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SLES256 – MAY 2010
ERROR REPORTING
The FAULT and OTW pins are both active-low,
open-drain
outputs.
Their
function
is
for
protection-mode signaling to a PWM controller or
other system-control device.
Any fault resulting in device shutdown, such as
overtemperatue shut down, overcurrent shut-down, or
undervoltage protection, is signaled by the FAULT pin
going low. Likewise, OTW goes low when the device
junction temperature exceeds 125°C (see
Table 1. Protection Mode Signal Descriptions
FAULT
0
OTW
0
DESCRIPTION
Overtemperature warning and
(overtemperature shut down or overcurrent
shut down or undervoltage protection) occurred
Overcurrent shut-down or GVDD undervoltage
protection occurred
Overtemperature warning
Device under normal operation
Bootstrap Capacitor Under Voltage Protection
When the device runs at a low switching frequency
(e.g. less than 10 kHz with a 100-nF bootstrap
capacitor), the bootstrap capacitor voltage might not
be able to maintain a proper voltage level for the
high-side gate driver. A bootstrap capacitor
undervoltage protection circuit (BST_UVP) will
prevent potential failure of the high-side MOSFET.
When the voltage on the bootstrap capacitors is less
than the required value for safe operation, the
DRV8312/32 will initiate bootstrap capacitor recharge
sequences (turn off high side FET for a short period)
until the bootstrap capacitors are properly charged for
safe operation. This function may also be activated
when PWM duty cycle is too high (e.g. less than 20
ns off time at 10 kHz). Note that bootstrap capacitor
might not be able to be charged if no load or
extremely light load is presented at output during
BST_UVP operation, so it is recommended to turn on
the low side FET for at least 50 ns for each PWM
cycle to avoid BST_UVP operation if possible.
For applications with lower than 10 kHz switching
frequency and not to trigger BST_UVP protection, a
larger bootstrap capacitor can be used (e.g., 1 uF cap
for 800 Hz operation). When using a bootstrap cap
larger than 220 nF, it is recommended to add 5 ohm
resistors between 12V GVDD power supply and
GVDD_X pins to limit the inrush current on the
internal bootstrap diodes.
Overcurrent (OC) Protection
The DRV8312/32 have independent, fast-reacting
current detectors with programmable trip threshold
(OC threshold) on all high-side and low-side
power-stage FETs. There are two settings for OC
protection
through
mode
selection
pins:
cycle-by-cycle (CBC) current limiting mode and OC
latching (OCL) shut down mode.
In CBC current limiting mode, the detector outputs
are monitored by two protection systems. The first
protection system controls the power stage in order to
prevent the output current from further increasing,
i.e., it performs a CBC current-limiting function rather
than prematurely shutting down the device. This
feature could effectively limit the inrush current during
motor start-up or transient without damaging the
device. During short to power and short to ground
conditions, the current limit circuitry might not be able
to control the current to a proper level, a second
protection system triggers a latching shutdown,
resulting in the related half bridge being set in the
high-impedance (Hi-Z) state. Current limiting and
overcurrent
protection
are
independent
for
half-bridges A, B, and C, respectively.
0
1
1
1
0
1
TI recommends monitoring the OTW signal using the
system microcontroller and responding to an OTW
signal by reducing the load current to prevent further
heating of the device resulting in device
overtemperature shutdown (OTSD).
To reduce external component count, an internal
pullup resistor to internal VREG (3.3 V) is provided on
both FAULT and OTW outputs. Level compliance for
5-V logic can be obtained by adding external pull-up
resistors to 5 V (see the
Electrical Characteristics
section of this data sheet for further specifications).
DEVICE PROTECTION SYSTEM
The DRV8312/32 contain advanced protection
circuitry carefully designed to facilitate system
integration and ease of use, as well as to safeguard
the device from permanent failure due to a wide
range of fault conditions such as short circuits,
overcurrent, overtemperature, and undervoltage. The
DRV8312/32 respond to a fault by immediately
setting the half bridge outputs in a high-impedance
(Hi-Z) state and asserting the FAULT pin low. In
situations other than overcurrent or overtemperature,
the device automatically recovers when the fault
condition has been removed or the gate supply
voltage has increased. For highest possible reliability,
reset the device externally no sooner than 1 second
after the shutdown when recovering from an
overcurrent shut down (OCSD) or OTSD fault.
Copyright © 2010, Texas Instruments Incorporated
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