AMC7812B Datasheet, PDF (37/92 Pages) TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS – 12-Bit Analog Monitoring and Control Solution with Multichannel ADC, DACs, and Temperature Sensors

AMC7812B
www.ti.com
SBAS625A – SEPTEMBER 2013 – 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 device 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 AMC7812B uses 6 μA for I_LOW and 120 μA for I_HIGH. The device is designed to function with
discrete transistors, such as the 2N3904 and 2N3906. If an alternative transistor is used, the device operates as
specified, as long as the following conditions are met:
1. Base-emitter voltage is greater than 0.25 V at 6 μA, at the highest sensed temperature.
2. Base-emitter voltage is less than 0.95 V at 120 μA, at the lowest sensed temperature.
3. Base resistance is less than 100 Ω.
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 device allows for different η-factor values, according to Table 3. The device 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.
BINARY
0111 1111
0000 1010
0000 1000
0000 0110
0000 0100
0000 0010
0000 0001
0000 0000
1111 1111
1111 1110
1111 1100
1111 1010
1111 1000
1111 0110
1000 0000
Table 3. η-Factor Range (Single Byte)
N_ADJUST
HEX
7F
0A
08
06
04
02
01
00
FF
FE
FC
FA
F8
F6
80
DECIMAL
127
10
8
6
4
2
1
0
–1
–2
–4
–6
–8
–10
–128
η_EFF
1.747977
1.042759
1.035616
1.028571
1.021622
1.014765
1.011371
1.008
1.004651
1.001325
0.994737
0.988235
0.981818
0.975484
0.706542
h_eff
=
1.008 ´ 300
300 - N_ADJUST
(2)
N_ADJUST = 300 -
300 ´ 1.008
h_eff
where:
• η_EFF is the actual ideality of the transistor used and
• N_ADJUST is the corrected ideality used in the calculation.
(3)
Copyright © 2013, Texas Instruments Incorporated
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