Datasheet

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Use of TAS5162 in High-Modulation-Index
Overcurrent (OC) Protection With Current
Overtemperature Protection
TAS5162
SLES194D – OCTOBER 2006 – REVISED JULY 2007
overload protection are independent for half-bridges
Capable Systems
A and B and, respectively, C and D. That is, if the
bridge-tied load between half-bridges A and B
This device requires at least 50 ns of low time on the
causes an overload fault, only half-bridges A and B
output per 384-kHz PWM frame rate in order to keep
are shut down.
the bootstrap capacitors charged. As an example, if
• For the lowest-cost bill of materials in terms of
the modulation index is set to 99.2% in the TAS5508,
component selection, the OC threshold measure
this setting allows PWM pulse durations down to 20
should be limited, considering the power output
ns. This signal, which does not meet the 50-ns
requirement and minimum load impedance.
requirement, is sent to the PWM_X pin and this
Higher-impedance loads require a lower OC
low-state pulse time does not allow the bootstrap
threshold.
capacitor to stay charged. In this situation, the low
voltage across the bootstrap capacitor can cause a
• The demodulation-filter inductor must retain at
failure of the high-side MOSFET transistor,
least 5 μ H of inductance at twice the OC
especially when driving a low-impedance load. The
threshold setting.
TAS5162 device requires limiting the TAS5508
Unfortunately, most inductors have decreasing
modulation index to less than 97.0% to keep the
inductance with increasing temperature and
bootstrap capacitor charged under all signals and
increasing current (saturation). To some degree, an
loads.
increase in temperature naturally occurs when
The device contains bootstrap capacitor under
operating at high output currents, due to core losses
voltage protection circuit (BST_UVP) that monitors
and the dc resistance of the inductor's copper
the voltage on the bootstrap capacitors. When the
winding. A thorough analysis of inductor saturation
voltage on the on the bootstrap capacitors is less
and thermal properties is strongly recommended.
than required for safe operation, the TAS5162 will
Setting the OC threshold too low might cause issues
initiate bootstrap capacitor recharge sequences until
such as lack of enough output power and/or
the bootstrap capacitors are properly charged for
unexpected shutdowns due to too-sensitive overload
safe operation. This function may be activated at a
detection.
modulation index of greater than 97.0%
In general, it is recommended to follow closely the
TI strongly recommends using a TI PWM processor,
external component selection and PCB layout as
such as TAS5508 or TAS5086, with the modulation
given in the Application section.
index set at 96.1% to interface with TAS5162.
For added flexibility, the OC threshold is
The Modulation Index is set by writing 0x04 to the
programmable within a limited range using a single
Modulation Index Limit Register (0x16) in the
external resistor connected between the OC_ADJ pin
TAS5508B or TAS5518A. In the case of the
and AGND. (See the Electrical Characteristics
TAS5086 a 0x04 is written to the Modulation Limit
section of this data sheet for information on the
Register (0x10).
correlation between programming-resistor value and
the OC threshold.) It should be noted that a properly
functioning overcurrent detector assumes the
Limiting and Overload Detection
presence of a properly designed demodulation filter
The device has independent, fast-reacting current
at the power-stage output. Short-circuit protection is
detectors with programmable trip threshold (OC
not provided directly at the output pins of the power
threshold) on all high-side and low-side power-stage
stage but only on the speaker terminals (after the
FETs. See the following table for OC-adjust resistor
demodulation filter). It is required to follow certain
values. The detector outputs are closely monitored
guidelines when selecting the OC threshold and an
by two protection systems. The first protection
appropriate demodulation inductor:
system controls the power stage in order to prevent
OC-Adjust Resistor Values Max. Current Before OC Occurs
the output current from further increasing, i.e., it
(k Ω ) (A)
performs a current-limiting function rather than
22 12.2
prematurely shutting down during combinations of
27 10.5
high-level music transients and extreme speaker
load impedance drops. If the high-current situation 47 6.4
persists, i.e., the power stage is being overloaded, a
68 4.0
second protection system triggers a latching
100 3.0
shutdown, resulting in the power stage being set in
the high-impedance (Hi-Z) state. Current limiting and
The TAS5162 has a two-level temperature-protection
22
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