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LT6118IMS8#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
LT6118
13
6118f
For more information www.linear.com/LT6118
applicaTions inForMaTion
can be multiplied by the θ
JA
value, 163°C/W for the MS8
package or 64°C/W for the DFN, to find the maximum
expected die temperature. Proper heat sinking and thermal
relief should be used to ensure that the die temperature
does not exceed the maximum rating.
LE Pin
The LE pin is used to enable the comparator output latch.
When the LE pin is high, the output latch is enabled and the
comparator output will stay low once it is tripped. When
LE is low, the comparator output latch is disabled and
the comparator operates transparently. To continuously
operate the comparator transparently, the LE pin should
be grounded. Do not leave the LE pin floating.
Power-On Reset
During startup the state of the comparator output can
-
not be guaranteed. To guarantee the correct state of the
comparator output on startup, a power-on reset (POR) is
required. A POR can be implemented by holding the LE pin
low until the LT6118 is in such a state that the compara
-
tor output is stable. This can be achieved by using an RC
network between the LE, V
+
and GND as shown in Figure 6.
When power is applied to the LT6118, the RC network
causes the voltage on the LE pin to remain below the V
IL
(0.5V) threshold long enough for the comparator output
to settle into the correct state. The LE pin should remain
below 0.5V for at least 100µs after power up in order to
guarantee a valid comparator output. The RC value can
be determined with the following equation:
RC =
t
ln
V
+
V
+
– 0.5V
; t ≥100µs
Figure 5. Gain Error vs Resistor Tolerance
RESISTOR TOLERANCE (%)
0.01
0.01
RESULTING GAIN ERROR (%)
0.1
1
10
0.1 1 10
6118 F05
R
IN
= 100Ω
R
IN
= 1k
of the maximum gain error which can be expected versus
the external resistor tolerance.
Output Current Limitations Due to Power Dissipation
The LT6118 can deliver a continuous current of 1mA to the
OUTA pin. This current flows through R
IN
and enters the
current sense amplifier via the SENSEHI pin. The power
dissipated in the LT6118 due to the output signal is:
P
OUT
= (V
SENSEHI
– V
OUTA
) • I
OUTA
Since V
SENSEHI
≈ V
+
, P
OUTA
≈ (V
+
– V
OUTA
) • I
OUTA
There is also power dissipated due to the quiescent power
supply current:
P
S
= I
S
• V
+
The comparator output current flows into the comparator
output pin and out of the V
–
pin. The power dissipated in
the LT6118 due to the comparator is often insignificant
and can be calculated as follows:
P
OUTC
= (V
OUTC
– V
–
) • I
OUTC
The total power dissipated is the sum of these
dissipations:
P
TOTAL
= P
OUTA
+ P
OUTC
+ P
S
At maximum supply and maximum output currents, the
total power dissipation can exceed 150mW. This will cause
significant heating of the LT6118 die. In order to prevent
damage to the LT6118, the maximum expected dissipa
-
tion in each application should be calculated. This number
Figure 6. RC Network Achieves Power-On Reset
6118 F06
V
LE
LE
V
+
LT6118
R
110k
60V
C
0.1µF
LT6118
14
6118f
For more information www.linear.com/LT6118
SENSEHI
LT6118
I
SENSE
R
SENSE
V
+
7
6
V
–
4
V
+
R
IN
V
BATT
SENSELO
8
1
OUTA
6118 F08
R
OUT
V
OUT
–
+
applicaTions inForMaTion
Output Filtering
The AC output voltage, V
OUT
, is simply I
OUTA
• Z
OUT
. This
makes filtering straightforward. Any circuit may be used
which generates the required Z
OUT
to get the desired filter
response. For example, a capacitor in parallel with R
OUT
will give a lowpass response. This will reduce noise at the
output, and may also be useful as a charge reservoir to
keep the output steady while driving a switching circuit
such as a MUX or ADC. This output capacitor in parallel
with R
OUT
will create an output pole at:
f
–3dB
=
1
2• π •R
OUT
•C
L
SENSELO, SENSEHI Range
The difference between V
BATT
(see Figure 8) and V
+
, as
well as the maximum value of V
SENSE
, must be considered
to ensure that the SENSELO pin doesn’t exceed the range
listed in the Electrical Characteristics table. The SENSELO
and SENSEHI pins of the LT6118 can function from 0.2V
above the positive supply to 33V below it. These operat
-
ing voltages are limited by internal diode clamps shown
in Figures 1 and 2. On supplies less than 35.5V
, the lower
range is limited by V
–
+ 2.5V. This allows the monitored
supply, V
BATT
, to be separate from the LT6118 positive
Figure 8. V
+
Powered Separately from Load Supply (V
BATT
)
Figure 9. Allowable SENSELO, SENSEHI Voltage Range
60
27
ALLOWABLE OPERATING VOLTAGE ON
SENSELO AND SENSHI INPUTS (V)
2.8
10
20
20.2V
40.2V
30
40
50
2.5
2.7 10 20 30 40 5035.5
V
+
(V)
60
6118 F09
VALID SENSELO/
SENSEHI RANGE
Figure 7. Minimum Resistance for Three Typical Capacitor Values
SUPPLY VOLTAGE (V)
1 10 100
RESISTOR VALUE (Ω)
6118 F07
C = 100nF
C = 10nF
C = 1nF
100,000,000
10,000,000
1,000,000
100,000
10,000
1000
supply as shown in Figure 8. Figure 9 shows the range of
operating voltages for the SENSELO and SENSEHI inputs,
for different supply voltage inputs (V
+
). The SENSELO and
SENSEHI range has been designed to allow the LT6118 to
monitor its own supply current (in addition to the load),
as long as V
SENSE
is less than 200mV. This is shown in
Figure 10.
Minimum Output Voltage
The output of the LT6118 current sense amplifier can
produce a non-zero output voltage when the sense voltage
is zero. This is a result of the sense amplifier V
OS
being
forced across R
IN
as discussed in the Output Voltage Er-
ror, ∆V
OUT(VOS)
section. Figure 11 shows the effect of the
input offset voltage on the transfer function for parts at
The RC will need to be chosen based on the supply voltage
of the circuit. Figure 7 can be used to easily determine an
appropriate RC combination for an applications supply
voltage range.
LT6118
15
6118f
For more information www.linear.com/LT6118
Figure 10. Supply Current Monitored with Load
Figure 11. Amplifier Output Voltage vs Input Sense Voltage
SENSEHI
LT6118
I
SENSE
R
SENSE
V
+
7
6
V
–
4
R
IN
V
BATT
SENSELO
8
1
OUTA
6118 F10
R
OUT
V
OUT
–
+
INPUT SENSE VOLTAGE (µV)
0
OUTPUT VOLTAGE (mV)
40
80
120
20
60
100
200 400 600 800
6118 F11
1000
1000 300 500 700 900
V
OS
= –200µV
G = 100
V
OS
= 200µV
applicaTions inForMaTion
the Typical Performance Characteristics are labeled with
respect to the initial sense voltage.
The speed is also affected by the external components.
Using a larger R
OUT
will decrease the response time, since
V
OUT
= I
OUTA
• Z
OUT
where Z
OUT
is the parallel combination
of R
OUT
and any parasitic and/or load capacitance. Note
that reducing R
IN
or increasing R
OUT
will both have the
effect of increasing the voltage gain of the circuit. If the
output capacitance is limiting the speed of the system, R
IN
and R
OUT
can be decreased together in order to maintain
the desired gain and provide more current to charge the
output capacitance.
The response time of the comparator is the sum of the
propagation delay and the fall time. The propagation delay
is a function of the overdrive voltage on the input of the
comparator. A larger overdrive will result in a lower propaga
-
tion delay. This helps achieve a fast system response time
to fault events. The fall time is affected by the load on the
output of the comparator as well as the pull-up voltage.
The L
T6118 amplifier has a typical response time of 500ns
and the comparators have a typical response time of 500ns.
When configured as a system, the amplifier output drives
the comparator input causing a total system response
time which is typically greater than that implied by the
individually specified response times. This is due to the
overdrive on the comparator input being determined by
the speed of the amplifier output.
Internal Reference and Comparator
The integrated precision reference and comparator com
-
bined with the high precision current sense allow for rapid
and easy detection of abnormal load currents. This is often
critical in systems that require high levels of safety and
reliability. The L
T6118 comparator is optimized for fault
detection and is designed with a latching output. The latch
-
ing output prevents faults from clearing themselves and
requires a separate system or user to reset the output. In
applications where the comparator output can inter
vene
and disconnect loads from the supply
, a latched output
is required to avoid oscillation. The latching output is
also useful for detecting problems that are intermittent.
In applications where a latching output is not desired the
LE pin can be tied low to disable the latch.
the V
OS
limits. With a negative offset voltage, zero input
sense voltage produces an output voltage. With a positive
offset voltage, the output voltage is zero until the input
sense voltage exceeds the input offset voltage. Neglect
-
ing V
OS
, the output circuit is not limited by saturation of
pull-down circuitry and can reach 0V.
Response Time
The LT6118 amplifier is designed to exhibit fast response
to inputs for the purpose of circuit protection or current
monitoring. This response time will be affected by the
external components in two ways, delay and speed.
If the output current is very low and an input transient
occurs, there may be an increased delay before the
output voltage begins to change. The Typical Performance
Characteristics show that this delay is short and it can
be improved by increasing the minimum output current,
either by increasing R
SENSE
or decreasing R
IN
. Note that
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LT6118IMS8#TRPBF

Mfr. #:
Buy LT6118IMS8#TRPBF
Manufacturer:
Analog Devices Inc.
Description:
Current Sense Amplifiers Current Sense Amp, Comparator with Latch Enable and Reference
Lifecycle:
New from this manufacturer.
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