CLmin

REF

N N_TOL

V

V

1 V_REF_TOL

COMP_OFFSET

32

u

u

CHmax

REF

N N_TOL

V

V

1 V_REF_TOL

COMP_OFFSET

32

u

u

16

LMT01-SP

SNIS205 – JANUARY 2019

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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-SP 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-

SP (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-SP is met with the

–

–

–

Now select the MSP430 comparator threshold voltage that enables the LMT01-SP to communicate to the

MSP430 properly.

settings for n of 1 to 31.

–

–

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

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,

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

(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 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

(6)

The noise margin is the minimum of the two differences:

(7)

which works out to be 145 mV.