LM49352: lm49352 Class D amplifier LS+/LS- waveform and failure issue.

Roger Luo

Part Number: LM49352

Hi,

As mentioned on datasheet, LS+ and LS- should have a 300KHz waveform after power on. But I get a 450KHz waveform as measured. Please explain how and why the frequency of the waveform is affected?

The speaker has voice for couple minutes even the frequency is 450KHz, but the waveform is gone forever after a few minutes. And it can't be recovered by cut off power. It seems class D amplifier is broken. Can you also explain this?

Thanks,

Roger Luo

over 5 years ago

0 Steve-Wilson over 5 years ago

TI__Mastermind 30030 points

Roger,

can you send me your LM49352 register configuration?

As far as I know there is no way to control the CLASS-D amplifier modulation frequency, and it should not be 450kHz.

best regards,
-STeve Wilson

0 Roger Luo over 5 years ago in reply to Steve-Wilson

Prodigy 160 points

Hi Steven,

The register cofiguration is as below:

(0x00, 0xa9)
(0x01, 0x01)
(0x02, 0x01)
(0x1b, 0x00)
(0x20, 0x12)
(0x21, 0x00)
(0x30, 0x11)
(0x31, 0x00)
(0x40, 0x0f)
(0x41, 0x01)
(0x44, 0x09)
(0x50, 0x2f)
(0x10, 0x03)
(0x14, 0x80)
(0x23, 0x1f)
(0x42, 0x05)
(0x89, 0x64)
(0x8a, 0x33)
(0x81, 0xf0)
(0x82, 0x1f)
(0x83, 0x1f)
(0xa0, 0x10)
(0xa1, 0x1f)
(0xa2, 0x00)
(0xa8, 0x70)
(0xa9, 0x33)
(0x15, 0x0c)
(0x16, 0x08)
(0x54, 0x00)
(0x55, 0x00)
(0x56, 0x00)
(0x70, 0x0e)
(0x71, 0x07)

0 Roger Luo over 5 years ago in reply to Roger Luo

Prodigy 160 points

Hi Steven,

As long as I set four registers below, LS+ and LS- have waveform output and the frequency is always 450KHz.

(0x00, 0xa9)
(0x01, 0x01)
(0x02, 0x01)
(0x10, 0x03)

Please help to find the cause. Is there any side-affect if class D is working on 450KHz?

Thanks,
Roger Luo

0 Roger Luo over 5 years ago in reply to Roger Luo

Prodigy 160 points

0 Roger Luo over 5 years ago in reply to Steve-Wilson

Prodigy 160 points

Hi Steven,

Any comments on this? The volume of speaker is also low. I set the registers 0x14, 0x40, 0xa8 and 0xa9 to maximum value but the volume is still low. Is this related to the frequency of square wave?

Thanks,

Roger Luo

0 Steve-Wilson over 5 years ago in reply to Roger Luo

TI__Mastermind 30030 points

Roger,

if you change reg 0x01 value to 0x02, do you see any change?

best regards,

Steve Wilson

0 Roger Luo over 5 years ago in reply to Steve-Wilson

Prodigy 160 points

Hi Steve,

The frequency of LS+/LS- has no change if I change reg 0x01 value to 0x02. I always get the same frequency 450KHz. And the volume of Speaker is always low even I set the LS amplifier to maximum. Do you think low volume is caused by frequency 450KHz? Are they related?

Thanks,

Roger

0 Steve-Wilson over 5 years ago in reply to Roger Luo

TI__Mastermind 30030 points

Roger,

Do you have a schematic you can share? This is not correct behavior there must be some explanation.
-Steve Wilson

0 Roger Luo over 5 years ago in reply to Steve-Wilson

Prodigy 160 points

0 Roger Luo over 5 years ago in reply to Steve-Wilson

Prodigy 160 points

Hi Steve,

Please check the schematic posted. I'm looking forward to your reply.

Thanks,
Roger

0 Roger Luo over 5 years ago in reply to Steve-Wilson

Prodigy 160 points

Hi Steve,

Any idea for this issue?

Thanks,
Roger

0 Steve-Wilson over 5 years ago in reply to Roger Luo

TI__Mastermind 30030 points

Roger,

two things:

1. You're class D filter does not follow the guidelines in the data sheet.

"A 10pF ceramic capacitor with a X7R dielectric is recommended as a starting point. Care needs to be taken to ensure that the value of the shunt capacitor does not exceed 47pF when using a low resistance ferrite bead in order to prevent permanent damage to the low side FETs of the Class D output stage."

2. I have noticed that you are not providing an external DAC reference. Its possible to use the internal bandgap reference, but you must set bit 1 of register 0xFE to '1'.

best regards,

-Steve wilson

0 Steve-Wilson over 5 years ago in reply to Steve-Wilson

TI__Mastermind 30030 points

Roger,

I finally got my hands on an EVM, and was able to take a look at this.

The issue isn't that the frequency is actually 450kHz, its just that the scope isn't smart enough to tell the frequency.

My scope tells me that it is 450kHz also, but when I capture a half second of data and measure, the frequency moves between 335-350khz or so.

best regards,
-Steve Wilson

0 Roger Luo over 5 years ago in reply to Steve-Wilson

Prodigy 160 points

Hi Steve,

LS+ and LS- are working well now after I change Speakers to 8ohm/1W. But the volume of speaker is always too low even I adjust the registers 0x14, 0x40 and 0xa8. There might be hardware problem to cause this issue. Can you advise?

For DAC reference, can you provide the value of reference voltage?

Thanks,
Roger

0 Steve-Wilson over 5 years ago in reply to Roger Luo

TI__Mastermind 30030 points

Roger,

The EVM allows the user to choose either GND or ADC REF. the ADC ref can either be GND or 3.3V.

if you leave the DAC floating as you have, I measure roughly 2V.

best regards,
-STeve Wilson

0 Steve-Wilson over 5 years ago in reply to Roger Luo

TI__Mastermind 30030 points

Roger,

regarding your low volume level, there are a number of reasons for this.

your register 0x40 setting is 0x0F which means that Port1_RX_L and Port1_RX_R input gain is -18dB.

next your register 0xa8 setting is 0x70 which means that you are linking the DACR_LEVEL to the DACL_LEVEL and that level = -7.5dB.

you do make up for a some of this on the Class-D amp settings in register 0x14 where the CLASS-D amp gets 8dB gain. But you are still attenuating the input signal pretty significantly.

try register 0x40 setting of 0x00, and a register 0xA8 setting of 0x33 (or 0x73 if you want the channel volumes linked)

best regards,
-Steve Wilson

Audio

Audio forum

LM49352: lm49352 Class D amplifier LS+/LS- waveform and failure issue.