Measuring Temperature with the

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Step 16. Measure V_BE3 using the ADC

The V_BE3 voltage is measured from the AIN1 to the AIN2 inputs to the ADC. The ADC is already configured and only needs to convert to get the resulting V_BE3 voltage. To start the ADC conversion, set the STRT bit in the ADC register. The ADC will do a conversion and the result will be in the DATA register. Read the DATA register value and save as a 16-bit integer named V_BE3 for later calculation.

Step 17. Set the ADC Input for V_R3

Set the ADC positive input multiplexer for AGND by setting MUXP[3:0] to 0x09 the MUX register. Set the ADC negative multiplexer for AIN2 by setting MUXN[3:0] to 0x07 the MUX register. To measure the AIN2 input relative to AGND, the polarity flipper bit is used. Set the POL bit in the ADC register. The ADC is now setup with AIN2 as its positive input and AGND as its negative input.

Step 18. Measure V_R3 using the ADC

To start the ADC conversion, set the STRT bit in the ADC register. The ADC will do a conversion and the result will be in the DATA register. Read the DATA register value and save as a 16-bit integer named V_R3 for later calculation.

Step 19. Set the Current Source for I₄ (120µA)

Set the current source for I₄ by setting the IVAL[1:0] bits to 0x11 in the TEMP_CTRL register.

Step 20. Set the ADC Input for TEMP+ to TEMP-

Set the polarity flipper back to normal by clearing the POL bit in the ADC register. Set the ADC positive input multiplexer for AIN1 by setting MUXP[3:0] to 0x00 the MUX register. Set the ADC negative multiplexer for AIN2 by setting MUXN[3:0] to 0x07 the MUX register.

Step 21. Measure V_BE4 Using the ADC

The V_BE4 voltage is measured from the AIN1 to the AIN2 inputs to the ADC. The ADC is already configured and only needs to convert to get the resulting V_BE4 voltage. To start the ADC conversion, set the STRT bit in the ADC register. The ADC will do a conversion and the result will be in the DATA register. Read the DATA register value and save as a 16-bit integer named V_BE4 for later calculation.

Step 22. Set the ADC Input for V_R4

Set the ADC positive input multiplexer for AGND by setting MUXP[3:0] to 0x09 the MUX register. Set the ADC negative multiplexer for AIN2 by setting MUXN[3:0] to 0x07 the MUX register. To measure the AIN2 input relative to AGND, the polarity flipper bit is used. Set the POL bit in the ADC register. The ADC is now setup with AIN2 as its positive input and AGND as its negative input.

Step 23. Measure V_R4 Using the ADC

To start the ADC conversion, set the STRT bit in the ADC register. The ADC will do a conversion and the result will be in the DATA register. Read the DATA register value and save as a 16-bit integer named V_R4 for later calculation.

Step 24. Calculate the Temperature

The temperature calculation is identical to the four-current internal method. Use that method with the eight saved measurements to calculate the external temperature.

Gain and Offset Correction

The TEMP_CAL register is provided to correct for gain and offset errors in the temperature-measurement circuit. To correct for the gain and offset errors, the following formula is used.

T_ACTUAL = g(T_MEAS) + T_OFFSET
Equation 3. Gain and offset correction equation.

Procedure for Gain and Offset Correction

The procedure for adding the gain and offset correction is detailed below. The procedure reads the values from the gain and offset correction registers. The gain and offset amounts are then computed using two formulas. The resulting correction values are applied to Equation 3 above.

Step 1. Measure and Calculate the Temperature

Use the procedure for the internal or the external transistor and save the result as T_MEAS for use in the correction formula above.

Step 2. Read the TGAIN Register

Read the TEMP_CAL register and save the upper byte TGAIN[7:0] for use in the formula below. It should be saved as a type-signed integer.

Step 3. Calculate the Gain Correction Factor

Using the value saved above, apply this value to the gain formula below.

Gain = 0.9025 + TGAIN × 0.000576
Equation 4. Gain formula equation.

Step 4. Read the TOFFS Register

Read the TEMP_CAL register and save the lower byte TOFFS[5:0] for use in the formula below. The data should be saved as a type-signed integer. Do not right-shift the data.

Step 5. Calculate the Offset Correction Factor

Using the offset value saved above, apply this value to the gain formula below.

Offset = -14.0 + TOFFS × 0.3125
Equation 5. Offset correction equation.

Step 6. Calculate the corrected Temperature

Use the gain and offset values calculated above in the temperature equation below.

T_ACTUAL = GAIN × T_MEAS + OFFSET

Conclusion

The MAX1358 features internal circuitry to measure the internal die temperature. The internal circuitry can also be used with an external transistor for a low-cost remote temperature sensor. The four-current temperature measurement method described in this application note can be used to achieve the ±0.5 and ±1.0 typical accuracy specified in the MAX1358 data sheet.

Panasonic is a registered trademark of Matsushita Electric Industrial Co., Ltd.

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