PRE-DRIVE tests

AN4370

14/23

DocID025336 Rev 1

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PRE-DRIVE tests

Before driving the MOSFETs, it is possible to detect whether any of the motor phases is

disconnected or short-circuited either to GND or to the supply voltage. This diagnosis can

be performed thanks to three switches that can be closed or opened through the following

control bits of Control Register 7:

ISTEST(x): control bit for the sink current of leg x:

•

0: sink current disabled

•

1: sink current enabled

ISTEST(x) = 0 means that the corresponding x switch is open.

The tests shown in Table 3 allow the detection of disconnected motor terminals and short

circuits of at least one motor terminal either to GND or to supply voltage, without turning on

any external MOSFET.

During the PRE-DRIVE tests, the DS Monitoring threshold (DSMTH[1,0]) has to be set to

2 V and the gate driver sink current bits (PCSI[3:0]) have to be set to the max value (1111b).

When changing the ISTEST bits, a certain time has to elapse before reading the result of

each test from Status Register 7 (SR7) in order to allow the diagnostic currents to settle in

the motor phases and yield a steady-state voltage drop over the external MOSFETs. In

order to perform the PRE-DRIVE tests, the ISTEST_EN bit has to be set to 1. This is to

allow the SR7 bits to change their state without turning on the external MOSFETs.

Figure 6. PRE-DRIVE tests functional block diagram

In this case, no flag is reported in the Global Status Byte (GSB) due to the SR7 bits.

When the PRE_DRIVE tests are complete, the SR7 bits have to be cleared right after

setting the ISTEST_EN to 0. Otherwise, the status of the SR7 bits would be reflected in the

GSB. It is also recommended to set the ISTEST(x) bits back to 0 at the end of the PRE-

DRIVE tests.