The Bill & Melinda Gates Foundation is planning to use its massive endowment to offer $400 million in new low-interest loans, loan guarantees and equity investments.

It's a significant step by the world's largest private charity to use more of its $30 billion in assets to finance program-related investing. I first wrote about the idea here.

The $400 million will be used for a range of new opportunities, including charter school expansion, agricultural financing for small farmers in Africa, and investment in global health technologies, according to people familiar with the plan.

The program-related investments or PRIs will be managed by Gates Foundation Chief Financial Officer Alexander Friedman and his staff. The Gates Foundation will seek partners for joint investments, aligned with its strategies in areas such as global health, U.S. education, agricultural development and financial services for the poor, to more than double the total funding.

U.S. foundations have hundreds of billions of dollars in their endowments, but pay out just 5 percent of the total each year. Meanwhile the economic downturn has forced them to think about new ways to achieve their charitable goals. Foundation endowments plunged an average of 26 percent last year.