LTC6115 datasheet(13/20 Pages) AD

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LTC6115

Rev. A

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APPLICATIONS INFORMATION: CURRENT SENSE

At maximum supply and maximum output current, the

total power dissipation can exceed 100mW. This will

cause significant heating of the die. In order to prevent

damage to the circuit, the maximum expected dissipa-

tion in each application should be calculated. This num-

ber can be multiplied by the

θJA value listed in the Pin

Configuration section 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 only the current sense is to run at 100V

supply voltage on V+, with 1mA output current at 125°C:

PQ(MAX) = IV+(MAX) • V+(MAX)

= 720μA • 100V

= 72mW

POUT(MAX) = IOUTI • V+(MAX)

= 1mA • 100V

= 100mW

TRISE =

θJA • PTOTAL(MAX)

= 135°C/W • 172mW

= 23.2°C

TMAX = TAMBIENT + TRISE

= 125°C + 23.2°C

= 148.2°C

The junction temperature will thus be very close to 150°C,

above the specified temperature range. In this case, the

maximum output current should be reduced, which can

be done by changing RIN.

It is important to note that the current sense amplifier has

been designed to provide at least 1mA to the output when

required, and can deliver more depending on the condi-

tions. Care must be taken to limit the maximum output

current by proper choice of sense resistor and, if input

fault conditions exist, external clamps.

Example

If a circuit has an expected ISENSE range of 1A to 1mA,

and (VOUTI/ISENSE) = 3V/1A, then, from the Electrical

Characteristics of the LTC6115 current sense amplifier,

RSENSE ≈ VSENSE(MAX)/ISENSE(MAX) = 500mV/1A = 500mΩ.

Gain = ROUT/RIN = VOUTI(MAX)/VSENSE(MAX) = 3V/500mV = 6.

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

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

3kΩ/6 ≈ 499Ω (1% value).

The output error due to DC offset is ±510µV (85µV typi-

cal • gain of 6) and the error due to offset current, IOS is

3kΩ • ±2nA = ±6µV (typical), provided RIN+ = RIN–.

The maximum output error can therefore reach ±516µV

or 0.017% (–75dB) of the output full-scale. Considering

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

1A), the 70dB performance of the LTC6115 current sense

amplifier makes this application feasible.

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

Gain, AOL, of the LTC6115 Current Sense Amplifier

This error is inconsequential as the AOL of the amplifier

is very large.

Output Current Limitations Due to Power Dissipation

The current sense amplifier can deliver up to 1mA contin-

uous current to the output pin. This current flows through

RIN and enters the current sense amplifier via the –IN pin.

The power dissipated in the current sense circuit due to

the output signal is:

POUTI = (V–IN – VOUTI) • IOUTI

Since V–IN ≈ V+, POUTI ≈ (V+ – VOUTI) • IOUTI

There is also power dissipated due to the quiescent sup-

ply current:

PQ = IV+ • V+

The total power dissipated is the output dissipation plus

the quiescent dissipation:

PTOTAL = POUTI + PQ

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