Aug 2002 Power Op Amp Protects Load Circuitry with Precise Current Limiting

LINEAR TECHNOLOGY
VOLUME XII NUMBER 3
AUGUST 2002
IN THIS ISSUE…
COVER ARTICLE
Power Op Amp Protects Load Circuitry
with Precise Current Limiting ....... 1
Tim Regan
Power Op Amp Protects
Load Circuitry with
Precise Current Limiting
Issue Highlights ............................ 2
LTC® in the News ........................... 2
DESIGN FEATURES
60V/3A Step-Down DC/DC Converter
Maintains High Efficiency over a Wide
Range of Input Voltages ................ 7
Mark W. Marosek
New Power for Ethernet—The
LTC4255 Delivers (Part 1 of a 3-Part
Series) ........................................... 9
Dave Dwelley
High Speed Low Noise Op Amp Family
Challenges Power and Distortion
Assumptions with Rail-to-Rail Inputs
and Output .................................. 12
John Wright and Glen Brisbois
Simplify Telecom Power Supply
Monitoring with the LTC1921
Integrated Dual –48V Supply and
Fuse Monitor ............................... 16
Brendan Whelan
DESIGN IDEAS
.............................................. 20–36
(complete list on page 20)
New Device Cameos ..................... 37
Design Tools ................................ 39
Sales Offices ............................... 40
by Tim Regan
Introduction
Snap, crackle and pop are the last
sounds you ever want to hear when
working with high power circuits, but
such disturbing noises can be prevented by the new LT1970 op amp
with variable current limiting.
Electronics designers do not often
celebrate at the sound of components
being overdriven to their demise. The
resulting lingering scent of melted
plastic and burnt metal can result in
wasteful hours of discussion with
curious co-workers who are interested in duplicating the explosive
circuit. This cost in man-hours, added
to the cost of the deceased components, can be staggering.
An important rule in working with
high power circuitry is that any device that provides a significant amount
of output power must provide some
measure of protection of the circuitry
it drives. Most power amplifiers only
limit output current to the maximum
current the amp can supply. This
simple measure primarily protects the
amplifier itself without much regard
to the downstream load circuitry.
Some power amplifiers provide slightly
more protection with a programmable
fixed current limit, where the maximum output current is fixed at a
(hopefully) safer level using an external resistor. The LT1970 500mA power
op amp takes load protection to the
next logical step by providing an on-
the-fly adjustable and precise output
current limit that can continuously
adapt to and protect load circuitry.
The current limit, both sourcing and
sinking, is adjusted through two 0V–
5V voltage inputs, making it easy to
create current limit control.
One obvious application of the
LT1970 is in Automatic Test Equipment (ATE). In ATE, power amplifiers
are used as pin drivers. These test
pins force conditions at numerous
points on a tested circuit board to
Snap, crackle and pop are
the last sounds you ever
want to hear when working
with high power circuits,
but such disturbing noises
can be prevented by the new
LT1970 op amp…
determine both continuity and functionality. As each test point presents
a unique load to the driver, the ability
to tailor the voltage and maximum
output current prevents damage to
the board being tested. Without this
flexibility, the tester itself could destroy the very unit it is testing should
any test node present an unexpected
load condition to the driver. ATE is
only one obvious example. Myriad
interesting applications are made
continued on page 3
, LTC, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered trademarks of Linear Technology
Corporation. Adaptive Power, C-Load, DirectSense, FilterCAD, Hot Swap, LinearView, Micropower SwitcherCAD,
Multimode Dimming, No Latency ∆Σ, No Latency Delta-Sigma, No RSENSE, Operational Filter, PowerSOT, SoftSpan,
SwitcherCAD, ThinSOT and UltraFast are trademarks of Linear Technology Corporation. Other product names may be
trademarks of the companies that manufacture the products.
DESIGN FEATURES
LT1970, continued from page 1
possible by the full and immediate
control of a power amplifier output
voltage and current.
A Look Inside the LT1970
The LT1970 is as easy to use as any
basic op amp. It is a unity gain stable
voltage feedback amplifier with good
performance characteristics. The input offset voltage is less than 1mV,
bias current is 160nA, gain bandwidth product is 3.6MHz and it slews
at 1.6V/µs. It can operate with a total
supply voltage of 36V over a
–40°C to 125°C temperature range. It
is also a power amplifier with a maximum output current limit of 800mA,
both sourcing and sinking, built in
thermal shutdown protection and
comes in a small 20-pin TSSOP power
package. The underside of the package has an exposed metal pad to
facilitate heat sinking. These are only
the basic amplifier characteristics;
there are other built-in features that
set the LT1970 apart.
Figure 1 is a block diagram of the
LT1970. A standard amplifier topology is composed of a differential-input
transconductance stage, gm1, driving
a unity gain high current output stage.
The inputs can handle 36 volts differentially without conducting any
current. This is an important feature
when the current limit amplifiers become active and take control of the
output voltage.
The current limit amplifiers, labeled ISINK and ISRC, provide the unique
output current limiting control in both
the sinking and sourcing direction.
These amplifiers connect to the high
impedance output of the input stage
and have a much higher transconductance than the gm1 stage. The
current limit amplifiers monitor the
voltage between two sense input pins,
SENSE+ and SENSE– (for simplicity
this voltage difference will be referred
to a simply VSENSE). These input pins
are typically connected across a small
external current sensing resistor, RCS.
As shown each amplifier has an independently controlled offset voltage,
VSNK and VSRC, which set the thresholds for the output current limit. When
VSENSE is less than either offset voltage, the current limit amplifiers are
disconnected from the signal path.
This functionality is indicated by diodes D1 and D2.
When VSENSE exceeds either current limit offset voltage the applicable
current limit amplifier becomes active and takes control of the signal
path from the input stage, gm1. Feedback control of the amplifier is now
through the current limit path and
the output current is regulated to a
value of VSENSE /RCS with VSENSE forced
to the value of the threshold voltage,
VSNK or VSRC depending on the direction of the output current flow. Voltage
control of these thresholds is the key
to on-the-fly current limit adjustments.
Two current limit control inputs,
VCSNK and VCSRC set the current limit
thresholds. These pins take a 0V to
5V input to independently control the
maximum sinking or sourcing current. The sinking current limit
threshold, VSNK, is equal to one tenth
the voltage applied to the VCSNK pin
(likewise for the sourcing current
limit). This sets the maximum output
current in either direction to a voltage-controlled value of:
IOUT (MAX) =
VCSNK or VCSRC
10 • RCS
If RCS is selected to be a 1Ω resistor,
a 0V to 5V control voltage adjusts the
current limit over the range of 4mA to
500mA. The accuracy of the current
limit at 500mA is guaranteed to be 2%
maximum or within 10mA. The lower
limit of 4mA, instead of 0mA, is intentional. A non-linearity with control
input voltages less than 0.1V is builtin to prevent the sourcing and sinking
limit amplifiers from ever being activated at the same time. This would
RFB
1k
VCC
7
V+
19
VIN
9
+IN
+
Q1
RG
1k
10k
–IN
OUT
1×
GM1
8
15V
–
3
Q2
ISNK
17
15V
18
15
5V
12
D1
ISINK
TSD
ENABLE
VCSNK
ENABLE
VCSNK
–
10k
ISRC
D2
13
+
–
VSNK
+
–
VSRC
SENSE+
+
16
–
10k
VCSRC
VCSRC
ISRC
FILTER
SENSE –
RFIL
1k
COMMON
4
5
6
RLOAD
1k
V–
2
+
14
RCS
1Ω
VEE
2, 10, 11, 20
–15V
Figure 1. The LT1970 is a basic power amplifier with built-in voltage control of the output current limit.
Linear Technology Magazine • August 2002
3
DESIGN FEATURES
VLIMIT
0V TO 5V
15V
15V
IOUT(LIMIT) = ±
VLIMIT
10 • RCS
VLOAD
3k
VCC
4V
2V
+
V
VCSRC
+IN
VCSNK
ISNK
VIN
0V
ISRC
TSD
OUT
LT1970
SENSE+
SENSE–
V–
–IN
VEE
COMMON
IOUT
RCS
1Ω
1/4W
–2V
R1
10k
LOAD
R2
10k
–15V
VIN
R4
10k
Application Ideas Abound
Having complete control over the voltage and current applied to a load in a
single device leads to innumerable
application possibilities. The ease of
limiting or modulating the output
current of the LT1970 solves many
circuit problems and can protect many
a load circuit. Here are a few ideas.
100Ω
R3
1.4k
VCSRC
VCSNK
ISNK
+IN
ISRC
tion. The VCC and VEE supply pins
power all of the internal circuitry
except for the high current output
stage. The output stage is powered
from the V+ and V– pins, which conduct all of the output current. Biasing
the output stage from lower supply
voltage levels can significantly reduce
the power dissipation in the output
stage in high current applications.
12V
R1
100k
R2
20k
IRFZ30
EN
VCC
V+
RSENSE
0.1Ω
5W FORCE
VOUT =
±10V at
±5A
OUT
LT1970
SENSE+
–IN
SENSE–
COMMON
100Ω
VEE
V–
100Ω
LOAD
IRF9540
100Ω
RG
10k
RF
10k
SENSE
Figure 2 shows the basic application of the LT1970 power amplifier.
This is a simple noninverting gain of
two amplifier until the current limiting is activated. Figure 3 shows the
separate current limiting control for
sourcing and sinking. With VCSRC set
to 4V, a sense resistor RCS of 1Ω and
a 10Ω load on the amplifier, the maximum output voltage is 4V due to
current limiting at 400mA. Setting
VCSNK to 2V sets the sinking current
in this example to 200mA. The three
error flags are ORed together to provide a single indication of the LT1970
reaching current or thermal limits.
Need More than 500mA?
The 500mA output stage of the LT1970
is adequate for many applications,
but there are also some higher current applications that can benefit from
the unique current limit control. Figure 4 shows how easy it is to boost the
output current to ±5A using an external complementary pair of MOSFETs.
The output current sense resistor is
VIN
5V/ 0V
DIV
VOUT
5V/ 0V
DIV
–12V
Figure 4. Boosting the output current capability to ±5A
4
20µs/DIV
Figure 3. Current limiting clamps the
output voltage of the circuit of Figure 2
at precise levels. Independent control
allows different sourcing and sinking
current limits.
Figure 2. A typical LT1970 circuit
result in an uncontrolled output. The
bandwidth from the control inputs to
the output is 2MHz, which can be
useful for AC current modulation.
The response time for the current
limit amplifiers to take control of the
output is fast, typically 4µs.
Other features include an active
high enable input, three open collector error flags and separate power
supply input lines. The enable input
turns off the LT1970 and drops the
supply current to 600µA. It also places
the output stage into a high impedance, zero output current, state. The
error flags, which can drive LEDs,
indicate that the driver is in current
limit, in either direction, or that a
load condition has caused the LT1970
to enter its thermal shutdown protec-
VCSRC = 4V
VCSNK = 2V
RCS = 1Ω
RLOAD = 10Ω
RLOAD = 5Ω
500µs/DIV
Figure 5. Snap-back current limiting provides
an added measure of safety.
Linear Technology Magazine • August 2002
DESIGN FEATURES
VOUT = 15V
OPTIONAL
APPLY LOAD
TEST PIN
DRIVE
5V
ON/OFF
CONTROL 0V
HI-Z
CODE C – CODE D
ISOURCE(MAX) = 0.5V
5V
ISINK(MAX) = 0.5V
VCC CLR VREF
DAC A
R5
3k
VCSRC
VCSNK
EN
+IN
DAC C
R6
3k
VCC
V+
ISNK
ISRC
TSD
OUT
LT1970
SENSE+
–
SENSE
FILTER
R3
3.4k
–IN
VEE
LTC1664
QUAD
10-BIT DAC
CODE A
≈ 4mA to 500mA
1024 • RS
V
+
0.1µF
10µF
LOAD FAULT
INDICATORS
R2
10.2k
DAC D
≈ –4mA to –500mA
1024 • RS
RS
1Ω
FORCE
TEST
PIN
LOAD
–
COM
SENSE
10µF
0.1µF
+
R1
3.4k
DECODER
CODE B
≈ ±15V
18V
DAC B
3-WIRE CS/LD
SERIAL SCLK
INTERFACE
DI
1024
–18V
R4
10.2k
Figure 6. An analog pin driver with DAC controlled parameters
scaled down to 0.1Ω to extend the
same 0V to 5V current limit control to
a range from 40mA to 5A. The gate
voltage drive is developed from the V+
and V– supply pins with the current
needed by the LT1970 output stage
as it drives a 100Ω load. This Class B
power stage is intended for DC and
low frequency, <1kHz, designs as
crossover distortion between sourcing and sinking current becomes
evident at higher frequencies. In very
high current designs, having externally connected gain-setting resistors
allows for Kelvin sensing at the load.
By connecting the feedback resistor
right at the load, the voltage placed on
the load is exactly what it should be.
Any voltage drop across the current
sense resistor is inside the feedback
loop and thus does not create a voltage error.
“Snap-Back” Current Limiting
Figure 4 also shows a unique way to
use the open-collector error flags to
provide extra protection to the load
circuitry. When the amplifier enters
current limit in either direction, the
appropriate error flag goes low. This
Linear Technology Magazine • August 2002
high impedance to 0V transition can
provide a large amount of hysteresis
to the current limit control inputs,
forcing a drastic reduction in output
current. Resistors R1, R2 and R3 in
this example set the current limit
control at 2V max and 200mV min.
Should the load current ever exceed
the predetermined maximum limit,
the output current snaps back to the
min level. The output current remains
at this lower level until the signal
drops to a point where the load current is less than the minimum set
value. When the signal is low enough,
the flag output goes open and the
current limit reverts to the maximum
value. This action simulates an automatically resetting fuse. Figure 5
shows the action of this hysteresis
with a maximum current limit of 2A
snapping back to 200mA when exceeded in either direction.
Digitally Controlled V and I
Figure 6 shows a way to combine a Dto-A converter such as the quad 10-bit
LTC1664 with an LT1970 to give complete control over output voltage and
current. This circuit could be applied
as an analog pin driver for ATE applications. The circuit is a difference
amplifier with a gain of three to produce ±15V output from 0V to 5V DAC
generated inputs. The two other DACs
control the maximum output current. Again, Kelvin sensing at the load
pin preserves precision voltage control across the load. The enable pin of
the LT1970 can be used to strobe new
voltage and current limit settings to
the load after each DAC update.
Power Comparator
The simple circuit shown in Figure 7
is a different type of comparator. This
comparator steers the direction of
current flow through the load, which
could be resistive, capacitive or inductive. The magnitude of the current
is controlled by the normal current
limit control input voltages and can
be DC or modulated up to 2MHz.
There is no voltage feedback so the
input voltage drives either the top or
the bottom output transistor fully on.
The output will source or sink the
load current depending on the polarity of the input voltage. On a
cautionary note, if the load cannot
5
DESIGN FEATURES
5V
IOUT
CONTROL 0V
VIN
12V
SOURCING
+V
–V
SINKING
VCSRC
VCSNK
EN
VCC
+IN
V+
ISNK
ISRC
TSD
OUT
SENSE+
–
SENSE
FILTER
D1
1N4001
RS
1Ω
±500mA
LT1970
–IN
VEE
LOAD
V–
COM
500pF
D2
1N4001
–12V
Figure 7. A power comparator steers a controlled amount of output current.
conduct the controlled current level
the output voltage will go to one supply rail or the other. Clamp diodes
from the output to the supplies are
shown together with a small frequency
compensation capacitor at the
SENSE– pin. This is for the case where
the load is highly inductive and able
to generate high voltage transients at
the moment of current reversal.
Symmetrical Voltage Clamp
Voltage clamping amplifier circuits
are often complicated designs requiring back to back diodes, Zeners or
references to limit the output swing
to a precise level. The ability to linearly vary the clamped voltage just
adds more to the challenge. A symmetrical clamp circuit (Figure 8) is
fairly simple to implement by using
12V
VCLAMP
0V–5V
VIN
RG
R3
3k
±CLAMP
VCSRC
REACHED
VCSNK
EN
VCC
+IN
V+
OUTPUT CLAMPS
ISNK
AT 2 × VCLAMP
ISRC
±80mV TO ±10V
TSD
OUT
LT1970
SENSE+
R1
SENSE–
21.5k
FILTER
RL
–IN
R2
V–
VEE
1.13k
COM
–12V
RF
the current limit sense amplifiers of
the LT1970 to monitor just the output voltage, instead of the output
current. The amplifier operates normally until the VSENSE+ voltage exceeds
the threshold controlled by the current limit control input voltages. The
internal divide by 10 from the control
input to the clamping threshold requires an external divide by 20 resistor
network between the circuit output
and the SENSE+ pin. This allows a 0V
to 5V control signal to produce an
output clamp voltage over the range
of ±80mV to ±10V. Since the threshold voltages are the same in either
direction the output clamping is symmetrical. Figure 9 illustrates this
clamping action.
Conclusion
The LT1970 is a versatile and easy to
use power op amp with a built-in
precision adjustable current limit,
which can protect load circuitry from
damage caused by excessive power
from the amplifier. This feature is
particularly useful in ATE systems
where the load is variable (and possibly faulty) at each tested node. Tight
control of the output current in these
systems is important to prevent damage to the tested unit. The LT1970’s
ability to control both output voltage
and current makes possible many
innovative applications that otherwise would be difficult or impracticle
to implement.
5V/DIV
Figure 8. Symmetrical output voltage clamping is easy to implement with the LT1970.
2ms/DIV
For more information on parts featured in this issue, see
http://www.linear.com/go/ltmag
6
Figure 9. Voltage clamping response of the
circuit of Figure 8
Linear Technology Magazine • August 2002
Similar pages