L9822EPD datasheet(6/11 Pages) STMICROELECTRONICS

6 / 11 page

device shift register (into the SI pin and out the SO

pin), allowing the other SPI devices to be cascaded

in a daisy chain with the L9822ESP.

CE Low to High Transition

Once the last data bit has been shifted into the

L9822ESP,the CE pin should be pulled high.

At the rising edge of CE the shift register data is lat-

ched into the parallel latch and the outputstageswill

be actuated by the new data. An internal 160

µsde-

lay timer will also be started at this rising edge (see

tUD). During the 160

µs period, the outputs will be

protected only by the analog current limiting circuits

since the resetting of the parallel latches by faults

conditionswill be inhibited duringthis period.This al-

lows the part to overcome any high inrush currents

that may flow immediately after turn on. Once the

delay period has elapsed, the output voltages are

sensed by the comparators and any output with vol-

tageshigher than 1.8V arelatched OFF. It shouldbe

noted that the SCLK pin should be low at both tran-

sitions of the CE pin to avoid any false clocking of

the shift register. The SCLK input is gatedby the CE

pin, so that the SCLK pin is ignored whenever the

CE pin is high.

FAULT CONDITIONS CHECK

Checking for fault conditions may be done in the fol-

lowing way. Clock in a new control byte. Wait 160

microseconds or so to allow the outputs to settle.

Clock in thesame controlbyte and observethe diag-

nostic data that comes out of the device. The diag-

nostic bits should be identical to the bits that were

first clocked in. Any differenceswould point to a fault

on that output.If the outputwas programmed ON by

clocking in a zero, and a one came back as the dia-

gnosticbit forthat output,the outputpinwasstill high

and a short circuit or overload condition exists. If the

output was programmed OFF by clocking in a one,

and a zero came back as the diagnostic bit for that

output, nothing had pulled the output pin high and it

Figure 1 : Byte Timing with Asynchronous Reset.

L9822E

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