4
DEMO MANUAL DC330
LOW DROPOUT REGULATOR
OPERATIO
U
Part Selection
Two versions of the LT1761 are provided for evaluation.
Both are adjustable versions, one with the low noise
option, and the other with the low current shutdown
option. Both allow selection of a number of common
output voltages or a custom output voltage. Fixed voltage
parts operate similarly to the adjustable parts, except that
fixed voltage LT1761 regulators feature both low current
shutdown and low noise operation.
Hook-Up
Solid turret terminals are provided for easy connection to
supplies and test equipment. Connect a 0V to 20V, 0.2A
power supply across the V
IN
and GND terminals and the
load across the V
OUT
and GND terminals. The SHDN pin
can be disconnected from V
IN
by removing JP2 to allow
separate shutdown control via a secondary control line.
JP1 and JP3 can be used to select a number of common
fixed output voltages or, in conjunction with R1 or R10, to
create a custom output voltage using the formula:
R1 or R10 = (V
OUT
– 1.22V)/4.93
µ
A
Output Capacitor Selection
The output capacitor C3 is a 10
µ
F X7R ceramic chip
capacitor and C5 is a 3.3
µ
F X7R ceramic chip capacitor.
Care must be exercised in the selection of output capaci-
tors should a different output capacitor be desired. Many
ceramic capacitor dielectrics exhibit undesirable tempera-
ture and voltage characteristics that reduce their effective
capacitance to as low as 10% to 20% of nominal value. For
further information, see Linear Technology Application
Note 83, “Performance Verification of Low Noise, Low
Dropout Regulators,” Appendix B, “Capacitor Selection
Considerations”; see also the Applications Information
Section of this manual.
Output Voltage Noise
Measuring output voltage noise can be a tricky process,
further complicated by the low levels of noise inherent in
a circuit such as this. Consideration must be given to
regulator operating conditions, as well as the noise band-
width of interest. Linear Technology has invested an
enormous amount of time to provide accurate, relevant
data to customers regarding noise performance. For fur-
ther information on measuring output voltage noise, see
Linear Technology Application Note 83, “Performance
Verification of Low Noise, Low Dropout Regulators.”
APPLICATIO S I FOR ATIO
W
U
U
U
prise a 60dB gain stage with a 5Hz highpass input. A3
provides a 10Hz, 2nd order Butterworth highpass charac-
teristic. The LTC
®
1562 filter block is arranged as a 4th
order Butterworth lowpass. Its output is delivered via the
330
µ
F-100
Ω
highpass network. The circuit’s output drives
a thermally responding RMS voltmeter.
3
Note that all
circuit power is furnished by batteries, precluding ground
loops from corrupting the measurement.
Note 1:
Switching regulators are an entirely different proposition,
requiring very broadband noise measurement. See Reference 1.
Note 2:
Component choice for the regulator, more critical than might
be supposed, is discussed in Appendix B, “Capacitor Selection
Considerations.”
Note 3:
The choice of the RMS voltmeter is absolutely crucial to
obtaining meaningful measurements. See Application Note 83
Appendix C, “Understanding and Selecting RMS Voltmeters.”
Noise Testing Considerations
What noise bandwidth is of interest and why is it interest-
ing? In most systems, the range of 10Hz to 100kHz is the
information signal processing area of concern. Addition-
ally, linear regulators produce little noise energy outside
this region.
1
These considerations suggest a measure-
ment bandpass of 10Hz to 100kHz, with steep slopes at the
band limits. Figure 2 shows a conceptual filter for LDO
noise testing. The Butterworth sections are the key to
steep slopes and flatness in the passband. The small input
level requires 60dB of low noise gain to provide adequate
signal for the Butterworth filters. Figure␣ 3 details the filter
scheme. The regulator under test is at the diagram’s
center.
2
A1–A3 make up a 60dB gain highpass section. A1
and A2, extremely low noise devices (<1nV/
√
Hz), com-
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