LTC1921 datasheet(13/22 Pages) LINER

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LTC6101/LTC6101HV

6101fh

APPLICATIONS INFORMATION

If the offset current, IOS, of the LTC6101 ampliﬁer is 2nA,

the 100 microvolt error above is reduced to 2 microvolts.

Adding RIN+ as described will maximize the dynamic

range of the circuit. For less sensitive designs, RIN+ is

not necessary.

Example:

If an ISENSE range = (1A to 1mA) and (VOUT/ISENSE) =

3V/1A

Then, from the Electrical Characteristics of the LTC6101,

RSENSE ≈ VSENSE (max) / ISENSE (max) = 500mV/1A =

500mΩ

Gain = ROUT/RIN = VOUT (max) / VSENSE (max) =

3V/500mV = 6

If the maximum output current, IOUT, is limited to 1mA,

ROUT equals 3V/1mA ≈ 3.01 kΩ (1% value) and RIN =

3kΩ/6 ≈ 499Ω (1% value).

The output error due to DC offset is ±900μVolts (typ) and

the error due to offset current, IOS is 3k x 2nA = ±6μVolts

(typical), provided RIN+ = RIN–.

The maximum output error can therefore reach ±906μVolts

or 0.03% (–70dB) of the output full scale. Considering

the system input 60dB dynamic range (ISENSE = 1mA to

1A), the 70dB performance of the LTC6101 makes this

application feasible.

Output Error, EOUT, Due to the Finite DC Open Loop

Gain, AOL, of the LTC6101 Ampliﬁer

This error is inconsequential as the AOL 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 RINandentersthe

current sense amp via the IN(–) pin. The power dissipated

in the LTC6101 due to the output signal is:

POUT = (V–IN – VOUT) • IOUT

Since V–IN ≈ V+, POUT ≈ (V+ – VOUT) • IOUT

There is also power dissipated due to the quiescent sup-

ply current:

PQ = IDD • V+

The total power dissipated is the output dissipation plus

the quiescent dissipation:

PTOTAL = POUT + PQ

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

θJA 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:

PQ(MAX) = IDD(MAX) • V+(MAX) = 41.4mW

POUT(MAX) = IOUT • V+(MAX) = 60mW

TRISE = θJA • PTOTAL(MAX)

TMAX = TAMBIENT + TRISE

TMAX must be < 150°C

PTOTAL(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 PQ, 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

θJA 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.

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