AN3338 datasheet(32/72 Pages) STMICROELECTRONICS

32 / 72 page

Electrical characteristics and functions

AN3338

32/72

Doc ID 18441 Rev 4

For a proper sizing of the voltage divider, first of all the maximum allowed temperature level

(TOT_Max) must be fixed, consequently the thermistor resistance is given by Equation 7, as

well as by

Figure 19. The value of ROT resistance can be calculated by using the voltage

divider formula:

Equation 8

taking into account that, if T = TOT_Max then V

-(T

OT_Max) = VNTC_th.

The maximum allowed power on the thermistor should not exceed 50 mW in all the

operating range, in order to guarantee a safe working condition and avoid power

consumption affecting the temperature measurement through self-heating. Therefore,

considering (T = TOT_Max), it must be:

Equation 9

Finally, to increase the noise immunity of the NTC thermistor, it is recommended to parallel a

decoupling capacitor (COT), whose value must be between 10 to 100 nF.

2.3.11

Op amps for advanced current sensing

The SLLIMM devices, in the SDIP-38L package, integrate also three operational amplifiers

optimized for field oriented control (FOC) applications. In a typical FOC application the

currents in the three half bridges are sensed using a shunt resistor. The analog current

information is transformed into a discontinuous sense voltage signal, having the same

frequency as the PWM signal driving the bridge. The sense voltage is a bipolar analog

signal, whose sign depends on the direction of the current (see

Figure 21):

NTC

)

(

)

(

⋅

−

NTC

≤













⋅

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