A3941 Datasheet, PDF (11/21 Pages) Allegro MicroSystems – Automotive Full Bridge MOSFET Driver

A3941
Automotive Full Bridge MOSFET Driver
the event that two or more faults are detected simultaneously, the
state of the fault flags will be determined by a logical OR of the
flag states for all detected faults.
Table 3. Fault Definitions
Flag State
FF1 FF2
Fault Description
Disable
Outputs*
Low Low No fault
No
Low High Short-to-ground
Yes
Low High Short-to-supply
Yes
Low High Shorted load
Yes
High Low Overtemperature
No
High High V5 undervoltage
Yes
High High VREG undervoltage
Yes
High High Bootstrap undervoltage
Yes
*Yes indicates all gate drives low, and all FETs off.
Fault
Latched
–
Yes
Yes
Yes
No
No
No
Yes
Fault States
Overtemperature If the junction temperature exceeds the over-
temperature threshold, typically 165°C, the A3941 will enter the
overtemperature fault state and FF1 will go high. The overtem-
perature fault state, and FF1, will only be cleared when the tem-
perature drops below the recovery level defined by T_JF – T_JFhys .
No circuitry will be disabled. External control circuits must take
action to limit the power dissipation in some way so as to prevent
overtemperature damage to the A3941 chip and unpredictable
device operation.
VREG Undervoltage VREG supplies the low-side gate driver
and the bootstrap charge current. It is critical to ensure that the
voltages are sufficiently high before enabling any of the outputs.
If the voltage at VREG, V_REG , drops below the falling VREG
undervoltage lockout threshold, V_REGUVoff , then the A3941 will
enter the VREG undervoltage fault state. In this fault state, both
FF1 and FF2 will be high, and the outputs will be disabled. The
VREG undervoltage fault state and the fault flags will be cleared
when V_REG rises above the rising VREG undervoltage lockout
threshold, V_REGUVon.
The VREG undervoltage monitor circuit is active during pow-
er-up, and the A3941 remains in the VREG undervoltage fault
state until V_REG is greater than the rising VREG undervoltage
lockout threshold, V_REGUVon.
Bootstrap Capacitor Undervoltage The A3941 monitors the
voltage across the individual bootstrap capacitors to ensure they
have sufficient charge to supply the current pulse for the high-
side drive. Before a high-side drive can be turned on, the voltage
across the associated bootstrap capacitor must be higher than the
turn-on voltage limit. If this is not the case, then the A3941 will
start a bootstrap charge cycle by activating the complementary
low-side drive. Under normal circumstances, this will charge the
bootstrap capacitor above the turn-on voltage in a few microsec-
onds and the high-side drive will then be enabled.
The bootstrap voltage monitor remains active while the high-side
drive is active and, if the voltage drops below the turn-off volt-
age, a charge cycle is initiated.
In either case, if there is a fault that prevents the bootstrap capaci-
tor charging, then the charge cycle will timeout, the fault flags
(indicating an undervoltage) will be set, and the outputs will be
disabled. The bootstrap undervoltage fault state remains latched
until RESET is set low.
V5 Undervoltage The output of the logic supply regulator volt-
age at V5 is monitored to ensure correct logical operation. If
the voltage at V5, V₅ , drops below the falling V5 undervoltage
lockout threshold, V_5UVoff , then the A3941 will enter the V5
undervoltage fault state. In this fault state, both FF1 and FF2 will
be high, and the outputs will be disabled. In addition, because
the state of other reported faults cannot be guaranteed, all fault
states and fault flags are reset and replaced by the fault flags cor-
responding to a V5 undervoltage fault state. For example, a V5
undervoltage will reset an existing short circuit fault condition
and replace it with a V5 undervoltage fault. The V5 undervoltage
fault state and the fault flags will be cleared when V5 rises above
the rising V5 undervoltage lockout threshold defined by V_5UVoff
+ V_5UVhys.
The V5 undervoltage monitor circuit is active during power-up,
and the A3941 remains in the V5 undervoltage fault state until V5
is greater than the rising VREG undervoltage lockout threshold,
V_5UVoff +V_5UVhys.
Short Fault Operation Shorts in the power bridge are determined
by monitoring the drain-souce voltage, V_DS , of each active FET
and comparing it to the fault threshold voltage at the VDSTH pin.
Because power MOSFETs take a finite time to reach the rated
on-resistance, the measured drain-source voltages will show a
fault as the phase switches. To avoid such false short fault detec-
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