Datasheet

300 1.008´
eff
h
N
ADJUST
=300 -
1.008 300
300 N
´
-
ADJUST
h
eff
=
AMC7812
www.ti.com 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 (I
LOW
) and high-level (I
HIGH
) current for the temperature-sensing substrate
transistors. The AMC7812 uses 6μA for I
LOW
and 120μA for I
HIGH
. 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 6μA, at the highest sensed temperature.
2. Base-emitter voltage < 0.95V at 120μA, at the lowest sensed temperature.
3. Base resistance < 100Ω.
4. Tight control of V
BE
characteristics indicated by small variations in h
FE
(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 (N
ADJUST
) 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)
N
ADJUST
BINARY HEX DECIMAL η
EFF
0111 1111 7F 127 1.747977
0000 1010 0A 10 1.042759
0000 1000 08 8 1.035616
0000 0110 06 6 1.028571
0000 0100 04 4 1.021622
0000 0010 02 2 1.014765
0000 0001 01 1 1.011371
0000 0000 00 0 1.008
1111 1111 FF –1 1.004651
1111 1110 FE –2 1.001325
1111 1100 FC –4 0.994737
1111 1010 FA –6 0.988235
1111 1000 F8 –8 0.981818
1111 0110 F6 –10 0.975484
1000 0000 80 –128 0.706542
(2)
Where:
η
EFF
is the actual ideality of the transistor being used
N
ADJUST
is the corrected ideality being used in the calculation (3)
Copyright © 2011–2013, Texas Instruments Incorporated Submit Documentation Feedback 37
Product Folder Links: AMC7812