TAS5342DDVRG4 datasheet(22/29 Pages) TI

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Overcurrent (OC) Protection With Current

Pin-To-Pin Short Circuit Protection System

TAS5342

SLAS557B–SEPTEMBER 2007–REVISED OCTOBER 2007

voltage on the bootstrap capacitors is less than

In general, it is recommended to follow closely the

required for proper control of the High-Side

external component selection and PCB layout as

MOSFETs, the device will initiate bootstrap capacitor

given in the Application section.

recharge sequences until the bootstrap capacitors are

For added flexibility, the OC threshold is

properly charged for robust operation. This function

programmable within a limited range using a single

may be activated with PWM pulses less than 30 nS.

external resistor connected between the OC_ADJ pin

Therefore, TI strongly recommends using a TI PWM

and AGND. (See the Electrical Characteristics section

processor, such as TAS5518, TAS5086 or TAS5508,

of this data sheet for information on the correlation

with the modulation index set at 97.7% to interface

between programming-resistor value and the OC

with TAS5342.

threshold.) It should be noted that a properly

functioning overcurrent detector assumes the

presence of a properly designed demodulation filter at

Limiting and Overload Detection

the power-stage output. Short-circuit protection is not

provided directly at the output pins of the power stage

The device has independent, fast-reacting current

but only on the speaker terminals (after the

detectors with programmable trip threshold (OC

demodulation filter). It is required to follow certain

threshold) on all high-side and low-side power-stage

guidelines when selecting the OC threshold and an

FETs. See the following table for OC-adjust resistor

appropriate demodulation inductor:

values. The detector outputs are closely monitored by

OC-Adjust Resistor Values

Max. Current Before OC Occurs

two protection systems. The first protection system

Ω)

(A), TC=75°C

controls the power stage in order to prevent the

10.1

output current from further increasing, i.e., it performs

a current-limiting function rather than prematurely

9.1

shutting down during combinations of high-level

7.1

music transients and extreme speaker load

impedance drops. If the high-current situation

The reported max peak current in the table above is

persists, i.e., the power stage is being overloaded, a

measured with continuous current in 1 Ω, one

second protection system triggers a latching

channel active and the other one muted.

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

(PPSC)

A and B and, respectively, C and D. That is, if the

The PPSC detection system protects the device from

bridge-tied load between half-bridges A and B causes

permanent damage in the case that a power output

an overload fault, only half-bridges A and B are shut

pin (OUT_X) is shorted to GND_X or PVDD_X. For

down.

comparison the OC protection system detects an over

• For the lowest-cost bill of materials in terms of

current after the demodulation filter where PPSC

component selection, the OC threshold measure

detects shorts directly at the pin before the filter.

should be limited, considering the power output

PPSC detection is performed at startup i.e. when

requirement and minimum load impedance.

VDD is supplied, consequently a short to either

Higher-impedance loads require a lower OC

GND_X or PVDD_X after system startup will not

threshold.

activate the PPSC detection system. When PPSC

• The demodulation-filter inductor must retain at

detection is activated by a short on the output, all half

least 5 μH of inductance at twice the OC threshold

bridges are kept in a Hi-Z state until the short is

setting.

removed, the device then continues the startup

sequence and starts switching. The detection is

Unfortunately, most inductors have decreasing

controlled globally by a two step sequence. The first

inductance

with

increasing

temperature

and

step ensures that there are no shorts from OUT_X to

increasing current (saturation). To some degree, an

GND_X, the second step tests that there are no

increase in temperature naturally occurs when

shorts from OUT_X to PVDD_X. The total duration of

operating at high output currents, due to core losses

this process is roughly proportional to the capacitance

and the dc resistance of the inductor's copper

of the output LC filter. The typical duration is < 15

winding. A thorough analysis of inductor saturation

ms/μF. While the PPSC detection is in progress, SD

and thermal properties is strongly recommended.

is kept low, and the device will not react to changes

Setting the OC threshold too low might cause issues

applied to the RESET pins. If no shorts are present

such as lack of enough output power and/or

the PPSC detection passes, and SD is released. A

unexpected shutdowns due to too-sensitive overload

device reset will not start a new PPSC detection.

detection.

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