L9805 datasheet(57/103 Pages) STMICROELECTRONICS

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L9805

5.5.3 Input Selections and Sampling

The input section of the ADC includes the analog

multiplexer and a buffer. The input of the buffer is

permanently connected to the multiplexer output.

The buffer output is fed to the sample and hold cir-

cuit.

The multiplexer is driven with CH1 and CH0 bit

only after ADST is set. Starting from this event, the

sampler follows the selected input signal for 2.5us

and then holds it for the remaining conversion time

(i.e. when the conversion is actually running).

5.5.4 Interrupt Management

If ADIE bit is set in register ADCCSR, an interrupt

is generated when a conversion is completed (i.e.

when COCO is set).

The interrupt request is cleared when any of the

ADC registers is access (either read or write).

Enabling/disabling the interrupt generation while

the conversion is running has no effect on the cur-

rent conversion. ADIE value is latched when

ADST is set and this internal value holds all the

conversion time long.

5.5.5 Temperature Sensing

The AD1 input is internally connected to the output

of a temperature sensing circuit.

The sensor generates a voltage proportional to the

absolute temperature of the die. It works over the

whole temperature range, with a minimum resolu-

tion of 1LSB/°K (5mV/°K) (Figure 33 shows the in-

dicative voltage output of the sensor).

Note The voltage output of the sensor is only relat-

ed to the absolute temperature of the silicon junc-

tions. Junction temperature and ambient tempera-

ture must be related taking in account the power

dissipated by the device and the thermal resist-

ance Rthje between the silicon and the environ-

ment around the application board.

Figure 33. Temperature Sensor output

The output of the sensor is not ratiometric with the

voltage reference for the ADC conversion (VCC).

When calculating the ADC reading error of this sig-

nal the variation of VCC must be accounted. Addi-

tional errors are due to the intrinsic spread of the

sensor characteristic.

5.5.6 Precise Temperature Measurement

To allow a more precise measurement of the tem-

perature a trimming procedure can be adopted (on

request).

The temperature is measured in EWS and two val-

ues are stored in four EEPROM bytes (see memo-

ry map):

T0L,T0H: temperature of the trimming measure-

ment (in Kelvin).

VT0L,VT0H: output value of the ADC correspond-

ing to T0 (in number of LSBs).

The corrected measurement of the temperature in

Kelvin must be accomplished in the following way:

where VTEMP is the output code in LSB of the

ADC corresponding to the measurement.

Example:

If the value stored in EEPROM are:

0C7Ch: 01h ->T0H

0C7Dh: 43h ->T0L

0C7Eh: 01h -> VT0H

0C7Fh: 5Ch -> VT0L

T0 = 0143h = 323K (50 Celsius)

VTo = 015Ch = 348 LSB (conversion of 1.7V, sen-

sor output)

and the sensor output is 2V, converted by the ADC

in code 0110011001 = 019Ah = 410LSB, the tem-

perature of the chip is

TEMP = 019Ah * 0143h / 015Ch = 017Ch

equivalent to:

TEMP = 410 * 323 / 348 = 380 K = 107

°C

Note The sensor circuit may have two kind of er-

ror: one translating its output characteristic up and

down and the other changing its slope. The de-

scribed trimming recovers only the translation er-

rors but can not recover slope error. After trim-

ming, being TTRIM the trimming temperature, the

specified precision can be achieved in the range

TTRIM-80, max[TTRIM+80, 150°C]. Precision is re-

lated to the read temperature in Kelvin.

VTEMP

Temperature (°K)

1.0

2.5

1.3

1.6

1.9

2.2

273

323

373

423

473

223

max

min

TEMP (in °K) = VTEMP * T0 / VT0

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