Story highlights The Gates Foundation has already contributed $650 million to the Global Fund

Gates announces a new donation of $750 million to fight AIDS, tuberculosis and malaria

"I can't think of more important work," the Microsoft co-founder says

The Bill & Melinda Gates Foundation will inject $750 million into the Global Fund to Fight AIDS, Tuberculosis and Malaria, Microsoft co-founder Bill Gates announced Thursday at the World Economic Forum.

The donation comes in the form of a promissory note, not as cash, which the Gates Foundation said "gives the Global Fund the flexibility and authority to distribute funds efficiently based on immediate needs."

"By supporting the Global Fund, we can help to change the fortunes of the poorest countries in the world," Gates said in a statement. "I can't think of more important work."

At a news conference at the World Economic Forum in Davos, Gates said the funds could be used immediately to "saves lives, whether it is bed nets (to protect against mosquitoes carrying malaria) or TB (tuberculosis) treatment, those are two diseases that don't get perhaps the visibility of the work done in HIV but they are every bit as important."

The investment comes on top of $650 million the Gates Foundation has already contributed since the Global Fund was launched 10 years ago.

The fund has been under scrutiny after controversy over the possible misuse of funds. Dr. Michel Kazatchkine, executive director of the fund, said Tuesday he would resign in March after leading the organization for five years. Kazatchkine cited the fund's decision to appoint a general manager as part of its "ambitions transformation plan" as the reason for his departure.

Speaking to journalists with Simon Bland, the Global Fund's chair, Gates downplayed the controversy.

The way it had been written about was "pretty disappointing," he said. "If you are going to do health programs in Africa, you are going to have some percentage that is misused."

"The interest is saving lives," Gates said, adding there were "all sorts of things that are going on that far overwhelm any amount of misdirection or whatever it was."