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Using NTC with bq51014
How does an NTC get connected to the bq51014? The bq51014 datasheet in figure 2 shows the NTC connected between TS-CTRL and ground. This more or less agress with the pin description of TS-CTRL. But in the "Simplified Block Diagram" the TS-CTRL pin is apparently a high-impedance input. Things are yet more confused by Figure 41 and accompanying text, which shows a network including VTSB (a pin that exists on other parts, but not on bq51014). The specifications table includes Vcold and Vhot which are percentages of Vts-bias (what's that?) and Vctrl thresholds for high and low. What's the function? hot / normal / cold? Or is it high / low? Is there in fact a curernt source at TS-CTRL? If it really is as simple as an NTC to ground, what nominal resistance and 'beta' should be used?
TS-CTRL is a voltage based input, therefore a high impedance is required. This pin is actively biased by the LDO shown in the block diagram. See figure 43 for the bias timing. During the 24ms when the TS pin is biased, it is looking for a reading on the NTC circuit. For the remaining 240ms, the TS pin acts as a CTRL pin where it looks for a voltage above 100mV to determine a logic 1 (end power transfer due to battery termination).
The TS pin as a temperature sense function is ratiometric around the 2.2V internal LDO. There is a fixed pull up resistor to the internal LDO of 20kΩ. %VCOLD is the ratiometric percentage for the cold voltage trip point and is fixed at 58.7% or a 0.587 voltage divide ratio. %VHOT is the ratiometric percentage for the hot voltage trip point and is fixed at 19.6% or a 0.196 voltage divide ratio. The designer picks the NTC resistor to set the β and the RNOM. Then the designer is to pick the desired TCOLD and THOT trip points for the design (in Kelvin). Then the designer uses equations 4 and 5 to solve for resistors R1 and R3 in figure 41. Figure 41 circuit allows the designer to use any NTC resistor available to them while setting the temperature profile to transition through the IC’s defined %VHOT and %VCOLD thresholds. This procedure is the same for the bq51013A and the bq51014.
If the temperature function is not desired, simply put a single 10kΩ resistor from TS-CTRL to GND.
Bill Johns, Applications Engineer
bqTESLA Wireless Power Products
Texas Instruments Inc
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