Datasheet

PRODUCTPREVIEW
7.307k
2.25V
43k 10k NTC
10
ADCDigital
TS
AGND2
TPS65186
SLVSB04 –JULY 2011 www.ti.com
NTC BIAS CIRCUIT
Figure 27 below shows the block diagram of the NTC bias and measurement circuit. The NTC is biased from an
internally generated 2.25-V reference voltage through an integrated 7.307-KΩ bias resistor. A 43-kΩ resistor is
connected parallel to the NTC to linearize the temperature response curve. The circuit is designed to work with a
nominal 10-kΩ NTC and achieves accuracy of ±1°C from 0°C to 50°C. The voltage drop across the NTC is
digitized by a 10-bit SAR ADC and translated into an 8-bit two’s complement by digital per Table 1.
Table 1. ADC Output Value vs Termperature
TEMPERATURE TMST_VALUE[7:0]
< -10°C 1111 0110
-10°C 1111 0110
-9°C 1111 0111
... ...
-2°C 1111 1110
-1°C 1111 1111
0°C 0000 0000
1°C 0000 0001
2°C 0000 0010
... ...
25°C 0001 1001
...
85°C 0101 0101
> 85°C 0101 0101
Figure 27. NTC Bias and Measurement Circuit
A temperature measurement is triggered by setting the READ_THERM bit of the TMST1 register to 1.During the
A/D conversion the CONV_END bit of the TMST1 register reads ‘0’, otherwise it reads ‘1’. At the end of the A/D
conversion the EOC bit in the INT2 register is set and the temperature value is available in the TMST_VALUE
register.
HOT, COLD, AND TEMPERATURE-CHANGE INTERRUPTS
Each temperature acquisition is compared against the programmable TMST_HOT and TMST_COLD thresholds
and to the baseline temperature, to determine if the display is within allowed operating temperature range and if
the temperature has changed by more than a user-defined threshold since the last update. The first temperature
reading after the WAKEUP pin has been pulled high automatically becomes the baseline temperature. Any
subsequent reading is compared against the baseline temperature. If the difference is equal or greater than the
threshold value, an interrupt is issued (DTX bit in register INT1 is set to ‘1’) and the latest value becomes the
new baseline. If the difference is less than the threshold value, no action is taken. The threshold value is defined
by DT[1:0] bits in the TMST1 register and has a default value of ±2°C. In summary:
• When the temperature is equal or less than the TMST_COLD[3:0] threshold, the TMST_COLD interrupt bit of
the INT1 register is set, and the nINT pin is pulled low.
• When the temperature is greater than TMST_COLD but lower then TMST_HOT, no action is taken.
22 Copyright © 2011, Texas Instruments Incorporated