FRANKFURT (Reuters) – China’s top state silicon chipmaker has indirectly raised its stake in Dialog Semiconductor (DLGS.DE), whose share price has plummeted on signs it could lose its top customer Apple Inc (AAPL.O).

According to a regulatory filing on Tuesday, a subsidiary of Chinese chipmaker Tsinghua Unigroup Ltd increased its stake to 7.15 percent on Nov. 30, the day Dialog shares fell by 20 percent on a report that Apple may design its own power-management chips.

As the purchases pushed Tsinghua’s holding above 7 percent, they triggered a compulsory filing. Anglo-German chipmaker Dialog declined further comment.

Tsinghua Unigroup, which could not immediately be reached for comment, has in the past been blocked on U.S. national security grounds in its attempts to take over Western chip makers, including Germany’s Aixtron SE (AIXGn.DE).

Its investment vehicle is now Dialog’s largest individual shareholder, having bought most of its stake on the open market earlier this year, according to a source familiar with the matter.

Dialog Semiconductor acknowledged for the first time on Monday that Apple could develop its own battery-saving chips used in iPhones, sending Dialog’s shares down another 24 percent.

Tsinghua Unigroup also controls a company called Spreadtrum Communications, a developer of mobile chipset platforms for smartphones with which Dialog does business in China.

Tsinghua Unigroup, which bought Spreadtrum in 2013, has said it wants to float the company next year.

Dialog shares recovered more than 5 percent on Tuesday, to trade just shy of 25 euros. They have shed around a third in value since Japan’s Nikkei business daily reported last week Apple may take power-management chip design for its iPhones in-house.

CEO Jalal Bagherli, acknowledging on Monday that Dialog might lose the Apple business, also said there was no risk to existing supply deals in 2018 and Dialog was in advanced work with Apple on designing 2019-type products.