8.2.1.2 Detailed Design Procedure

First, select the R and determine the maximum logic low voltage and the minimum logic high voltage while ensuring that when the LMT01 is converting, the minimum (VP – VN) requirement of 2.15 V is met.

1. Select R using minimum VP-VN during data transmission (2 V) and maximum output current of the LMT01 (143.75 µA)
   • R = (3.0 V – 2 V) / 143.75 µA = 6.993 k the closest 1% resistor is 6.980 k
   • 6.993 k is the maximum resistance so if using 1% tolerance resistor the actual resistor value needs to be 1% less than 6.993 k and 6.98 k is 0.2% less than 6.993 k thus 6.81 k must be used.
2. Check to see if the 2.15-V minimum voltage during conversion requirement for the LMT01 is met with the maximum I_OL of 39 µA and maximum R of 6.81 k + 1%:
   • V_LMT01 = 3 V – (6.81 k × 1.01) × 39 µA = 2.73 V
3. Find the maximum low level voltage range using the maximum R of 6.81 k and maximum I_OL of 39 µA:
   • V_RLmax = (6.81 k × 1.01) × 39 µA = 268 mV
4. Find the minimum high level voltage using the minimum R of 6.81 k and minimum I_OH of 112.5 µA:
   • V_RHmin = (6.81 k × 0.99) × 112.5 µA = 758 mV

Now select the MSP430 comparator threshold voltage that enables the LMT01 to communicate to the MSP430 properly.

1. The MSP430 voltage is selected by selecting the internal V_REF and then choosing the appropriate 1 of n/32 settings for n of 1 to 31.
   • V_MID= (V_RLmax – V_RHmin) / 2 + V_RHmin = (758 mV – 268 mV) / 2 + 268 mV = 513 mV
   • n = (V_MID / V_REF ) × 32 = (0.513 / 2.5) × 32 = 7
2. To prevent oscillation of the comparator, output hysteresis must be implemented. The MSP430 allows this by enabling different n for the rising edge and falling edge of the comparator output. For a falling comparator output transition, N must be set to 6.
3. Determine the noise margin caused by variation in comparator threshold level. Even though the comparator threshold level theoretically is set to V_MID, the actual level varies from device to device due to V_REF tolerance, resistor divider tolerance, and comparator offset. For proper operation, the COMP_B worst case input threshold levels must be within the minimum high and maximum low voltage levels presented across R, V_RHmin and V_RLmax, respectively
   Equation 5.
   where
   • VREF is the MSP430 COMP_B reference voltage for this example at 2.5 V
   • V_REF_TOL is the tolerance of the VREF of 1% or 0.01,
   • N is the divisor for the MSP430 or 7
   • N_TOL is the tolerance of the divisor or 0.5
   • COMP_OFFSET is the comparator offset specification or 10 mV
   Equation 6.
   where
   • VREF is the MSP430 COMP_B reference voltage for this example at 2.5 V,
   • V_REF_TOL is the tolerance of the VREF of 1% or 0.01,
   • N is the divisor for the MSP430 for the hysteresis setting or 6,
   • N_TOL is the tolerance of the divisor or 0.5,
   • COMP_OFFSET is the comparator offset specification or 10 mV

   The noise margin is the minimum of the two differences:

   Equation 7. (V_RHmin – V_CHmax) or (V_CHmin – V_RLmax)

   which works out to be 145 mV.

Figure 29. Pulse Count Signal Amplitude Variation