AMC7812SPAPR datasheet(37/92 Pages) TI

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300

1.008

eff

ADJUST

=300 -

1.008

300

ADJUST

heff =

AMC7812

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SBAS513E – JANUARY 2011 – REVISED SEPTEMBER 2013

Remote Sensing Diode

Errors in remote temperature sensor readings are typically the consequence of the ideality factor and current

excitation used by the AMC7812 versus the manufacturer-specified operating current for a given transistor. Some

manufacturers specify a low-level (ILOW) and high-level (IHIGH) current for the temperature-sensing substrate

transistors. The AMC7812 uses 6

μA for ILOW and 120μA for IHIGH. The AMC7812 is designed to work with

discrete transistors, such as the 2N3904 and 2N3906. If an alternative transistor is used, the AMC7812 operates

as specified, as long as the following conditions are met:

1. Base-emitter voltage > 0.25V at 6

μA, at the highest sensed temperature.

2. Base-emitter voltage < 0.95V at 120

μA, at the lowest sensed temperature.

3. Base resistance < 100

Ω.

4. Tight control of VBE characteristics indicated by small variations in hFE (that is, 50 to 150).

Ideality Factor

The ideality factor (

η) is a measured characteristic of a remote temperature sensor diode as compared to an

ideal diode. The AMC7812 allows for different

η-factor values, according to Table 3. The AMC7812 is trimmed for

a power-on reset (POR) value of

η = 1.008. If η is different, the η-Factor Correction Register can be used. The

value (NADJUST) written in this register must be in twos complement format, as shown in Table 3. This value is

used to adjust the effective

η-factor according to Equation 2 and Equation 3.

Table 3.

η-Factor Range (Single Byte)

NADJUST

BINARY

HEX

DECIMAL

ηEFF

0111 1111

127

1.747977

0000 1010

1.042759

0000 1000

1.035616

0000 0110

1.028571

0000 0100

1.021622

0000 0010

1.014765

0000 0001

1.011371

0000 0000

1.008

1111 1111

–1

1.004651

1111 1110

–2

1.001325

1111 1100

–4

0.994737

1111 1010

–6

0.988235

1111 1000

–8

0.981818

1111 0110

–10

0.975484

1000 0000

–128

0.706542

(2)

Where:

ηEFF is the actual ideality of the transistor being used

NADJUST is the corrected ideality being used in the calculation

(3)

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