 500mV
– REF
09924-005
1.0µV rms
Figure 7. RMS Noise vs. Load Current and Output Voltage Plot (ADP223)
Rev. 0 | Page 5 of 12
AN-1120
Application Note
The noise spectral density plot of the ADP223 in Figure 8 shows
how the noise spectral density varies with the output voltage over
the frequency range of 10 Hz and 100 kHz. Integrating the data
in this graph over the same bandwidth yields the rms noise. Use the
following formula to estimate the rms noise for an arbitrary
frequency range:
Vn = √BW × √(NFL2 × NFU2)
(5)
where:
BW = NFU − NFL.
NFL is the noise in μV/√Hz at the lower frequency limit
NFU is the noise in μV/√Hz at the upper frequency limit
For example, the rms noise between 10 Hz and 100 Hz of the
1.2 V output in Figure 8 is approximately
(6)
Vn = 8.9 μV rms
The noise spectral density measurements are taken at a load
current that is high enough to ensure that the LDO is operating
at full bandwidth but not so high as to induce significant selfheating. For most LDOs with a maximum output current of 1 A
or less, 10 mA is sufficient.
1
0.1
10k
where VEIN and VEOUT are the extraneous signals appearing at
the input and output, respectively.
For circuits such as ADCs, DACs, and amplifiers, this PSRR
applies to the inputs that supply power to the inner working of
the circuit in question. In the case of an LDO, the input power
pin supplies power to the regulated output voltage as well as to
the internal circuitry.
PSRR AS A FUNCTION OF FREQUENCY
100k
The relationship between the error amplifier gain bandwidth
and PSRR is shown in Figure 9. This example is a highly
simplified case where the output capacitor and pass element
parasitics are ignored.
The PSRR is equal to the reciprocal of the open-loop gain of
60 dB until the gain starts to roll off at 3 kHz. The PSRR decreases
at a rate of 20 dB/decade until, at 3 MHz, the PSRR reaches
0 dB, where it stays for all higher frequencies.
Figure 8. Noise Spectral Density vs. Output Voltage Plot (ADP223)
Comparing LDO Noise Specifications
Because the rms noise is expressed as a single number, it is a
useful metric for comparing the performance of different LDOs.
However, it is imperative that the noise specifications of the
LDOs being compared are made under the same test conditions.
100dB
0dB
80dB
20dB
For example, the 1.2 V output ADP223 rms noise is about
27.7 μV rms from 10 Hz to 100 kHz. If the noise bandwidth is
reduced to 100 Hz to 100 kHz, the rms noise decreases to about
26.2 μV rms. The rms noise falls because the 8.9 μV rms of
noise in the 10 Hz to 100 Hz range is no longer included in the
noise measurement.
Vn = √(27.72 – 8.92) = 26.2 μV rms
(8)
(7)
It is also important to pay attention to any noise reduction features
of the LDOs under consideration. LDOs that require external
capacitors for noise reduction can be as much as 100× as noisy
without the capacitor. In applications where small footprint and
Rev. 0 | Page 6 of 12
OPEN-LOOP GAIN
60dB
40dB
40dB
60dB
PSRR
20dB
0.1
80dB
1
10
100
1k
10k
100k
1M
FREQUENCY (Hz)
Figure 9. Simplified LDO Gain vs. PSRR
10M
09924-009
1k
FREQUENCY (Hz)
The PSRR of an LDO is often confused with its intrinsic noise.
Simply put, PSRR is a measure of how well a circuit suppresses
or rejects extraneous signals (noise and ripple) appearing at the
power supply input and keeps these unwanted signals from
corrupting the output of the circuit. The PSRR of a circuit is
defined as
PSRR is not defined by a single value because it is frequency
dependent. Figure 1 shows that an LDO consists of a reference
voltage, error amplifier, and a power pass element, such as a
MOSFET or bipolar transistor. The error amplifier provides the
dc gain to regulate the output voltage. The ac gain characteristic
of the error amplifier in large part determines the PSRR of the
LDO. A typical LDO can have as much as 80 dB of PSRR at
10 Hz; however, the PSRR can fall to as little as 20 dB at a few
tens of kilohertz.
1.2V ADJ
1.8V ADJ
2.8V ADJ
3.3V ADJ
09924-008
NOISE SPECTRAL DENSITY (µV/ Hz)
10
100
LDO PSRR
PSRR = 20log(VEIN/VEOUT)
Vn = √(100 − 10) × √(1.182 × 0.82)
0.01
10
cost are a factor, an LDO that does not require an external noise
reduction capacitor, but is slightly noisier than one that does
require a capacitor, may be selected due to PCB area and cost
savings.
Application Note
AN-1120
The PSRR plot in Figure 10 shows three main frequency domains
that characterize the PSRR of an LDO: the reference PSRR
region, the open-loop gain region, and the output capacitor region.
REFERENCE
PSRR
0
OUTPUT
CAPACITOR
LDO OPEN LOOP GAIN
–10
–50
–60
–70
–80
100
1k
10k
100k
FREQUENCY (Hz)
1M
10M
09924-010
–90
Figure 10. Typical LDO PSRR vs. Frequency
The reference PSRR region is dependent on the PSRR of the
reference amplifier and the LDO open-loop gain. Ideally, the
reference amplifier is fully isolated from perturbations in the
power supply. In practice, the reference need only reject power
supply noise up to a few tens of hertz because the error amplifier
feedback ensures high PSRR at low frequencies.
Above 10 Hz or so, PSRR in the second region is dominated by
the open-loop gain of the LDO. The PSRR in this region is a
function of the error amplifier gain bandwidth up to the unity
gain frequency. At low frequencies, the ac gain of the error
amplifier is equal to the dc gain and remains constant until it
reaches the 3 dB roll-off frequency. At frequencies above the
3 dB roll-off point, the ac gain of the error amplifier decreases
with frequency, typically at a rate of 20 dB/decade.
Above the unity gain frequency of the error amplifier, the feedback of the control loop has no effect on the PSRR. PSRR is
determined by the output capacitor and any parasitics between
the input and output voltages. Output capacitor ESR and ESL, as
well as board layout, strongly affect the PSRR at these frequencies.
Careful attention to layout is essential to reduce the effect of any
high frequency resonances.
At heavy loads currents, the LDO output looks less like an ideal
current source, and the output impedance of the pass element is
relatively low, which causes the gain of the output stage to
decrease. The drop in gain of the output stage reduces the PSRR
from dc to the unity-gain frequency of the feedback loop.
Figure 11 shows how dramatically the dc gain can fall as a function
of load current. Between 100 mA and 200 mA, the dc gain of the
ADP151 decreases over 20 dB.
The output stage bandwidth increases because the output pole
increases in frequency. At high frequencies, it seems that the PSSR
should increase due to the increased bandwidth of the loop. In
practice, the high frequency PSRR may not improve because of
the decrease in overall loop gain. In general, PSRR at light loads is
better than it is at heavy loads.
0
200mA
100mA
10mA
1mA
100µA
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
10
100
1k
10k
100k
FREQUENCY (Hz)
1M
Figure 11. Typical LDO PSRR vs. Load Current (ADP151)
Rev. 0 | Page 7 of 12
10M
09924-011
–40
PSRR (dB)
–30
PSRR (dB)
As shown in the PSRR as a Function of Frequency section, the
PSRR of an LDO is dependent on the gain bandwidth of the error
amplifier feedback loop. Anything that affects the gain of this
loop affects the PSRR of the LDO. The load current can affect the
PSRR in two ways.
At light load currents, typically less than 50 mA, the output
impedance of the pass element is high. The LDO output appears
to be an ideal current source due to the negative feedback of the
control loop. Because of the pole formed by the output capacitor
and the pass element, the output impedance occurs at a relatively
low frequency and tends to increase the PSRR at low frequencies.
The high dc gain of the output stage at low currents also tends
to increase the PSRR at frequencies below the error amplifier
unity-gain point.
–20
–100
10
PSRR AS A FUNCTION OF LOAD CURRENT
AN-1120
Application Note
PSRR AS A FUNCTION OF LDO HEADROOM
The PSRR of an LDO is also a function of the input-to-output
voltage differential, or headroom. For a fixed headroom voltage,
PSRR decreases as the load current increases; this is especially
apparent at heavy load currents and small headroom voltages.
Figure 12 shows the difference in PSRR for a 2.8 V output
ADP151 at 500 mV and 1 V of headroom with a load of 200 mA.
The gain of the pass element (PMOSFET for the ADP151)
decreases as it leaves saturation and into the triode region of
operation as the load current increases. This causes the overall
loop gain of the LDO to decrease, resulting in a lowering of the
PSRR. The smaller the headroom is, the more dramatic the
reduction in gain. At some small headroom voltages, the control
loop has no gain at all, and the PSRR falls to zero.
0
–20
•
•
Operate the LDO with at least 1 V of headroom. Some
LDOs, such as the ADP151, perform well with as little as
500 mV.
Use an LDO with a maximum load current rating of at
least 1.5× greater than the expected load.
Add external filtering to the input or output of the LDO.
Cascade two or more LDOs, if there is adequate headroom.
The addition of external filtering can greatly improve the PSRR
of an LDO circuit; however, it comes at the cost of additional
circuit complexity and a decrease in headroom and efficiency.
Depending on the application, the additional filtering can be added
to the input (prefiltering) or output (postfiltering) of the LDO.
–40
–50
–60
–70
Postfiltering is often used if there is significant low frequency
noise present at the output of the LDO. Postfiltering is no
longer necessary with modern low noise LDOs, such as the
ADP151. The disadvantage of postfiltering is that it incurs an
additional load regulation error due to the resistance of the
filter inductor.
–80
100
1k
10k
100k
FREQUENCY (Hz)
1M
10M
09924-012
–90
–100
10
Figure 12. Typical LDO PSRR vs. Headroom (ADP151)
Another factor that reduces the gain of the loop is that the pass
element has a nonzero resistance, or RDSON. The RDSON includes
the MOSFET on resistance, the on-chip interconnect resistance,
and the wire bonds. The RDSON is estimated by the dropout voltage
of the LDO. For example, the ADP151 in the WLCSP has a worstcase dropout voltage of 200 mV at a 200 mA load. This means that
the RDSON is about 1.0 Ω. Figure 13 shows a simplified schematic
of the pass element and RDSON.
VARIABLE
RDS ON RESISTANCE
Adding prefiltering is preferable when high frequency noise,
such as the output voltage ripple of a switching converter, must
be suppressed. It is also desirable because it does not affect load
regulation.
Figure 14 shows an LDO circuit with both prefiltering and postfiltering; however, only one external filter is typically used.
POSTFILTER
PREFILTER
LF
VIN
CIN
VOUT
LF
LDO
CF
CD
COUT CF
RD
GND
VOUT
CD
RD
09924-014
PSRR (dB)
For a given load current, the PSRR of an LDO can be improved
in several ways:
Adding External Filtering to Increase PSRR
–30
VIN
IMPROVING PSRR
•
•
200mA, 0.5V HR
200mA, 1V HR
–10
Any voltage drop across the RDSON due to the load current,
subtracts from the headroom of the active portion of the pass
element. For example, if the pass element is a 1 Ω device, a load
current of 200 mA reduces the headroom by 200 mV. When
operating LDOs at headroom voltages of 1 V or less, this voltage
drop must be accounted for when estimating the LDO PSRR.
Figure 14. LDO with External Prefiltering and Postfiltering
The main filter components are LF and CF, which set the corner
frequency of the filter. CD and RD damp the resonance of LF and
CF. CIN and COUT are the typical input and output capacitors
used for the LDO, although CIN is not necessary.
Figure 13. Simplified LDO Showing Pass Element Resistances
09924-013
REFERENCE
NOTES
1. ERROR AMP CONTROLS VALUE OF VARIABLE
RESISTOR TO REGULATE OUTPUT VOLTAGE.
2. AT LOW HEADROOM VOLTAGE, THE VARIABLE
RESISTOR IS NEARLY 0Ω.
The following equations can be used to determine the values for
CF, LF, CD, and RD:
1/(2π ×√(LF × CF)) = Corner Frequency
Rev. 0 | Page 8 of 12
(9)
CD = 10 × CF
(10)
RD = √(LF/CF)
(11)
Application Note
AN-1120
From Equation 9, assuming CF = 1 μF and LF = 1 μH, fC = 160 kHz.
From Equation 10, CD = 10 μF, and from Equation 11, RD = 1 Ω.
Figure 15 shows the response of the example filter. The attenuation
at 1 MHz is about 33 dB, and the maximum peaking is about
0.7 dB at 81 kHz.
The inductor LF should have as low a dc resistance as possible to
minimize the reduction in headroom, and in the case of a postfilter, load regulation error. The saturation current of the inductor
must also be at least as high as the maximum expected load
current of the circuit.
40
PROBE CURSOR
The output of LDO1 is chosen so that the headroom across
LDO2 is at least 500 mV. The headroom across LDO1 should
also be at least 500 mV for best results. Figure 17 compares the
PSRR of a single 1.8 V ADP151 and two cascaded ADP151s.
The load current and headroom are 200 mA and 1 V, respectively,
in both cases. It can be clearly seen in Figure 17 that cascading
two LDOs can improve PSRR by as much as 30 dB over a wide
frequency range.
0
SINGLE LDO/200mA
CASCADED LDOs/200mA
–20
–40
–60
PSRR (dB)
For example, assume that 1 MHz ripple from a switching converter
must be reduced by at least 30 dB. A corner frequency between
100 kHz and 200 kHz should be adequate.
A1 = 91.284k, 671.618m
A2 = 10.000k, 29.032m
DIF = 81.284k, 642.586m
–80
–100
–120
–140
–160
0
–200
10
–40
100
1k
10k
100k
FREQUENCY (Hz)
1M
10M
09924-017
dB (V(2))
–180
Figure 17. PSRR of a Single LDO and Cascaded LDOs
–80
10k
30k
100k
300k
1M
FREQUENCY (Hz)
3M
10M
09924-015
Comparing LDO PSRR Specifications
Figure 15. Response of the Example Ripple Filter
CASCADING LDOs TO INCREASE PSRR
In applications with adequate headroom, cascading LDOs, such
as the ADP151, can greatly improve PSRR while preserving the
low output noise characteristics of the ADP151. Figure 16
shows the schematic for two cascaded LDOs. The bypass
capacitors, CIN, COUT, and CO, are equal to the value
recommended in the ADP151 data sheet, in this case 1 μF.
CIN
LDO1
LDO2
CO
Figure 16. Cascaded LDOs
VOUT
COUT
09924-016
VIN
When comparing LDO PSRR specifications, ensure that the
measurements are made under the same test conditions. Many
older LDOs specify PSRR at only 120 Hz or 1 kHz with no mention
of headroom voltage or load current. At the least, PSRR in the
electrical specification table should be listed for different
frequencies. Ideally, typical characteristic plots of PSRR under
different load and headroom voltages should be used to make
meaningful comparisons.
The output capacitor also affects the LDO PSRR at high frequency.
For example, a 1 μF capacitor has 10 times the impedance of a
10 μF capacitor. The capacitor value is especially important at
frequencies above the error amplifier 0 dB crossover frequency,
where the attenuation of power supply noise is a function of the
output capacitance. When comparing PSRR figures, the output
capacitor must be the same type and value for the comparison
to be valid.
Rev. 0 | Page 9 of 12
AN-1120
Application Note
TOTAL LDO NOISE
SUMMARY
The intrinsic noise and PSRR both contribute to the total output
noise of an LDO. Depending on the application, the contribution of
intrinsic noise, PSRR, one or the other, or both may be important.
When both PSRR and internally generated noise contribute to
the overall performance of an application, a single figure for
noise cannot be applied.
In general, LDO noise comprises two components: intrinsic, or
internally generated noise, and extrinsic, or externally generated
noise.
A typical application is a switching converter powering an RF
PLL. To surpress the ripple from the switching converter, the
output is regulated with an LDO. The intrinsic noise of the
LDO slightly modulates the supply to the PLL and causes phase
noise in the output of the PLL. Phase noise in the PLL is caused
by the shift in the VCO frequency as a function of the supply
voltage. This is expressed as Δf/ΔV and is often referred to as
the pushing gain of the VCO.
Extrinsic noise has many sources, but the most common is the
noise on the input power supply to the LDO.
The PSRR of the LDO reduces the switching converter noise up
to the unity-gain frequency of the LDO. Beyond the unity-gain
frequency of the LDO, the switching converter noise is attenuated
by the LDO output capacitor, or by any passive filtering following
the LDO. Harmonics of the switching converter frequency that
are not adequately attenuated show up as spurs on either side of
the PLL frequency.
Thermal and 1/f noise are the main contributors to internally
generated noise and are a function of the design and semiconductor technology of the LDO.
Because an LDO has high gain to ensure good line and load
regulation, it is able to attenuate the noise and ripple from the
input power supply. This is known as the PSRR of the LDO. The
PSRR of the LDO decreases as a function of frequency because the
LDO has a finite bandwidth. Noise beyond the bandwidth of the
LDO is not attenuated and may be reduced with passive filter
components.
Rev. 0 | Page 10 of 12
Application Note
AN-1120
NOTES
Rev. 0 | Page 11 of 12
AN-1120
Application Note
NOTES
©2011 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN09924-0-6/11(0)
Rev. 0 | Page 12 of 12
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