A3924 datasheet(30/55 Pages) ALLEGRO

30 / 55 page

Automotive, Full-Bridge MOSFET Driver

A3924

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

To comply with various aspects of safe system design, it is neces-

sary for higher-level safety systems to verify that any diagnostics

or functions used to guarantee safe operation must also be veri-

fied to ensure that theses critical functions are operating within

specified tolerances.

There are four basic aspects to verification of diagnostic func-

tions:

1. Verify connections

2. Verify comparators

3. Verify thresholds

4. Verify fault propagation

These have to be completed for each diagnostic. In addition, the

operation of system functions not directly covered by diagnostics

should also be verified.

The A3924 includes additional verification functions to help the

system design comply with any safety requirements. Many of

these functions can only be completed when the diagnostics are

not required and must be commanded to run by the main system

controller. These functions are referred to as “off-line” verification.

A few of the functions can be continuously active, but the results

must be checked by the main system controller on a regular basis.

These functions are referred to as “on-line” verification.

The frequency with which these off-line verification functions are

run, or on-line verifications results are checked, will depend on

the safety requirements of the system using the A3924.

Example methods of how to use these verification functions to

verify system diagnostics are documented in the A3924 Safety

Manual.

On-Line Verification

The following functions are permanently active and will set the

appropriate bit in the verification result register to indicate that

the verification has failed. No other action will be taken by the

A3924. These verification functions verify that certain of the

A3924 terminals are correctly connected to the power bridge

circuit.

BRIDGE: VBRG DISCONNECTED

The VBRG terminal provides the common drain voltage refer-

ence for the high-side MOSFET VDS overvoltage detectors. If

this becomes disconnected, then the high-side VDS detection will

Table 6: Verification Functions

Type

Function Verified

Operation

Connection VBRG Connection

On-line

Connection Phase connection

Off-line

Connection Sense Amp Connection

On-line

Connection LSS Connection

On-line

Monitor

Over current detectors

Off-line

Monitor

Phase state monitor

On-line

Diagnostic

Over temperature diagnostic

Off-line

Diagnostic

Temperature warning monitor

Off-line

Diagnostic

VBB undervoltage diagnostic

Off-line

Diagnostic

VBB overvoltage diagnostic

Off-line

Diagnostic

VREG diagnostics

Off-line

Diagnostic

VGS undervoltage diagnostic

Off-line

Diagnostic

Logic terminal diagnostic

Off-line

Diagnostic

Open load detectors

Off-line

Diagnostic

Bootstrap capacitor diagnostic

Off-line

Diagnostic

VDS overvoltage diagnostic

Off-line

Diagnostic

V3 regulator diagnotics

Off-line

be invalid, and VDS overvoltage faults may not be detected. If

VBRG is disconnected, the internal current sink from the input

will ensure that the voltage at the VBRG terminal will fall. A

comparator is provided to monitor the voltage between the main

supply connection at the VBB terminal, and the voltage at VBRG

(VBB – VBRG) is compared to the VBRG open threshold voltage

(VBRO), determined by the variable VTB[1:0] as:

VBRO = (n + 1) × 2 V

where n is a positive integer defined by VTB[1:0] giving thresh-

olds at 2 V, 4 V, 6 V, and 8 V.

If VBB – VBRG exceeds the VBRG open threshold voltage, then

the VBR bit will be set in the verification result register, all

high-side VDS fault bits will be set in the Diag 1 register, and

the general fault flag will be active. If ESF = 1, then all gate

drive outputs will be disabled. When VBB – VBRG falls below the

falling VBRG open threshold voltage, VBRO – VBROHys, the fault

flag will be reset and the outputs will be reactivated. The VBR

bit remains in the verification result register until cleared and the

VDS diagnostic bits remain in the Diag 1 register until cleared.

Note that, for accurate VBRG disconnect detection at VBB less

than 12 V, it is important to ensure the selected VBRG disconnect

threshold (VBRO) is no more than 4 V less than VBB.

DIAGNOSTIC AND SYSTEM VERIFICATION

A3924 Datasheet(PDF) 30 Page - Allegro MicroSystems

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