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SLOS556 – NOVEMBER 2008
DETAILED DESCRIPTION
POWER SUPPLY
To facilitate system design, the TAS5707 needs only a 3.3-V supply in addition to the (typical) 18-V power-stage
supply. An internal voltage regulator provides suitable voltage levels for the gate drive circuitry. Additionally, all
circuitry requiring a floating voltage supply, e.g., the high-side gate drive, is accommodated by built-in bootstrap
circuitry requiring only a few external capacitors.
In order to provide good electrical and acoustical characteristics, the PWM signal path for the output stage is
designed as identical, independent half-bridges. For this reason, each half-bridge has separate bootstrap pins
(BST_X), and power-stage supply pins (PVDD_X). The gate drive voltages (GVDD_AB and GVDD_CD) are
derived from the PVDD voltage. Special attention should be paid to placing all decoupling capacitors as close to
their associated pins as possible. In general, inductance between the power-supply pins and decoupling
capacitors must be avoided.
For a properly functioning bootstrap circuit, a small ceramic capacitor must be connected from each bootstrap pin
(BST_X) to the power-stage output pin (OUT_X). When the power-stage output is low, the bootstrap capacitor is
charged through an internal diode connected between the gate-drive regulator output pin (GVDD_X) and the
bootstrap pin. When the power-stage output is high, the bootstrap capacitor potential is shifted above the output
potential and thus provides a suitable voltage supply for the high-side gate driver. In an application with PWM
switching frequencies in the range from 352 kHz to 384 kHz, it is recommended to use 33-nF ceramic capacitors,
size 0603 or 0805, for the bootstrap supply. These 33-nF capacitors ensure sufficient energy storage, even
during minimal PWM duty cycles, to keep the high-side power stage FET (LDMOS) fully turned on during the
remaining part of the PWM cycle.
Special attention should be paid to the power-stage power supply; this includes component selection, PCB
placement, and routing. As indicated, each half-bridge has independent power-stage supply pins (PVDD_X). For
optimal electrical performance, EMI compliance, and system reliability, it is important that each PVDD_X pin is
decoupled with a 100-nF ceramic capacitor placed as close as possible to each supply pin.
The TAS5707 is fully protected against erroneous power-stage turnon due to parasitic gate charging.
ERROR REPORTING
Any fault resulting in device shutdown is signaled by the FAULT pin going low (see
A sticky version of
this pin is available on D1 of register 0X02.
Table 1. FAULT Output States
FAULT
0
1
DESCRIPTION
Overcurrent (OC) or undervoltage (UVP) error or overtemperature error (OTE) or over
voltage ERROR
No faults (normal operation)
DEVICE PROTECTION SYSTEM
Overcurrent (OC) Protection With Current Limiting
The device has independent, fast-reacting current detectors on all high-side and low-side power-stage FETs. The
detector outputs are closely monitored by two protection systems. The first protection system controls the power
stage in order to prevent the output current further increasing, i.e., it performs a cycle-by-cycle current-limiting
function, rather than prematurely shutting down during combinations of high-level music transients and extreme
speaker load impedance drops. If the high-current condition situation persists, i.e., the power stage is being
overloaded, a second protection system triggers a latching shutdown, resulting in the power stage being set in
the high-impedance (Hi-Z) state. The device returns to normal operation once the fault condition (i.e., a short
circuit on the output) is removed. Current limiting and overcurrent protection are not independent for half-bridges.
That is, if the bridge-tied load between half-bridges A and B causes an overcurrent fault, half-bridges A, B, C,
and D are shut down.
Copyright © 2008, Texas Instruments Incorporated
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