Dec 2006 - Precise Current Sense Amplifiers Operate from 4V to 60V

L DESIGN FEATURES
Precise Current Sense Amplifiers
Operate from 4V to 60V
Introduction
The LTC6103 and LTC6104 are
versatile, precise high side current
sense amplifiers with a wide operation
range. The LTC6103 is a dual current
sense amplifier, while the LTC6104 is
a single, bi-directional current sense
amplifier—it can source or sink an
output current that is proportional to
a bi-directional sense voltage.
Due to the amplifiers’ wide supply
range (60V), fast speed (1µs response
time), low offset voltage (85µV typical),
low supply current (275µA/channel
typical) and user-configurable gains,
they can be used in precision industrial
and automotive sensing applications,
as well as current-overload protection
circuits.
Other features include high PSRR,
low input bias current and wide input
sense voltage range. Both parts are
available in an 8-lead MSOP.
VBATT_A
VBATT_B
VSENSE
ILOAD
–
VSENSE
+
+
RSENSE
LOAD
6
–INA
IOUT =
5k 5k
+INB
5k 5k
+ –
ISB
– +
VSA
VSB
10V
10V
V–
OUTA
1
OUTB
4
2
IOUT
IOUT
ROUT
VOUT = VSENSE •
ROUT
ROUT
RIN
Figure 1. The LTC6103 block diagram and typical connection
–
RIN
7
+INA
6
VS
–INA
10V
5k
5
–INB
5k
–
5k
+
–
V+
A
+INB
10V
5k
+
V–
ILOAD
+
RSENSE
RIN
8
VSENSE
V+
B
V–
(ILOAD + IS ) RSENSE
RIN
flows through RIN. The high impedance inputs of the sense amplifier do
not conduct this input current, so
the current flows through an internal
MOSFET to the OUT pin. In most application cases, IS << ILOAD, so
IOUT
5
–INB
LTC6103 Theory of Operation
Figure 1 shows a block diagram of the
LTC6103 in a basic current sense circuit. A sense resistor, RSENSE, is added
in the load path, thereby creating a
small voltage drop proportional to the
load current.
An internal sense amplifier loop
forces –IN to have the same potential
as +IN. Connecting an external resistor, RIN, between –IN and VBATT forces
a potential across RIN that is the same
as the sense voltage across RSENSE. A
corresponding current
LOAD
RIN
7
+INA
ISA
ILOAD
–
RSENSE
RIN
8
by Jun He
I
•R
≈ LOAD SENSE
RIN
10V
V–
OUT
1
4
VOUT
R
VOUT = VSENSE • OUT + VREF
RIN
ROUT
+
–
VREF
Figure 2. The LTC6104 block diagram and typical connection
Linear Technology Magazine • December 2006
DESIGN FEATURES L
The output current can be transformed into a voltage by adding a
resistor from OUT to V–. The output
voltage is then
Sources of Current
Sensing Error
As the output voltage is defined by
ILOAD • R SENSE • ROUT
RIN
VOUT = (V–) + (IOUT • ROUT)
VOUT =
LTC6104 Theory of Operation
any error of the external resistors contributes to the ultimate output error.
If current flowing through the sense
resistor is high, Kelvin connection of
the –IN and +IN inputs to the sense
resistor is necessary to avoid error
introduced by interconnection and
trace resistance on the PCB.
Besides external resistors, the
dominant error source is the offset
voltage of the sense amplifier. Since
this is a level independent error,
Figure 2 shows a block diagram of
the LTC6104 in a basic current sense
circuit.
Similar to the operation of the
LTC6103, the LTC6104 can transfer a high side current signal into a
ground-referenced readout signal.
The difference is that the LTC6104
can sense the input signal in both
polarities.
Only one amplifier is active at a
time in the LTC6104. If the current
direction activates the “B” amplifier,
the “A” amplifier is inactive. The signal current goes into the –INB pin,
through the MOSFET, and then into a
current mirror. The mirror reverses the
polarity of the signal so that current
flows into the “OUT” pin, causing the
output voltage to change polarity. The
magnitude of the output is
VOUT =
10µF
63V
VLOGIC
3
FAULT
RS
LT1910
1µF
1
–IN
V–
5
LOAD
10k
VOUT
VLOGIC
(3.3V TO 5V)
7
RSENSE(LO)
100mΩ
RSENSE(HI)
10mΩ
3
1
VS
6
+
–
LTC1540
8
Q1
CMPT5551
4.7k
6
1.74M
5
HIGH
RANGE
INDICATOR
(ILOAD > 1.2A)
619k
HIGH CURRENT
RANGE OUT
250mV/A
4
7.5k
BAT54C
VLOGIC
V–
R5
7.5k
ROUT
5
301Ω
7
2
4
VIN
40.2k
301Ω
+ –
OUT
FOR RS = 5mΩ,
VO = 2.5V AT IL = 10A (FULL SCALE)
CMPZ4697
LTC6103
LTC6103
VO = 49.9 • RS • IL
Figure 4. Automotive smart-switch with current readout
8
IS
IL
6103 TA06
ILOAD
+IN
VO
4.99k
M1
Si4465
RIN
OUT
SUB85N06-5
VBATT
RT
1/2
LTC6103
+IN
6
2
RSENSE
RT
100Ω
1% –IN
8
4
OFF ON
Keep in mind that the OUT voltage
cannot swing below V–, even though
it is sinking current. A proper VREF
and ROUT need to be chosen so that
the designed OUT voltage swing does
not go beyond the specified voltage
range of the output.
LOAD
14V
47k
VSENSE • ROUT
+ VREF
RIN
ILOAD
maximizing the input sense voltage
improves the dynamic range of the
system. If practical, the offset voltage
error can also be calibrated out.
Care should be taken when designing the printed circuit board layout.
As shown in Figure 3, supply current
flows through the +IN pin, which is
also the positive amplifier input pin
(for the LTC6104, this applies to the
+INB pin only). The supply current
can cause an equivalent additional
input offset voltage if trace resistance
between RSENSE and +IN is significant.
Trace resistance to the -IN terminals is
added to the value of RIN. In addition,
the internal device resistance adds
approximately 0.3Ω to RIN.
(VLOGIC + 5V) ≤ VIN ≤ 60V
0A ≤ ILOAD ≤ 10A
6103 F03b
LOW CURRENT
RANGE OUT
250mV/A
6103 F04
Figure 3. Error Due to PCB trace resistance
Linear Technology Magazine • December 2006
Figure 5. The LTC6103 allows high-low current ranging
L DESIGN FEATURES
ICHARGE
The LTC6103 supplies a current
output, rather than a voltage output, in
proportion to the sense resistor voltage
drop. The load resistor for the LTC6103
may be located at the far end of an
arbitrary length connection, thereby
preserving accuracy even in the presence of ground-loop voltages.
0.01Ω
CHARGER
IDISCHARGE
249Ω
8
7
+INA
ILOAD
249Ω
6
–INA
5
–INB
+INB
+ –
LOAD
VS
LTC6104
VOUT
2.5V±2V
(±10A FS)
– +
A
B
CURRENT
MIRROR
OUT
1
+
VS
High-Low Range
Current Measurement
Figure 5 shows LTC6103 used in
a multi-range configuration where
a low current circuit is added to a
high current circuit. A comparator
(LTC1540) is used to select the range,
and transistor M1 limits the voltage
across RSENSE(LO).
V–
4
2.5V 6
4.99k
LT1790-2.5
1µF
1
2
4
3V TO
18V
1µF
Figure 6. The LTC6104 bi-direction current sense circuit with combined charge/discharge output
Applications
The LTC6103 and LTC6104 operate
from 4V to 60V, with a maximum
supply voltage of 70V. This allows
them to be used in applications that
require high operating voltages, such
as motor control and telecom supply
monitoring, or where it must survive
in the face of high-voltages, such as
with automotive load dump conditions.
The accuracy is preserved across this
supply range by a high PSRR of 120dB
(typical).
Fast response time makes the
LTC6103 and LTC6104 the perfect
choice for load current warnings and
shutoff protection control. With very
low supply current, they are suitable
for power sensitive applications.
The gain of the LTC6103 and
LTC6104 is completely controlled
by external resistors, making them
flexible enough to fit a wide variety of
applications.
Monitor the Current of
Automotive Load Switches
With its 60V input rating, the LTC6103
is ideally suited for directly monitoring currents on automotive power
systems without need for additional
supply conditioning or surge protection components.
Figure 4 shows an LT1910-based
intelligent automotive high side switch
with an LTC6103 providing an analog
current indication. The LT1910 high
Battery Charge/Discharge
Current Monitor
Figure 6 shows the LTC6104 used in
monitoring the charge and discharge
current of a battery. The voltage reference LT1790 provides a 2.5V offset
so that the output can swing above
side switch controls an N-channel
MOSFET that drives a controlled load
and uses a sense resistor to provide
overload detection. The sense resistor
is shared by the LT6103 to provide the
current measurement.
continued on page 28
VBATTERY (6V–60V)
+
VSENSE(A)
–
10mΩ
10mΩ
200Ω
8
7
+INA
LTC6104
6
–INA
5
–INB
+INB
– +
A
B
CURRENT
MIRROR
OUT
VS
V–
1
4
VOUT
±2.5V
(±10A FS)
VEE
(–5V)
4.99k
M
VSENSE(B)
–
200Ω
+ –
VS
+
DC MOTOR OR
PELTIER DEVICE
P
–+
ILOAD
P
M
Figure 7. Current monitoring for an H-bridge application
Linear Technology Magazine • December 2006
L DESIGN IDEAS
V1 rises above 10.8V. The transition
from the V2 to V1 is accomplished by
slowly (10ms) turning off Q2 and Q3
allowing the Q1 to turn on rapidly
when VS matches V1. The H1 output
is open until the E1 input drops below
the VREF voltage level. The V1 VFAIL is
determined by:
R2A + R2C
R2C
158k + 24.9k
= 1.222V •
24.9kk
= 8.98 V
VFAIL = VETH •
input drops to 12V and the V2 path
is enabled. Finally, the load will be
removed from the input supply when
the voltage drops below 5V.
Undervoltage
R1A + R1C
R1C
75k + 24.3k
= 1.222V •
24.3k
= 4.99 V
VFAIL = VETH •
VRESTORE = VETH •
VRESTORE = VETH
(R2A + (R2C R2E))
•
= 1.222V •
R2C R2E
(
1558k + 24.9k 105k
= 1.222V •
)
24.9k 105k
= 10.81V
Undervoltage and
Overvoltage Shutdown
Figure 2 shows an application that
disables the power to the load when the
input voltage gets too low or too high.
When VIN starts from zero volts, the
load to the output is disabled until VIN
reaches 5.5V. The V1 path is enabled
and the load remains on the input
until the supply exceeds 13.5V. At
that voltage, the V2 path is disabled.
As the input falls, the voltage source
is reconnected to the load when the
LTC6103/LTC6104, continued from page and below this point. Make sure that
the lowest expected output level is
higher than pin 4 (V–) by at least 0.3V
to ensure that negative going output
swings remain linear.
H-Bridge Load Current Monitor
The H-bridge power-transistor topology remains popular as a means
of driving motors and other loads
bi-directionally from a single supply
potential. In most cases, monitoring
the current delivered to the load allows
for real-time operational feedback to
a control system.
28
Conclusion
(R1A + (R1C R1D))
Determine V1 VRESTORE by:
R1C R1D
(
755k + 24.3k 182k
lockout by using only one of the voltage
paths and eliminating the components
from the other. Only one PFET is required in this case. The LTC4416-1
should be used in this configuration
rather than the LTC4416 because
the LTC4416-1 turns off rapidly if
an over or undervoltage condition is
detected.
)
24.3k 182k
= 5.497 V
Overvoltage
R2A + R2C || R2E
R2C || R2E
221k + 24.9k || 187k
= 1.222V •
24.9k || 187k
VFAIL = VETH •
= 13.51V
R2A + R2C
R2C
221k + 24.9k
= 1.222V •
244.9k
= 12.07 V
VRESTORE = VETH •
The over and undervoltage lockout
circuits are shown here working in
tandem. It is possible to configure the
circuit for either over or undervoltage
Figure 7 shows the LTC6104 used
in monitoring the load current in an
H-bridge. In this case, the LTC6104
operates with dual supplies. The output resistance is connected directly
to ground instead of connected to a
voltage reference. The output ranges
from 0V to 2.5V for VSENSE_A = 0mV
to 100mV, and from 0V to –2.5V for
VSENSE_B = 0mV to 100mV.
Conclusion
The LTC6103 and LTC6104 are precise
high side current sensing solutions.
The parts can operate to 60V, making
The LTC4416 provides power supply switchover solutions that cannot
be easily generated using off-theshelf components. The LTC4416
also provides power efficiencies not
available with traditional NFET Hot
Swap controllers. These efficiencies
reduce the IDD of the solution by not
having active switching gate drivers.
The power losses are also reduced by
decreasing the voltage drop across the
PFETs to 25mV. The LTC4416 provides a smoother transition between
the backup and the secondary power
supplies.
The LTC4416-1 dual gate drivers
provide a single controller solution to
not only protect loads from overvoltage
conditions, but also undervoltage
conditions. The user can externally program the overvoltage and
undervoltage thresholds using simple
external resistor networks. These resistor networks also provide hysterisis
to prevent chattering between the
power source and the load. L
them ideal for high voltage applications
such as those found in automotive,
industrial and telecom systems. Low
DC offset allows the use of a small
shunt resistor and large gain-setting resistors. The fast response time
makes them suitable for overcurrentprotection circuits. Configurable gain
means design flexibility. In addition,
the open-drain output architecture
provides an advantage for remotesensing applications. L
Authors can be contacted
at (408) 432-1900
Linear Technology Magazine • December 2006
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