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AN1596 - APPLICATION NOTE

voltage can go low and signaling an open load condition (we remind that in ON state the status signal has

a maximum delay of 200

µs).

- Analog HSDs

In the previous chapter the current sense feature in M0-3 HSDs has been shown; this is active during the

ON state only (INPUT=high) whilst in the OFF phase (INPUT=low) the current sense circuit is inactivated.

This means that it is possible to detect an open load in ON state fault but not during OFF state. Two

possible solutions can be thought of:

Solution 1: external comparator (see figure 15). Each load is connected to a pull up resistor supplied by

controller with the fault signal. As seen for digital HSDs, the pull up disable network is supplied when the

ignition key is in, but the car is off.

Solution 2: two additional signal bipolar transistor T3 and T4 can be used (see figure 16). The emitter of

the T4 transistor (PNP type) is connected to a positive 5V line, and its base is connected to the HSD input

line. This means that T4 is off when the HSD is on whilst it is on when the HSD is off. T3 (NPN type)

collector is connected to the T4 one while T3 base to the output pin. T3 doesn’t conduct until the output

which the voltage drop on the load is almost zero.

conducts and a voltage of about 5V appears on the sense pin. Therefore the micro-controller will detect

the open load fault in OFF mode.

Figure 15: Open load cicuitry for analog HSDs (OFF state) with an external comparator

Pull up disable

Vcc

SENSE

INPUT

4 Channels HSD

T1

T2

+V ref.

µ C

Loads

DRIVER