New Linear Regulators Solve Old Problems

January 2014
I N
T H I S
I S S U E
µModule® regulator
combines a switcher with
five 1.1A LDOs 12
Volume 23 Number 4
New Linear Regulators
Solve Old Problems
Bob Dobkin
fully differential op amp
drives high res ADCs 14
Silent Switcher™ regulator
meets CISPR Class 5 25
high speed ADC sampling
transients 34
The architecture of linear regulators has remained virtually
unchanged since the introduction of the 3-terminal floating
voltage regulator in 1976. Architectures have settled on either the
floating architecture (LT317) or an amplifier loop with feedback
from the output to the amplifier. These architectures inherently
limit regulator versatility, regulation and accuracy. In 2007, Linear
Technology released the LT®3080, introducing a new linear regulator
architecture that expanded versatility,
significantly improved performance,
and allowed easy parallel operation.
The LT3081 is one of this new
class of linear regulator, specifically
featuring a wide safe operating
area for industrial applications.
THE OLD WAY
In the old linear regulator architecture (Figure 1),
feedback resistors set the output voltage and attenuate
the feedback signal into the amplifier. In this scheme,
regulation accuracy at the output is a function of the
percentage of the output voltage, so absolute regulation accuracy (in volts) degrades as output voltage
increases, even though percentage accuracy is maintained. The bandwidth of the regulator also changes
with voltage—as the loop gain is decreased, the bandwidth decreases at higher output voltages, resulting
in relatively slow transient response and high ripple.
The LT3081 is one of a new breed of linear regulator, specifically featuring a wide safe
operating area for industrial applications.
Caption
w w w. li n ea r.com
(continued on page 4)
The LT3081 is an industrial regulator with a wide safe operating area (SOA). It provides
1.5A of output current, is adjustable to zero output voltage, is reverse protected
and has monitor outputs for temperature and output current. Furthermore, the
current limit can be adjusted by connecting an external resistor to the device.
(LT3081 continued from page 1)
Furthermore, in the architecture shown
in Figure 1, current limiting is fixed in
the IC, requiring alternative devices or
different output currents. An additional
external circuit is required to match
the current limit to an application or
if accurate current limit is needed.
THE NEW WAY
In 2007, Linear Technology released the
LT3080, introducing a new linear regulator architecture featuring a current source
as reference and a voltage follower for
the output amplifier (Figure 2). This new
architecture has a number of advantages,
including easy regulator paralleling for
increased output current and operation down to zero output voltage. Since
the output amplifier always operates
at unity gain, bandwidth and absolute
regulation are constant across the output voltage range. Transient response
is independent of output voltage and
regulation can be specified in millivolts
rather than as a percent of output.
Table 1 shows the family of regulators
based on this architecture and their main
features. Along with different output
current variations, these regulators are
specifically designed to add functional
features not previously available in linear
regulators. There are monitor outputs for
temperature, current and external control of current limit. One device (LT3086)
also has external control of thermal
shutdown. A new negative regulator
provides monitoring and can operate
as a floating regulator or an LDO. All of
these new regulators can be paralleled to
4 | January 2014 : LT Journal of Analog Innovation
VIN
VIN
REF
REF
IREF
+
–
+
( )
OUTPUT
VOUT = REF 1 + R1
R2
–
R1
R1
OUTPUT
VOUT = IREF • R1
R2
Figure 1. The traditional linear regulator architecture
suffers from limitations on versatility, regulation and
accuracy.
Figure 2. The new linear regulator architecture
features a precision current source reference. This
improves regulation and transient response over the
old architecture. Output voltages down to zero are
now possible. Paralleling for greater output current
and user-defined current limiting are now easy.
increase current capability, provide balanced current sharing, and spread heat.
Temperature and Current Monitor
Outputs
THE LT3081 INDUSTRIAL REGULATOR
WITH WIDE SAFE OPERATING AREA
The LT3081 is an industrial regulator with
a wide safe operating area (SOA). It provides 1.5A of output current, is adjustable
to zero output voltage, is reverse protected
and has monitor outputs for temperature
and output current. Furthermore, the current limit can be adjusted by connecting
an external resistor to the device. Figure 3
shows the basic hookup for the LT3081.
Temperature and current monitor outputs
are current sources configured to operate from 0.4V above VOUT to 40V below
VOUT. Temperature output is 1µ A /°C
per degree and the current monitor is
IOUT/5,000. These current sources are
measured by tying a resistor to ground
in series with the current source and
reading across the resistor. The current
sources must continue to work even if
the output is shorted. The dynamic range
for the monitor outputs is 400mV above
the output so, with the output shorted
VIN
Figure 3. Basic regulator using the LT3081
IN
LT3081
ISET
50µA
+
–
TEMP
RTEMP
1k
OUT
SET IMON
RSET
30.1k
RIMON
1k
ILIM
RILIM
6.04k
COUT*
10µF
*OPTIONAL
IOUT
1.5V
RLOAD* 1.5A
5mA
MIN
design features
The benefit of using an internal true current source as the reference, rather
than a bootstrapped reference, as in prior regulators, is not obvious.
A true reference current source allows the regulator to have gain and
frequency response independent of the impedance on the positive input.
or set to zero, temperature and current
can still be measured. Using a 1k resistor
provides sufficient margin and ensures
operation when the output is shorted.
Benefits of Internal Current Source as
Reference
3.0
One Resistor Sets the Output Voltage
The output is set with a resistor from
the SET pin to ground and a 50µ A precision current source set to the output.
The internal follower amplifier forces the
output voltage to be the same voltage
as the SET pin. Unique to the LT3081, an
output capacitor is optional. The regulator is stable with or without input and
output capacitors. All the internal operating current flows through the output
pin and minimum load is required to
maintain regulation. Here, a 5m A load
is required at all output voltages to
maintain the device in full regulation.
The set resistor can add to the system
temperature drift. Commercially available surface mount resistors have a
wide range of temperature coefficients.
CURRENT LIMIT (A)
2.5
LT1963A
2.0
INCREASED
SAFE AREA
1.5
1.0
LT3081
0.5
0
LT1086
10 15 20 25 30 35 40
0
5
INPUT-TO-OUTPUT DIFFERENTIAL VOLTAGE (V)
Figure 4. Comparative safe operating area
performance
Depending on the manufacturer, these
can go from 100ppm up to over 500ppm.
While the resistor is not heated by
power dissipation in the regulator, over
a wide ambient temperature range its
temperature coefficient can change the
output by one to four percent. Lower
temperature coefficient thin film resistors
are available for precision applications.
The benefit of using an internal true current source as the reference, rather than
a bootstrapped reference, as in prior
regulators, is not obvious. A true reference current source enables the regulator to have gain and frequency response
independent of the impedance on the
positive input. With all previous adjustable regulators, such as the LT1086, loop
gain and bandwidth change with variations in output voltage. If the adjustment
pin is bypassed to ground, bandwidth
also changes. For the LT3081, the loop
gain is unchanged with output voltage
or bypassing. Output regulation is not a
fixed percentage of output voltage, but is
a fixed number of millivolts. Use of a true
current source allows all of the gain in
the buffer amplifier to provide regulation,
and none of that gain is needed to amplify
the reference to a higher output voltage.
Table 1. Linear regulators featuring updated architecture
DEVICE
OUTPUT CURRENT
I SET
ADJUSTABLE CURRENT
LIMIT/CURRENT MONITOR
TEMPERATURE
MONITOR
LDO
LT3080
1.1A
10µA
No/No
No
Yes
LT3081
1.5A
50µA
Yes/Yes
Yes
No
LT3082
200mA
10µA
No/No
No
No
LT3083
3A
50µA
No/No
No
Yes
LT3085
600mA
10µA
No/No
No
Yes
LT3086
2.1A
Yes/Yes
Yes + Temp Limit
Yes
LT3090
600mA
–50µA
Yes/Yes
Yes
Yes
Negative Regulator
LT3092
200mA
10µA
No/No
No
No
Current Source Operation Needs No Output CAP
COMMENTS
Output CAP Optional
January 2014 : LT Journal of Analog Innovation | 5
The LT3081 extends the safe operating area, offering nearly 1A of
output current at 25V of differential. Even above 25V, 500mA is still
available. This allows the regulator to be used in applications where
widely varying input voltages can be applied during operation.
Wide Safe Operating Area
Industrial applications require a wide
safe operating area. Safe operating area
(SOA) is the ability to carry large currents at high input-output differentials.
The safe operating area for several
regulators is compared in Figure 4.
The LT1086, introduced in the mid-1980s,
is a 1.5A regulator in which output current drops very low above 20V input/
output differential. Above 20V only about
100m A of output current is available. This
causes output voltage to go unregulated
if the load current is above 100m A and
transients on the input cause the high
voltage current limit to be exceeded. The
LT1963A is a low dropout regulator that
also has a limited safe operating area.
The LT3081 extends the safe operating
area, offering nearly 1A of output current at 25V of differential. Even above
25V, 500m A is still available. This allows
the regulator to be used in applications
where widely varying input voltages
can be applied during operation. Wide
operating safe area is obtained by using
a large structure for the PNP pass device.
Also, the LT3081 is protected (along with
the load) for reverse input voltage.
Figure 5 shows a block diagram of
the LT3081. There are three current
sources: two that report output current and temperature and a third that
supplies the 50µ A reference current.
The LT3081, while not a low dropout
regulator, operates down to 1.2V across
the device — slightly better than
older devices such as the LT1086.
6 | January 2014 : LT Journal of Analog Innovation
Figure 5. Block diagram
of the LT3081
IN
50µA
IMON
+
TEMPERATURE
DEPENDENT
CURRENT SOURCE
1µA/°C
CURRENT
MONITOR
IMON = ILOAD/5000
TEMP
–
SET
PROGRAMMABLE
CURRENT LIMIT
ILIM
No Input or Output Capacitors Needed
Paralleling Devices for More Current
The internal amplifier configuration, in
conjunction with well regulated internal
bias supplies, allows the LT3081 to be
stable with no external capacitors. One
caveat: it cannot be designed to tolerate
all possible impedances in the input and
load, so it is important to test the stability
in the actual system used. If instability is
found, external capacitors will ensure that
the device is stable at all output currents.
External capacitors also improve the transient response since it is no longer limited
by the bandwidth of the internal amplifier.
Paralleling devices—typically forbidden
with prior regulators since they do
not share current—is easy with these
new current source reference regulators. Paralleling is useful for increasing
output current or spreading the heat.
Since it is set up as a voltage follower,
tying all the SET pins together makes
the outputs the same voltage. If the
outputs are at the same voltage, only
a few milliohms of ballast are needed
to allow them to share current.
Figure 6. Offset voltage distribution
N = 3195
–2
1
–1
0
VOS DISTRIBUTION (mV)
2
OUT
Figure 6 shows a distribution of the
offset voltage for the LT3081. The distribution is within 1mV so ensuring 10%
sharing requires no more than 10mΩ of
ballast resistance. The ballast resistor
can be less than an inch of a trace on a
PC board or a small piece of wire, and
provides good current balance from
parallel devices. Even at 1V output, this
degrades the regulation by only about
1.5%. Table 2 shows PC board resistance.
Figure 7 shows a schematic of two
LT3081s paralleled to obtain 3A output. The set resistor now has twice the
set current flowing through it, so the
design features
Table 2. PC board trace resistance
WEIGHT (oz)
10MIL WIDTH
20MIL WIDTH
1
54.3
27.1
2
27.1
13.6
Paralleling devices—typically not possible
with prior regulators since they do not share
current—is easy with these new current source
reference regulators. Paralleling is useful for
increasing output current or spreading the heat.
Trace resistance is measured in mΩ/in.
output is 100µ A times RSET and the
10mΩ output resistors ensure ballasting
at full current. Any number of devices
can be paralleled for higher current.
The I pins can be paralleled (if used)
so one resistor sets the current limit.
Figure 7. Paralleling devices
50µA
+
–
50µA
+
–
1µF
SET
33k
output voltages. Cleaning of all insulating surfaces to remove fluxes and other
residues is required. Surface coating
may be necessary to provide a moisture
barrier in high humidity environments.
Minimize board leakage by encircling
the SET pin and circuitry with a guard
ring tied to the OUT pin. Increasing the
set current as shown also decreases
the effects of spurious leakages.
With the 50µ A current source used to
generate the reference voltage, leakage paths to or from the SET pin can
create errors in the reference and
IN
LT3081
ISET
50µA
+
–
IMON
SET TEMP
OUT
20mΩ
ILIM
20mΩ
LT1963-3.3
10µF
10mΩ
LT3081
IN
VIN
4.8V TO 40V
LAYOUT CONSIDERATIONS
5V
OUT
SET
Figure 8 shows the LT3081 paralleled with
a fixed regulator. This is useful when a
system that has been designed has insufficient output current available. It provides
a quick fix for higher output current.
The output voltage of the fixed device is
divided down by just a few millivolts by
the divider. The SET pin of the LT3081 is
tied about 4mV below the fixed output.
This ensures no current flows from the
LT3081 under a no-load condition. Then
the 20mΩ resistors provide sufficient ballast to overcome this offset and ensure current matching at higher output currents.
Figure 8. Increasing the output
current of a fixed regulator
LT3081
IN
8.2Ω
47µF
3.3VOUT
3A
47µF
OUT
10mΩ
10µF
VOUT
3.3V
3A
The low 50µ A SET current can cause problems in some applications. High value film
potentiometers are not as stable as lower
value wire wounds. Board leakage can
also introduce instabilities in the output.
Problems can be minimized by increasing
the set current above the nominal 50µ A.
Figure 9 shows a solution using lower
value set resistors. Here an increased current is generated through R2 and summed
with the SET pin current, giving a much
larger current for adjusting the output.
SET current flows through a 4k resistor,
generating 200mV across R1. Then the
current through R2 adds to the SET current,
giving a total of 1.05m A flowing through
ISET to ground. This makes the voltage less
sensitive to leakage currents around RSET.
Care should be taken to Kelvin connect
R2 directly to the output. Voltage drops
from the output to R2 affect regulation.
6.2k
January 2014 : LT Journal of Analog Innovation | 7
The LT3081 is one of a new family of linear regulators that
yields an order-of-magnitude better regulation against
load and line changes compared to traditional devices.
Regulation and transient response, measured in millivolts,
are now maintained regardless of output voltage.
CONCLUSION
VIN
12V
4.7µF
The LT3081 is one of a new family of
linear regulators that yields an order-ofmagnitude better regulation against load
and line changes compared to traditional
devices. Regulation and transient response,
measured in millivolts, are now maintained regardless of output voltage.
IN
LT3081
ISET
50µA
+
–
IMON
Figure 9. Using a lower value set resistor
OUT
SET TEMP
1k
R1
4.02k
Another configuration uses an LT3092 as
an external current source of 1m A. This
provides increased set current and allows
the output to be adjusted down to zero.
Figure 10 shows an LT3092 current source
used to provide the current reference to
an LT3081. The 1m A generated reference
current allows the adjustment set resistor
to be much lower in value while enabling
the device to be adjusted down to zero.
R2
40.2Ω
1k
RSET
2k
VOUT
0.2V TO 10V
ILIM
4.7µF
These new regulators are far more robust
and versatile than previous generations,
offering features that were previously
unavailable. Temperature and current
monitoring and adjustable current limiting are now added. Paralleling these new
regulators no longer requires external
current balance circuitry to prevent current hogging. Line drops can be easily
compensated. Current limit thresholds are
now user-defined, as opposed to fixed in
the regulator, and outputs are adjustable
to zero. The LT3801, in particular, features
a wide safe operating area to support load
currents in the face of wide input swings. n
VOUT = 0.2V + 5mA • RSET
The current monitor output can be
used to compensate for line drops, as
shown in Figure 11. Feeding the current monitor through a portion of the
set resister generates a voltage at the
SET pin that raises the output as a function of current. The value of the comp
resistor is R2 = 5000 • RCABLE(TOTAL) and
VOUT = 50µ A • (RSET + RCOMP). Several volts
of line drop can be compensated this way.
Figure 10. Using an external
reference current
Figure 11. Using current monitor
output to compensate for line drops
VIN ≥ 7V
VIN
1µF
LT3092
IN
20k
ISET
50µA
+
–
OUT
OUT
215Ω
IMON
SET TEMP
1k
1k
20k
8 | January 2014 : LT Journal of Analog Innovation
IN
LT3081
ISET
50µA
+
–
SET
IN
LT3081
10µA
1mA
ILIM
VOUT
0V TO 20V
1µF
+
–
TEMP
1k
SET IMON
R1
98k
R2
2k
OUT
ILIM
RLINE
0.2Ω
1µF
RLINE
0.2Ω
RL
VOUT
5V
1.5A