UPDATE 1-ST-Ericsson CEO to step down as overhaul looms

* Didier Lamouche to leave end-March

* STMicro has said it will exit joint venture

* Unit faces shutdown or sale in parts

STOCKHOLM/PARIS, March 11 (Reuters) - The chief executive of STMicroelectronics' unprofitable mobile chip-making joint venture with Sweden's Ericsson will step down at the end of the month, the companies said on Monday, ahead of a planned overhaul.

ST-Ericsson said Didier Lamouche, who took over as CEO of the joint venture in late 2011, was leaving to pursue "other opportunities." His departure will take effect at end-March.

STMicro has already said it plans to exit the joint venture, which has not won enough new customers to compensate for a major drop in business from embattled Nokia. The exit is set to cost the Franco-Italian chipmaker up to $500 million this year.

Ericsson, the company's other 50 percent shareholder, has said it does not want to take over the operation.

Analysts have said ST-Ericsson, which has around 5,000 employees, could be shut down entirely, or parts could be sold to competitors such as Intel, Broadcom or Samsung.

A spokesman for STMicro was unavailable for comment.