DN220 - Lowest Noise SOT-23 LDOs Have 20µA Quiescent Current, 20µVrms Noise

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Lowest Noise SOT-23 LDOs Have 20µA Quiescent Current,
20µVRMS Noise – Design Note 220
Todd Owen and Jim Williams
Applying the Regulators
Applying the regulators is simple. Figure 1 shows a
minimum parts count, 3.3V output design. This circuit
appears similar to conventional approaches with a
1μF
OUT
0.01μF
LT1761-3.3
SHDN
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of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
500
450
3.3V/100mA
20μVRMS
NOISE (TYP)
VIN > 3.7V
IN
notable exception: a bypass pin (BYP) is returned to
the output via a 0.01μF capacitor. This path filters the
internal reference’s output, minimizing regulator output
noise. It is the key to the 20μVRMS noise performance.
A shutdown pin (SHDN), when pulled low, turns off the
regulator output while keeping current drain inside 1μA.
Dropout characteristics appear in Figure 2. Dropout
scales with output current, falling to less than 100mV
at low currents.
DROPOUT VOLTAGE (mV)
Telecom and instrumentation applications often require
a low noise voltage regulator. Frequently this requirement coincides with the need for low regulator dropout
and small quiescent current. LTC recently introduced
a family of devices to address this problem. Table 1
shows a variety of packages, power ranges and features
in three basic regulator types. The SOT-23 packaged
LT®1761 has only 20μVRMS noise with 300mV dropout
at 100mA. Quiescent current is only 20μA.
+
BYP
10μF
DN220 F01
GND
400
350
300
IL = 100mA
250
IL = 50mA
200
150
IL = 10mA
100
IL = 1mA
50
Figure 1. Applying the Low Noise, Low Dropout,
Micropower Regulator. Bypass Pin and Associated
Capacitor are Key to Low Noise Performance
0
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
DN220 F02
Figure 2. Figure 1’s Dropout Voltage at Various Currents
Table 1. Low Noise LDO Family Short-Form Specifications. Quiescent Current Scales
with Output Current Capability, Although Noise Performance Remains Constant
OUTPUT
CURRENT
RMS NOISE
(10Hz to 100kHz)
CBYP = 0.01μF
PACKAGE
OPTIONS
LT1761
100mA
20μV
SOT-23
LT1762
150mA
20μV
LT1763
500mA
20μV
REGULATOR
TYPE
12/99/220_conv
QUIESCENT
CURRENT
SHUTDOWN
CURRENT
Shutdown, Reference Bypass,
Adjustable Output. SOT-23 Package
Mandates Selecting Any Two Features
20μA
<1μA
MS8
Shutdown, Reference Bypass,
Adjustable Output
25μA
<1μA
SO-8
Shutdown, Reference Bypass,
Adjustable Output
30μA
<1μA
FEATURES
Noise Performance
Noise performance is displayed in Figure 3. This measurement was taken in a 10Hz to 100kHz bandwidth
with a “brick wall” multipole filter1. The photo’s trace,
applied to a thermally responding RMS voltmeter, contains less than 20μVRMS noise. Figure 4 shows noise
in the frequency domain with noise power falling with
increasing frequency.
Other Advantages
The LT1761 family is stable (no output oscillation)
even when used with low ESR ceramic output capacitors. This is in stark contrast to LDO regulators from
other manufacturers that often oscillate with ceramic
capacitors.
The unique internal architecture provides an added
bonus in transient performance when adding a 0.01μF
noise capacitor. Transient response for a 10mA to 100mA
step with a 10μF output capacitor is shown in Figure 5.
Figure 6 shows the same setup with the addition of a
0.01μF bypass capacitor. Settling time and amplitude
are markedly reduced.
Conclusion
These devices provide the lowest available output noise
in a low dropout regulator without compromising other
parameters. Their performance, ease of use and versatility allow use in a variety of noise-sensitive applications.
1Noise measurement and specification of regulators requires care and will be
comprehensively treated in a forthcoming LTC Application Note.
OUTPUT VOLTAGE NOISE (μV/√Hz)
10
ENOISE
100μV/DIV
1ms/DIV
DN220 F03
LT1761-5
LT1761-3.3
1
LT1761
0.1
0.01
10
Figure 3. LT1761 Output Voltage Noise in a
10Hz to 100kHz Bandwidth. 20μVRMS Noise
is the Lowest Available in an LDO
100
1k
10k
FREQUENCY (Hz)
100k
DN220 F04
Figure 4. Output Noise Spectral Density for
Figure 1’s Circuit. Curves for Three Output
Versions Show Dispersion Below 200Hz
VOUT
20mV/DIV
VOUT
100mV/DIV
200μs/DIV
DN220 F05
Figure 5. Transient Response with
No Noise Bypass Capacitor
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200μs/DIV
DN220 F06
Figure 6. Noise Bypass Capacitor Improves Transient
Response. Note Change in Voltage Scale
For applications help,
call (408) 432-1900
dn220f_conv LT/TP 1299 340K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1999
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