LM5066: Negative local temperature reading issue !

NPN transistor MMBT3904LT1G collector and base connected to diode pin of lm5066. The emitter is grounded. Board placed in a thermal chamber at -20degC. While reading the local temperature register 0x8Dh, we received data as 0xF0Ah. How to convert the data to negative temp. At room temp +25, data was received properly from LM5066 as 0x190h.

Since it is a 12bit A/D presumably 0xF0A is a twos complement negative value (since the MSB is set) and thus you should convert that using the formula and values in table 42 in the lm5066 data sheet to get the value in degrees C.