Raj Rajaratnam, the Galleon Group hedge fund founder convicted of insider trading in May, asked a federal judge to impose a sentence substantially below what U.S. guidelines recommend, saying a long prison term would be tantamount to a death sentence.

Under federal rules, prosecutors have said, the one-time billionaire faces between 15-1/2 and 19-1/2 years in prison.

In a court filing, lawyers for the 54-year-old Rajaratnam said such a sentence would be unfair and overstate the seriousness of the offense.

Mr. Rajaratnam's failing health and the unique constellation of ailments ravaging his body mean, quite simply, that a lengthy period of imprisonment will constitute a death sentence and result in the permanent and final separation of Mr. Rajaratnam from his family, said the filing.

U.S. District Judge Richard Holwell is expected to sentence Rajaratnam on September 27 and need not follow the federal guidelines.

Rajaratnam was found guilty on five counts of conspiracy and nine counts of securities fraud in May.

Prosecutors accused him of trading on inside information from corporate executives, traders and others, resulting in $63.8 million of illegal profit.

The government is expected to file its own sentencing memorandum later on Tuesday.

The case is USA v Raj Rajaratnam et al, U.S. District Court for the Southern District of New York, No. 09-01184.

(Editing by Richard Chang and Steve Orlofsky)