Noty an142f Linear Technology

Application Note 142

August 2013

New Linear Regulators Solve Old Problems

Bob Dobkin, Vice President, Engineering and CTO, Linear Technology Corp.

Regulators regulate but are capable of doing much more.
The architecture of linear regulators has remained virtually
unchanged since the introduction of the three terminal
floating voltage regulator in 1976. Regulators were either

®

a floating architecture (LT

317) or else an amplifier loop
with feedback from the output to the amplifier. Both of
these architectures suffer from limitations on versatility,
regulation and accuracy.

The feedback resistors set the output voltage and
attenuate the feedback signal into the amplifier. Therefore
the regulation at the output is a percentage of the output
voltage, so higher output voltages have worse regulation
in “Volts” while the percentage may be the same. Also, the
bandwidth of the regulator changes with voltage. Since the
loop gain is decreased, the bandwidth is decreased as well
at higher output voltages. This makes transient response
slower and ripple worse as output voltage goes up.

The regulator fixes current limiting and it has no adjust­ment. It is built into the IC and different devices must be
used for different output currents. So, if the current limit
needs to be matched to the application or accurate current
limit is needed, an external circuit must be used. Figure 1a
shows the basic architecture of older regulators.

A new architecture was introduced in 2007 in the LT3080.
It used a current source for the reference and a voltage
follower for the output amplifier. Two advantages of this
architecture are the ability to parallel the regulators for
more output current and the ability for the regulator to
operate down to zero. Since the output amplifier always
operates at unity gain, bandwidth is constant and regulation
is constant as well. Transient response is independent of
output voltage and regulation can be specified in millivolts
rather than a percent of output. Figure 1b shows the new
regulator architecture.

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.

V

IN

REF

REF

+

–

Figure 1a. Older Regulators Figure 1b. New Architecture Regulator

R1

R2

AN142 F01a

OUTPUT
V

= REF

OUT

R1

1 +



R2

V

IN

I

REF

+

R1

–

OUTPUT
V

= I

• R1

OUT

REF

AN142 F01b

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Application Note 142

Table 1 shows the new regulators and main features. Along
with different output current variations, these regulators
were specifically designed to add functional features not
previously available in existing 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
for higher current, current sharing, and heat spreading.

Temperature and current monitor outputs are current
sources configured to operate from 0.4V above V
below V
the current monitor is I

. Temperature output is 1µA/°C per degree and

OUT

/5,000. These current sources

OUT

are measured by tying a resistor to ground in series with
the current source and reading across the resistor. The
current source has a range of –40V to 0.4V referred to
the output and it continues 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 or
set to zero, temperature and current can still be measured.

A New Industrial Regulator

The LT3081 is a wide safe operating area industrial regulator.
It provides 1.5A of output current, is adjustable to zero, is
reverse protected and has monitor outputs for temperature
and output current. In addition, the current limit can be
adjusted by connecting an external resistor to the device.
Figure 2 shows the basic hookup for the LT3081.

Table 1

ADJUSTABLE CURRENT

DEVICE OUTPUT CURRENT I

LT3080 1.1A 10µA No/No No Yes

LT3081 1.5A 50µA Yes/Yes Yes No Output CAP Optional

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

SET

LIMIT/CURRENT MONITOR

Using a 1k resistor provides sufficient margin and ensures
operation when the output is shorted.

The output is set with a resistor from set pin to ground
and a 50µA 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

TEMPERATURE

MONITOR LDO

Needs No Output CAP

OUT

to 40V

AN142-2

R

TEMP

V

IN

I

SET

50µA

IN

LT3081

+

TEMP

1k

SET

R

30.1k

SET

I

–

MON

R
1k

IMON

OUT

I

LIM

R

ILIM

6.04k

Figure 2. Basic Regulator Using the LT3081

C

OUT*

10µF

*OPTIONAL

R

LOAD*

5mA
MIN

AN142 F02

I

OUT

1.5V

1.5A

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Application Note 142

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 5mA 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. 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 1 to 4 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 so obvious. A true reference current
source allows the regulator to have gain and frequency

3.0

2.5

2.0

1.5

1.0

CURRENT LIMIT (A)

0.5

0

0
INPUT-TO-OUTPUT DIFFERENTIAL VOLTAGE (V)

Figure 3. Comparative Safe Operating Area Performance

LT1963A

LT1086

10 20

515

INCREASED
SAFE AREA

LT3081

30

25

35

AN142 F03

40

response independent of the impedance on the positive
input. With all previous adjustable regulators, such as the
LT1086, loop gain and bandwidth change with output volt­age changes. 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 up
the reference to a higher output voltage.

Industrial applications require large safe operating area.
Safe operating area is the ability to carry large currents at
high input-output differentials. The safe operating area for
several regulators is compared in Figure 3. 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 100mA of output current
is available. This causes output voltage to go unregulated
if the load current is above 100mA 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, the output current
of 500mA is still usable. 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 4 shows a block diagram of the LT3081. There are
three current sources — two that report output current and

I

MON

CURRENT
MONITOR
I

= I

MON

LOAD

/5000

IN

50µA

+

TEMPERATURE
DEPENDENT
CURRENT SOURCE
1µA/°C

SETTEMP

–

PROGRAMMABLE

CURRENT LIMIT

Figure 4. Block Diagram of the LT3081

I

LIM OUT

AN142 F04

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AN142-3