LTC6101/LTC6101HV
13
Rev I
For more information www.analog.com
APPLICATIONS INFORMATION
If the offset current, I
OS
, of the LTC6101 amplifier is 2nA,
the 100 microvolt error above is reduced to 2 microvolts.
Adding R
IN
+
as described will maximize the dynamic
range of the circuit. For less sensitive designs, R
IN
+
is
not necessary.
Example:
If an I
SENSE
range = (1A to 1mA) and (V
OUT
/I
SENSE
) =
3V/1A
Then, from the Electrical Characteristics of the LTC6101,
R
SENSE
≈ V
SENSE
(max) / I
SENSE
(max) = 500mV/1A =
500mΩ
Gain = R
OUT
/R
IN
= V
OUT
(max) / V
SENSE
(max) =
3V/500mV = 6
If the maximum output current, I
OUT
, is limited to 1mA,
R
OUT
equals 3V/1mA ≈ 3.01 kΩ (1% value) and R
IN
=
3kΩ/6 ≈ 499Ω (1% value).
The output error due to DC offset is ±900µVolts (typ) and
the error due to offset current, I
OS
is 3k x 2nA = ±6µVolts
(typical), provided R
IN
+
= R
IN
–
.
The maximum output error can therefore reach ±906µVolts
or 0.03% (–70dB) of the output full scale. Considering
the system input 60dB dynamic range (I
SENSE
= 1mA to
1A), the 70dB performance of the LTC6101 makes this
application feasible.
Output Error, E
OUT
, Due to the Finite DC Open Loop
Gain, A
OL
, of the LTC6101 Amplifier
This error is inconsequential as the A
OL
of the LTC6101
is very large.
Output Current Limitations Due to Power Dissipation
The LTC6101 can deliver up to 1mA continuous current to
the output pin. This current flows through R
IN
and enters the
current sense amp via the IN(–) pin. The power dissipated
in the LTC6101 due to the output signal is:
P
OUT
= (V
–IN
– V
OUT
) • I
OUT
Since V
–IN
≈ V
+
, P
OUT
≈ (V
+
– V
OUT
) • I
OUT
There is also power dissipated due to the quiescent sup-
ply current:
P
Q
= I
DD
• V
+
The total power dissipated is the output dissipation plus
the quiescent dissipation:
P
TOTAL
= P
OUT
+ P
Q
At maximum supply and maximum output current, the
total power dissipation can exceed 100mW. This will
cause significant heating of the LTC6101 die. In order to
prevent damage to the LTC6101, the maximum expected
dissipation in each application should be calculated. This
number can be multiplied by the θ
JA
value listed in the
package section on page 2 to find the maximum expected
die temperature. This must not be allowed to exceed 150°C,
or performance may be degraded.
As an example, if an LTC6101 in the S5 package is to be
run at 55V ±5V supply with 1mA output current at 80°C:
P
Q(MAX)
= I
DD(MAX)
• V
+
(MAX)
= 41.4mW
P
OUT(MAX)
= I
OUT
• V
+
(MAX)
= 60mW
T
RISE
= θ
JA
• P
TOTAL(MAX)
T
MAX
= T
AMBIENT
+ T
RISE
T
MAX
must be < 150°C
P
TOTAL(MAX)
≈ 96mW and the max die temp
will be 104°C
If this same circuit must run at 125°C, the max die
temp will increase to 150°C. (Note that supply current,
and therefore P
Q
, is proportional to temperature. Refer
to Typical Performance Characteristics section.) In this
condition, the maximum output current should be reduced
to avoid device damage. Note that the MSOP package
has a larger θ
JA
than the S5, so additional care must be
taken when operating the LTC6101A/LTC6101HVA at high
temperatures and high output currents.
The LTC6101HV can be used at voltages up to 105V. This
additional voltage requires that more power be dissipated
for a given level of current. This will further limit the allowed
output current at high ambient temperatures.
It is important to note that the LTC6101 has been designed
to provide at least 1mA to the output when required, and
can deliver more depending on the conditions. Care must
be taken to limit the maximum output current by proper
choice of sense resistor and, if input fault conditions exist,
external clamps.
