L9780 datasheet(14/56 Pages) STMICROELECTRONICS

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Main functionalities

L9780

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DocID026356 Rev 3

4.6

Pin RCT1 – RCT2 functionalities: RCT2 amplifier, RCT1

switch, RCT1 band-gap switch

RCT2 pin is the input of an amplifier with typical gain 4.2 (electrical characteristics of RCT2

amplifier are specified in Table 10). The output of this amplifier is evaluated during the

impedance test and converted in order to communicate its value via SPI to the μP: the value

of RCT2 amplifier output, measured in 2 different conditions (with RCT1 switch open and

closed), can in fact give a feedback about the impedance of the reference cell, and

consequently about the temperature of the sensor.

RCT1 pin is connected through a switch (RCT1 switch) to ground and through another

switch (RCT1 band-gap switch) to the band-gap voltage; both these functions are used

during the reference cell impedance test, in particular the second one is used to restore the

correct charge of the reference cell after the impedance test (charge balance mechanism).

RCT1 switches resistance range and RCT1 voltage during charge balance are specified in

Table 10. For the time diagram of the sequence and the related description see

Section 4.12. Values of external components connected to pins RCT1 and RCT2 are

specified in Table 19.

4.7

Pin C1A, C2A, C1B, C2B, C3 functionalities: compensation

network connection

These pins are used to connect externally the compensation network to the device; via SPI

(using COMPSEL bit) it is possible to select the desired channel (A or B) and connect the

external network to the pins C1(A or B), C2(A or B) and C3. The pins of the non selected

channel are in high impedance state. C1 is internally connected to the output of INRC

amplifier through the INRC switch (driven by the logic during the impedance test, for details

see Section 4.12), while C2 and C3 are internally connected to the inverting input and the

output of an integrator amplifier (C3 is also the VCCS control voltage pin). The non inverting

input of the amplifier is connected to REF voltage (typ 1.89 V). The integrator amplifier and

the VCCS create a loop (with the external compensation network) of a PI controller. When

the loop reaches a stable condition (

 = 1), on INRC pin there are 450 mV (if 4.2 gain for

INRC amplifier is selected), while the inputs of integrator amplifier are both to 1.89 V

(450 mV * 4.2). In this condition there is no output current from the VCCS cell (2.5 V on C3

pin).

The values of the external components that must be used in compensation network depend

on the sensor that will be driven by the device.

Two examples of possible compensation networks (with defined external components

values) are shown below.

Figure 3. Typical sensor D compensation network

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