33797 datasheet(30/34 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33797

FUNCTIONAL DEVICE OPERATION

PROTECTION AND DIAGNOSIS FEATURES

FET DRIVER CURRENT LIMIT

A single resistor is used to set the current limit protection

of the high-side drivers of both squib channels. The low-side

current limit is never less than the high-side current limit.

Example of current limit conditions:

RR_LIMIT_X =10 kΩ, IHS= A ± A

The high-side driver controls the current through the squib.

The current limit for the low-side driver is only to protect the

low-side driver stage from excessive current in the event of a

short to battery.

With RR_LIMIT_X conditions <4.32 kΩ or shorted to ground,

the current limit will default to the RR_LIMIT_X = 10 kΩ current

limit, not to exceed. With RR_LIMIT_X resistance value >60 kΩ

or open, the current limit will default to the RR_LIMIT_X = 10 kΩ

maximum current limit.

FET DRIVER CURRENT LIMIT MEASUREMENT

($7X COMMAND)

This function measures the firing current in each squib line

and records the “ON” time in which the IMEAS is above the

threshold for each squib. (Refer to Dynamic Electrical

Characteristics table, page 13.) The timing registers can be

reset via SPI command so additional current measurements

can be made.

An 8-bit message will be used to determine 255 time

steps. The driver current limit measurement is activated when

each individual high-side driver is activated. Each time the

squib current is measured above the IMEAS threshold during

the timer activation, a status bit will be set to “1”. If the current

measured is not above the IMEAS threshold during the timer

activation, the timing data log bit will not increment. Each

squib timing register can be reset via SPI command so

additional current measurements can be made. Initial squib

IC power-up will reset the timing registers (i.e., “Power-ON

Reset”). When reset, the current limit measurement register

byte will be set to $00.

Command $79 will indicate the status of the current limit

measurement comparator. The current limit measurement

from the test is captured and loaded into the register on the

next valid SPI command. When the firing current is above

IMEAS, the current limit is activated and the status bit will be

set to “1”. If the firing current is below IMEAS, the current limit

status bit will be set to “0”.

FET DRIVER CURRENT LIMIT MEASUREMENT

RESET COMMAND ($3X COMMAND)

The current limit status registers can be individually reset

with the command set found in Table 8. When the register bit

is set to “1” for squib X, the current measurement register will

be reset to $00.

SQUIB DRIVER THERMAL SHUTDOWN

($7F COMMAND)

With a nominal squib load, the FET squib driver will not

enter thermal shutdown until the driver has been active for a

minimum of 2.09 ms. The individual squib driver thermal

shutdown will not affect other squib drivers firing “ON” times.

With a shorted squib load, the FET squib driver will not

enter thermal shutdown until the driver has been active for a

minimum of 2.090 ms. For the shorted squib load, the

associated FET squib driver may enter thermal shutdown

with an “ON” time of 2.09 ms

≤ t

ON ≤ 2.82 ms.

When the thermal shutdown limit is exceeded, the thermal

status will be set to “1”. The thermal shutdown status ($7F)

diagnostics latch the thermal bit status when executed. The

Squib Driver Thermal shutdown status latch will be cleared

after the information is transmitted on the next valid SPI

command (i.e., TX: NOP or next $7F, latch cleared on rising

edge of chip select).

The FET squib driver can be activated through the arm/fire

command when the TEMPRENABLE (MIN) is reached (thermal

shutdown status “0”).

VTRANTSTX, HIGH- AND LOW-SIDE SQUIB DRIVER

FET TEST and STATUS ($82 TO $83 COMMAND)

This function checks the squib driver FET transistor status.

The high- and low-side squib driver FET test requires

FEN_1 and FEN_2 pins to be low and two separate 8-bit write

commands to be made to the shift register. With the FEN_1

and FEN_2 pins status LOW, the first write is to unlock in

preparation of receiving the diagnostic command for testing

the high- and low-side squib drivers. The unlock command

($82 and $83) is an “AND” function with the FEN_1 and

FEN_2 BAR. All transistor test unlock commands ($82 and

$83) will be echoed back on the SPI Data output.

The high- or low-side squib driver FET test will be aborted

if firing from any FET is enabled.

During the first write (unlock command), all diagnostic

functions are cleared. After the second write is completed, all

other diagnostic functions are made available again.

Squib 1A, squib 1B, squib 2A, and squib 2B high-side

squib drivers will be activated and diagnosed by the $82

followed by $1X diagnostic command (refer to Table 8,

page 25). A load from the SQB_HI_XX pin to the SENSE_XX

pin is required for the high-side squib driver to be tested.

Squib 1A, squib 1B, squib 2A, and squib 2B low-side

squib drivers will be activated and diagnosed by the $83

followed by $2X diagnostic command (Table 8).

When enabled the high- or low-side FET driver will be

enabled and current limited to a nominal current limit of

10 mA. The high- and low-side driver test time is not

automated and is controlled through SPI.

Table 9. RR_LIMIT_X Current Limit

RR_LIMIT_X

VVFIRE = 7.0 V

VVFIRE = 35 V

4.32

Ω

0.92 A

Ω

1.37 A

45.3

Ω

2.0 A

33797 Datasheet(PDF) 30 Page - Freescale Semiconductor, Inc

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