LTC6103CMS8 datasheet(11/16 Pages) LINER

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LTC6103

6103f

APPLICATIONS INFORMATION

If the power dissipation of the sense resistor is chosen to

be less than 0.5W then:

SENSE

SENSE MAX

≤=

500

()

Ω

VSENSE(MAX) = ISENSE(MAX) • RSENSE = 500mV

Gain

OUT

OUT MAX

SENSE MAX

()

500

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

value and

OUT

=≈

301 1

499

.( %

)

Ω (%

)

value

The output error due to DC offset is ±510µV (typ) and the

error due to offset current:

IOS is 3kΩ × 100nA = ±300µV (typical)

The maximum output error can therefore reach ±810µV

or 0.027% (–71dB) of the output full scale. Considering

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

1A), the 71dB performance of the LTC6103 makes this

application feasible.

In many applications the power dissipation of the sense

resistor is of greater importance than the precision of the

measurement. Designing for a VSENSE(MAX) of as low as

100mV is recommended in such cases.

Output Current Limitations Due to Power Dissipation

The LTC6103 can deliver up to 1mA continuous current to

the output pin. This current ﬂows through RIN and enters

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

in the LTC6103 due to the output signal is:

POUT = (VIN

– – VOUT) • IOUT

Since VIN

– ≈ VS, POUT ≈ (VS – VOUT) • IOUT

There is also power dissipated due to the quiescent sup-

ply current:

PQ = IS • VS

The total power dissipated is the output dissipation plus

the quiescent dissipation:

PTOTAL = POUTA + POUTB + PQA + PQB

At maximum supply and maximum output current, the

total power dissipation can exceed 100mW. This will

cause signiﬁcant heating of the LTC6103 die. In order to

prevent damage to the LTC6103, the maximum expected

dissipation in each application should be calculated. This

number can be multiplied by the

θJA value listed in the

Package/Order Information 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 LTC6103 in the MS8 package is to

be run at 55V ±5V supply with 0.5mA output current in

both ampliﬁers at 80°C:

PQ(MAX) = IS(MAX) • V

+ (MAX) • 2 = 82.8mW

POUT(MAX) = IOUT • V

+ (MAX) • 2 = 60mW

TRISE = θJA • PTOTAL(MAX) = 300°C/W • (82.8mW +

60mW) ≈ 43°C

TMAX = TAMBIENT + TRISE = 80°C + 43°C = 123°C

TMAX must be <150°C

PTOTAL(MAX) ≈ 143mW and the maximum die tempera-

ture will be 123°C

If this same circuit must run at 125°C, the maximum die

temperature will exceed 150°C. (Note that supply current,

and therefore PQ, is proportional to temperature. Refer to

the Typical Performance Characteristics.) In this condition,

the maximum output current should be reduced to avoid

device damage. It is important to note that the LTC6103

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 resistors and, if input

fault conditions exist, external clamps.

Output Filtering

The output voltage, VOUT, is simply IOUT • ZOUT. This

makes ﬁltering straightforward. Any circuit may be used

which generates the required ZOUT to get the desired ﬁlter

response. For example, a capacitor in parallel with ROUT

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