FAULT Pin This is an open drain output fault flag, which indi-
cates a fault condition by its state, as shown in table 2. It must be
pulled to V
DD
with an external resistor, typically 10 to 47 kΩ.
Fault States
Overtemperature If the junction temperature exceeds the
over-temperature threshold, 170°C typical, the A4957 will enter
the overtemperature fault state and FAULT will go low. The
overtemperature fault state, and FAULT, will only be cleared
when the temperature 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 A4957 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 A4957 will
enter VREG undervoltage, a latched fault state. In this fault state
FAULT will be low, and the outputs will be disabled. The VREG
undervoltage fault state and the fault flags will be cleared on
either an xHI or xLO rising edge or when RESETn is pulled low
for for the reset pulse time, t
RES
.
The VREG undervoltage monitor circuit is active during power-
up, and the A4957 remains in the VREG undervoltage fault state
until VREG is greater than the rising VREG undervoltage lockout
threshold, V
REGUVON
.
Bootstrap Capacitor Undervoltage The A4957 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 A4957 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 will be initiated.
In either case, if there is a fault that prevents the bootstrap capaci-
tor charging, then the charge cycle will timeout, FAULT will be
low, and the outputs will be disabled. The bootstrap undervoltage
fault state remains latched until RESET is set low.
VDD Undervoltage The logic supply at VDD is monitored
to ensure correct logical operation. If the voltage at VDD, V
DD
,
drops below the falling VDD undervoltage lockout threshold,
V
DDUVOFF
, then the A4957 will enter the VDD undervoltage
fault state. In this fault state, FAULT will be low, and the outputs
will be disabled. In addition, because the state of other reported
faults cannot be guaranteed, all fault states are reset and replaced
by a VDD undervoltage fault state. For example, a VDD under-
voltage will reset an existing boostrap undervoltage fault condi-
tion and replace it with a VDD undervoltage fault. The VDD
undervoltage fault state and the fault flag will be cleared when
V
DD
rises above the rising VDD undervoltage lockout threshold
defined by V
DDUVOFF
+V
DDUVHYS
.
The VDD undervoltage monitor circuit is active during power-up,
and the A4957 remains in the VDD undervoltage fault state until
V
DD
is greater than the rising VDD undervoltage lockout thresh-
old, V
DDUVOFF
+V
DDUVHYS
.
Full Bridge MOSFET Driver
A4957
8
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Table 2. Fault Definitions
FAULT Pin Setting Fault Description Disable Outputs* Fault Latched Fault Reset Action
High No Fault No – N.A.
Low Overtemperature No No N.A.
Low VDD undervoltage Yes No N.A.
Low VREG undervoltage Yes Yes
Rising Edge on xHI or xLO
or Reset
Low Bootstrap undervoltage Yes Yes
Rising Edge on xHI or xLO
or Reset
*”Yes” indicates all gate drives low, and all MOSFETs off.
