AN10327 TDA856x and TDA8571J power amplifiers

AN10327 TDA856x and TDA8571J power amplifiers

Application information for the power amplifier TDA856x series. These amplifiers are mainly used in automotive applications such as car radios, boosters and multimedia applications.

AN10327 TDA856x and TDA8571J power amplifiers Rev. 01.00 — 15 October 2004 Application note Document information Info Content Keywords Automotive, audio, power amplifier, Stereo, Quad, BTL, class AB, bipolar Abstract This document contains application information for the power amplifier TDA856x series and the TDA8571J Philips Semiconductors AN10327 TDA856x ,TDA8571J Revision history Rev Date Description 1.0 20041015 First version Contact information For additional information, please visit: http://www.semiconductors.philips.com sales.addresses@www.semiconductors.philips.comFor sales office addresses, please send an email to: Ã'© Koninklijke Philips Electronics N.V. 2004. All rights reserved. Application note Rev. 01.00 — 15 October 2004 2 of 19 Philips Semiconductors AN10327 TDA856x ,TDA8571J 1. Introduction 1.1 Amplifier overview This document describes the application specific subjects of the following audio power amplifiers : TDA856x family and TDA8571J. These amplifiers, which are made in a bipolar process, are mainly used in automotive applications such as car radios, boosters and multimedia applications. The differences between the types are mainly the number of output channels, different load values and output power. The following matrix shows an overview of the mentioned amplifiers and their properties. Table 1: Amplifier Overview * DDD = Dynamic Distortion Detection Channels Gain [dB] Load [Ohm] Inputs Output power [W] DDD* [%] Package TDA8560Q 2 x BTL 40 2 2 x SE 2 X 40 10 DBS13P TDA8562Q 4 x SE 20 4 4 x SE 4 x 12 10 DBS17P TDA8563Q 2 x BTL 20 2 2 x SE 2 x 40 10 DBS13P TDA8563AQ 2 x BTL 20 2 2 x SE 2 x 40 2.2 DBS13P TDA8566Q 2 x BTL 26 2 2 x diff. 2 x 40 7.5 DBS17P TDA8566TH 2 x BTL 26 2 2 x diff. 2 x 40 10 HSOP20 TDA8567Q 4 x BTL 26 4 4 x SE 4 x 25 10 DBS23P TDA8568Q 4 x BTL 40 4 4 x SE 4 x 25 10 DBS23P TDA8569Q 4 x BTL 26 2 4 X SE 4 x 40 10 DBS23P TDA8571J 4 x BTL 34 4 4 x SE 4 x 26 10 DBS23P Ã'© Koninklijke Philips Electronics N.V. 2004. All rights reserved. Application note Rev. 01.00 — 15 October 2004 3 of 19 Philips Semiconductors AN10327 TDA856x ,TDA8571J 2. Application Information 2.1 Input capacitors The amplifiers need capacitors on the inputs to get a DC decoupling of the input source (pre-amplifier stage). The impedance of the input stage together with the input capacitors, create a low frequency roll-off point. A larger input capacitor means a lower frequency roll-off point. The values that should be used are mentioned in the datasheet of the amplifier type. The following figure shows the influence of the input capacitors on the frequency roll-off point for the TDA8566TH. (A) Input capacitor 470nF (B) Input capacitor 220nF (C) Input capacitor 100nF Fig 1. Roll-off frequency at different input capacitor values The low frequency roll-off point can easily be calculated : totinin dBlow CZ f _ 3_ 2 1 =- Ã'© Koninklijke Philips Electronics N.V. 2004. All rights reserved. Application note Rev. 01.00 — 15 October 2004 4 of 19 Philips Semiconductors AN10327 TDA856x ,TDA8571J For example the low frequency roll-off point for the TDA8566TH, when using 220nF input capacitors, equals : Hzf dBlow 12 10110101202 1 933_ = = — In this case the total input capacitance is halved since the input source is “seeing” the input capacitors in series. This is due to the differential input configuration which is drawn in the next figure. Fig 2. Differential input stage TDA8566TH Furthermore it is recommended to use input capacitors with a low DC leakage (film capacitors), since any DC leakage at the inputs will result in a DC offset at the outputs. Electrolytic capacitors usually have a relatively high DC leakage and should therefore not be used. 2.2 Differential inputs The TDA8566 is provided with differential input circuits. This has the advantage that disturbances on the inputs, with relation to ground, are greatly eliminated. However, if there's a mismatch of the input capacitors, the common mode rejection ratio (CMRR) decreases for low frequencies, since the impedance of the input capacitors will increase then. Ã'© Koninklijke Philips Electronics N.V. 2004. All rights reserved. Application note Rev. 01.00 — 15 October 2004 5 of 19 Philips Semiconductors AN10327 TDA856x ,TDA8571J The next figure shows the CMRR of the different input capacitor configurations : (A) 100nF input capacitors, unmatched ( Ã'© Koninklijke Philips Electronics N.V. 2004. All rights reserved. Application note Rev. 01.00 — 15 October 2004 6 of 19 Philips Semiconductors AN10327 TDA856x ,TDA8571J The ground of the power supply (car-battery) is connected to the output of the amplifier, instead of to the amplifier-ground, after which the amplifier is switched on. In a practical situation a loss of ground condition could occur during assembly in the factory, the car manufacturer (OEM) or in the case of an aftersales customer. The following picture shows a loss of ground condition Fig 4. Loss of ground 0 Power Amplifier Vp loss of ground + Vp 14.4V V1Cvp Ground + Input Output According to figure 4, during a LOG, the peak current which charges the buffer capacitor Cvp, will flow from Cvp into the amplifier ground pin and can destroy the amplifier. Ã'© Koninklijke Philips Electronics N.V. 2004. All rights reserved. Application note Rev. 01.00 — 15 October 2004 7 of 19 Philips Semiconductors AN10327 TDA856x ,TDA8571J Fig 5. Amplifier simplified internal schematic Q6 NPN Lower power D1 Diode parasitic + Q1 PNP Vp – 0 current mirror Q5 NPN parasiticQ4 NPN Vp Q2 PNP Q3 NPN Upper power Output power stage one channel C Vp output Gnd According to the internal schematic of the amplifier, the failure mechanism is described step by step. During a Loss of ground, when the amplifier is turned on : 1. The buffer capacitor Cvp is charged and the current flows from Cvp to the amplifier ground pin via the parasitic diode D1 to ground 2. Since D1 is conducting, the voltage on the collector of the lower power Q6 equals Ã'­ 0.7V (under substrate level) 3. This causes a turn-on of a parasitic NPN Q5 4. The current mirror is `activated' and pulls a current 5. Then the upper power Q3 will be turned on and a very large current will flow, since the full Vp is across it 6. This will destroy the upper power transistor In order to withstand the LOG it has to be prevented that the upper power is conducting. The root cause is the conduction of the parasitic diode D1, which causes a substrate level of Ã'­0.7V. To prevent the conduction of D1 it is adviced to use a schottky diode between each of the outputs and ground, according to figure 6. (So for a 4 channel BTL amplifier 8 schottky diodes are to be used) Ã'© Koninklijke Philips Electronics N.V. 2004. All rights reserved. Application note Rev. 01.00 — 15 October 2004 8 of 19 Philips Semiconductors AN10327 TDA856x ,TDA8571J Since the schottky diode has a lower treshold (0.1 .. 0.3V) it will prevent a current flow through D1 and so the turn-on of the upper power. During turn-on of the amplifier, the capacitor will be charged via the schottky diode instead of via D1. For the schottky D2 it is recommended to use a Philips type BYV10-40 or a double SMD type BAT140A. Fig 6. Schottky diode D1 Diode parasitic output Q2 PNP Q4 NPN 0 Vp Vp Q6 NPN Lower power current mirror BAT140A Output power stage one channel C Vp BYV10-40 Gnd D2 DIODE SCHOTTKY Q5 NPN parasitic + Q3 NPN Upper power -Q1 PNP 2.4 Critical conditions 2.4.1 Stability When using capacitors from the outputs to ground (EMC) one must consider that the TDA856x / TDA8571 is stable for capacitances smaller than 2.2nF and larger than 100nF. So, when capacitors are used outside of this range, boucherot filters at the outputs could be necessary. Ã'© Koninklijke Philips Electronics N.V. 2004. All rights reserved. Application note Rev. 01.00 — 15 October 2004 9 of 19 Philips Semiconductors AN10327 TDA856x ,TDA8571J 2.4.2 Ground loops Ground loops are unwanted signal paths that can occur during measurements of the power amplifier, which can result in a higher THD performance of the amplifier. A many seen fault is after connecting two ground connectors of an oscilloscope probe : one at the signal ground of the input of the amplifier and one on the ground of the power supply. The same condition holds when connecting an audio analyser (Audio Precision). In this case when the ground connector (cable shield) is connected to the amplifier input signal ground and when the output is measured, while its ground connector (cable shield) is connected to the power supply ground. The following drawing shows such a ground loop condition Fig 7. Ground loop In practice one should always try various ground connections when measuring THD. However, in many cases it is adviced to use only one ground connection from the measuring device to the power amplifier board. To check if a ground loop is present, measure the distortion residue on an oscilloscope together with the output signals of the amplifier. The distortion residue is usually a monitor output on an audio analyser, eg. Audio Precision System Two, which shows the difference between the shape of the original waveform that is put on the input of the power amplifier and the waveform that is present on the output. (be aware of that the Audio Precision System Two does not scale this distortion residue !) The distortion residue shows a groundloop; the waveform shows the rectified frequency of the signal that is put on the amplifier inputs. The following picture shows an example of a ground loop. Ã'© Koninklijke Philips Electronics N.V. 2004. All rights reserved. Application note Rev. 01.00 — 15 October 2004 10 of 19