SML2108
PRELIMINARY
APPLICATIONS
APPLICATION EXAMPLE USING EXTERNAL TEM-
PERATURE INPUT
5V
TheEXTTEMPpinoftheSML2108allowstheinputofthe
internal ADC to be driven from an external device, rather
than a mirrored version of the bias current. Figure 25
shows an example using a National Semiconductor
LM334 to deliver a current into the SML2108 that is
proportional to absolute temperature. The scale and
offset features of the ADC input can be used to center this
current and maximize the full range of the look-up table.
LM334
+
–
R
210Ω ±1%
SET
SML2108
I
SET
EXT TEMP
For this application the current ISET coming out of the
LM334 and into the EXT TEMP pin of the SML2108 is
given by the following equation:
2053 Fig25
Figure 25. Example of an External Temperature
Sensing Device
I
SET = 227µV / oK / RSET
For example, using a value of 210Ω for RSET yields a
current of 295µA at 0oC and 387µA at 85oC.
Next select scale and offset values of the ADC input that
willoptimizethecurrentrangeoftheLM334. Nominalfull-
scale input current of the ADC is 390.6µA (= 100mA/256).
By setting the input offset to ¾ scale (293µA) and the full-
scalerangeto¼scale(97.6µA), thenthezeroscaleofthe
internal ADC becomes 293µA, and the full-scale is
390.6µA. Thisrepresentsatemperaturerangeofapproxi-
mately –2oC to 88oC using a 210Ω resistor in the configu-
rationshown. (Thesesettingscorrespondtoconfiguration
Register 0, Bits 3 - 0 set to 7HEX.)
SUMMIT MICROELECTRONICS, Inc.
2053 2.2 11/07/00
19
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
