LTC2991IMS#PBF Linear Technology, LTC2991IMS#PBF Datasheet - Page 14

• Manufacturer Part Number: LTC2991IMS#PBF
• Manufacturer: Linear Technology

Specifications of LTC2991IMS#PBF

• Operating Temperature (max): 85C
• Operating Temperature (min): -40C
• Pin Count: 16
• Mounting: Surface Mount
• Package Type: MSOP
• Case Height: 0.86mm
• Screening Level: Industrial
• Lead Free Status / Rohs Status: Compliant

LTC2991
In the case of current measurements, the external sense
resistor is typically small, and determined by the full-scale
input voltage of the LTC2991. The full-scale differential
voltage is 0.300V. The external sense resistance, is then a
function of the maximum measurable current, or R
= 0.300V/I
current range of ±5A, the external shunt resistance would
equal 0.300V/5A = 60mΩ.
There exists a way to improve the sense resistor’s precision
using the LTC2991. The LTC2991 measures both differential
voltage and remote temperature. It is therefore, possible
to compensate for the absolute resistance tolerance of the
sense resistor and the temperature coefficient of the sense
resistor in software. The resistance would be measured
by running a calibrated test current through the discrete
resistor. The LTC2991 would measure both the differential
voltage across this resistor and the resistor temperature.
From this measurement, R
tion would be known. Using the two equations, the host
microprocessor could compensate for both the absolute
tolerance and the TCR.
Device Configuration
The LTC2991 is configured by writing the channel control
registers through the serial interface. Refer to Tables 5, 6
and 7 for control register bit definition. The device is ca-
pable of many application configurations including voltage,
temperature and current measurements. It is possible to
configure the device for single or repeated acquisitions. For
repeated acquisitions, only the initial trigger is required,
and new data is written over the old data. Acquisitions
are frozen during serial read data transfers, to prevent the
upper and lower data bytes for a particular measurement
from becoming out of sync. Internally, both the upper and
lower bytes are written at the same instant. Since serial data
transfer timeout is not implemented, failure to terminate a
read operation will yield an indefinitely frozen wait state.
The device can also make single measurements, or with
one trigger, all of the measurements for the configuration.
When the device is configured for multiple measurements,
APPLICATIONS INFORMATION
14
R
α = 3930ppm/°C for copper trace
α = ±2 to ~200ppm/°C for discrete R
I = (V1 – V2)/R
T
= R
O
MAX
• [1 + α(T – T
. For example, if you wanted to measure a
T
O
O
)],where
and T
O
in the following equa-
EXT_MAX
(7)
(8)
the order of measurements is fixed. As each new data
result is ready, the MSB of the corresponding data reg-
ister is set, and the corresponding status register bit is
set. These bits are cleared when the corresponding data
register is addressed. The configuration register value at
power-up yields the measurement of the internal tempera-
ture sensor and V1 through V8 as single-ended voltages,
if triggered. The eight input pins V1 through V8 will be in
a high impedance state, until configured otherwise, and
a measurement triggered.
Data Format
The data registers are broken into 8-bit upper and lower
bytes. Voltage and temperature conversions are 13-bits.
The upper bits in the MSB registers provide status on the
resulting conversions. These status bits are different for
temperature and voltage conversions.
Temperature
Temperature conversions are reported as Celsius or Kelvin
results described in Tables 11 and 12, each with 0.0625
degree weighted LSBs. The format is controlled by the
control registers, xxx. The temperature MSB result register
most significant bit (Bit 7) is the DATA_VALID bit, which
indicates whether the current register contents have been
accessed since the result was written to the register. This
bit will be set when new data is written to the register,
and cleared when accessed. The LTC2991 internal bias
circuitry maintains this voltage above this level during
normal operating conditions. Bit 4 through bit 0 of the
MSB register are the conversion result bits D[12:8], in
two’s compliment format. Note in Kelvin results, the
result will always be positive. The LSB register contains
temperature result bits D[7:0]. To convert the register
contents to temperature, use the following equation: T =
D[12:0]/16. See Table 16 for conversion value examples.
Remote diode voltage is digitized at ~50μA of bias current.
The ADC LSB value during these conversions is typically
38.15μV. Voltages are only available for the remote di-
odes, not the internal sensor. This code repeats at a diode
voltage of approximately 0.625V (see Tables 13 and 14).
The absolute temperature of the diode can be used to
detect whether the diode is operating (≤0.62501V or
≥ 0.62505V). This mode is useful for testing small relative
2991f