Data Sheet

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Use of TAS5142 in High-Modulation-Index
ERROR REPORTING
Overcurrent (OC) Protection With Current
DEVICE PROTECTION SYSTEM
TAS5142
SLES126B – DECEMBER 2004 – REVISED MAY 2005
Characteristics section of this data sheet). Although temperature has dropped or the supply voltage has
not specifically required, it is a good practice to hold increased. For highest possible reliability, recovering
RESET_AB and RESET_CD low during power down, from an overload fault requires external reset of the
thus preventing audible artifacts including pops or device (see the Device Reset section of this data
clicks. sheet) no sooner than 1 second after the shutdown.
When the TAS5142 is being used with TI PWM
modulators such as the TAS5508, no special
Capable Systems
attention to the state of RESET_AB and RESET_CD
This device requires at least 50 ns of low time on the
is required, provided that the chipset is configured as
output per 384-kHz PWM frame rate in order to keep
recommended.
the bootstrap capacitors charged. As an example, if
the modulation index is set to 99.2% in the TAS5508,
this setting allows PWM pulse durations down to 20
The SD and OTW pins are both active-low,
ns. This signal, which does not meet the 50-ns
open-drain outputs. Their function is for
requirement, is sent to the PWM_X pin and this
protection-mode signaling to a PWM controller or
low-state pulse time does not allow the bootstrap
other system-control device.
capacitor to stay charged. In this situation, the low
voltage across the bootstrap capacitor can cause a
Any fault resulting in device shutdown is signaled by
failure of the high-side MOSFET transistor, especially
the SD pin going low. Likewise, OTW goes low when
when driving a low-impedance load. The TAS5142
the device junction temperature exceeds 125 ° C (see
device requires limiting the TAS5508 modulation
the following table).
index to 96.1% to keep the bootstrap capacitor
SD OTW DESCRIPTION
charged under all signals and loads.
0 0 Overtemperature (OTE) or overload (OLP) or
Therefore, TI strongly recommends using a TI PWM
undervoltage (UVP)
processor, such as TAS5508 or TAS5086, with the
0 1 Overload (OLP) or undervoltage (UVP)
modulation index set at 96.1% to interface with
1 0 Junction temperature higher than 125°C
TAS5142.
(overtemperature warning)
1 1 Junction temperature lower than 125°C and no
OLP or UVP faults (normal operation)
Limiting and Overload Detection
Note that asserting either RESET_AB or RESET_CD
The device has independent, fast-reacting current
low forces the SD signal high, independent of faults
detectors with programmable trip threshold (OC
being present. TI recommends monitoring the OTW
threshold) on all high-side and low-side power-stage
signal using the system microcontroller and
FETs. See the following table for OC-adjust resistor
responding to an overtemperature warning signal by,
values. The detector outputs are closely monitored by
e.g., turning down the volume to prevent further
two protection systems. The first protection system
heating of the device resulting in device shutdown
controls the power stage in order to prevent the
(OTE).
output current from further increasing, i.e., it performs
a current-limiting function rather than prematurely
To reduce external component count, an internal
shutting down during combinations of high-level
pullup resistor to 3.3 V is provided on both SD and
music transients and extreme speaker load
OTW outputs. Level compliance for 5-V logic can be
impedance drops. If the high-current situation
obtained by adding external pullup resistors to 5 V
persists, i.e., the power stage is being overloaded, a
(see the Electrical Characteristics section of this data
second protection system triggers a latching
sheet for further specifications).
shutdown, resulting in the power stage being set in
the high-impedance (Hi-Z) state. Current limiting and
overload protection are independent for half-bridges
The TAS5142 contains advanced protection circuitry A and B and, respectively, C and D. That is, if the
carefully designed to facilitate system integration and bridge-tied load between half-bridges A and B causes
ease of use, as well as to safeguard the device from an overload fault, only half-bridges A and B are shut
permanent failure due to a wide range of fault down.
conditions such as short circuits, overload,
• For the lowest-cost bill of materials in terms of
overtemperature, and undervoltage. The TAS5142
component selection, the OC threshold measure
responds to a fault by immediately setting the power
should be limited, considering the power output
stage in a high-impedance (Hi-Z) state and asserting
requirement and minimum load impedance.
the SD pin low. In situations other than overload, the
Higher-impedance loads require a lower OC
device automatically recovers when the fault
threshold.
condition has been removed, i.e., the junction
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