LMH1251, wrong pulse width of V sync.

Shinichi Inoue

Guru 19671 points

Other Parts Discussed in Thread: LMH1251

Dear Specialists,

My customer is using DM365 and LMH1251.

Sometimes V Sync pulse width is doubled and LCD.

He can't find what is the reason.

Could you please see attached file and advise?

What point should he confirm?

I appreciate your great help.

Best regards,

Shinichi

Problem of LMH1251.xlsx

over 8 years ago

0 Hooman Hashemi over 8 years ago

TI__Mastermind 22825 points

Hello Shinichi,
The LMH1251 produces accurate H and V sync leading / rising edge, but the trailing / falling edge is set by RC delay (in certain cases) which may not be accurate enough. This is also discussed briefly in the datasheet section “H SYNC AND V SYNC CHARACTERISTICS” on page 9.

If your LCD monitor is using the negative edge for syncing, I recommend that you invert the H and V sync pulses before presenting them to the LCD monitor.

Regards,
Hooman

0 Shinichi Inoue over 8 years ago in reply to Hooman Hashemi

Guru 19671 points

Dear Hooman

Thank you for your reply.

In this case, Vsync pulse is produced by V, sometimes Vsync pulse width became twice.

Is this an assumable result?

The customer wants to know why this phenomenon is happened and what is a measure.

I appreciate your great help.

Best regards,

Shinichi

0 Hooman Hashemi over 8 years ago in reply to Shinichi Inoue

TI__Mastermind 22825 points

Hi Shinichi,
From talking to the designer, the answer I got was "the trailing edge is not going to occur at an accurate point, but the leading edge will be". Since he is no longer in my group, unfortunately I cannot push him for more detail.
I hope you understand.

Regards,
Hooman

0 Shinichi Inoue over 8 years ago in reply to Hooman Hashemi

Guru 19671 points

Dear Hooman

Thank you for your quick reply.

I understand It must use leading edge.

I'll send your information to the customer.

I appreciate your great help.

Best regards,
Shinichi

Data converters

Data converters forum

LMH1251, wrong pulse width of V sync.