L9780 datasheet(25/56 Pages) STMICROELECTRONICS

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L9780

Main functionalities

4.13

Pin HD, HG functionalities: heater FET driver and diagnostic

As shown in the block diagram of Figure 2, HG pin is externally connected to the gate of the

external heater FET (an NMOS in low-side configuration, with the drain directly connected

to the heater element), while HD is connected to the drain through the external resistance

RHD (possible range for this resistor is specified in Table 19).

The heater is necessary in order to make the sensor work at a correct temperature. The

sensor can only operate in high temperature conditions, but as it is not able to reach the

desired range of temperature passively, an active heater is usually incorporated into the

sensor structure. This heater is normally composed by an ohmic resistor through which the

heater FET current is forced. The μP is able to obtain information regarding the sensor

temperature from the results of the impedance test performed into the device and

communicated to the μP via SPI (the higher is the sensor temperature, the lower is its

impedance); consequently, the μP can intervene on sensor temperature acting on PW pin.

PW is a digital input pin of the device and can be driven with a PWM signal by the μP. PW

pin signal is then internally inverted by the logic and the consequent signal is then applied

on HG pin, conditioning the activation/deactivation of the external heater FET (and so the

power consumption necessary to heat the sensor).

Capacitive coupling due to heater commutations can cause undesired signals on the

reference cell; in order to avoid it, the logic freezes the current state of HG signal during the

measurement cycle. HG status will be driven again by PW signal once the measurement

cycle is completed.

In order to avoid that the external FET switches on when VIGN voltage is present but the

device is un-powered, a 100 kΩ pull-down resistor is permanently connected to HG pin.

A diagnostic circuitry is also present into the heater driver, in order to detect open circuit,

short to ground or short to battery fault conditions; the logic into the device can

communicate via SPI to the μP the fault detection through OCH, STGH, STBH bit status.

The actions related to the diagnostic fault detection are described in Section 5.4, while all

the heater diagnostic thresholds are specified in Table 13. More details about the heater

diagnostic bits can be found in Section 6.4.

The ranges of the electrical parameters of the heater FET driver are specified in Table 12

and Table 15.

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