The VC firm that warned its portfolio companies of tough times ahead led a Series C investment round in mobile ad network provider AdMob.

Sequoia Capital’s Growth Fund led the $15.7 million round with existing investor Accel Partners. The round is intended to fund AdMob’s global expansion.

AdMob reports strong growth over the past year, more than tripling the number of ads served on a monthly basis to 4.5 billion in September. 

AdMob plans to boost growth by hiring local staff in key markets and adding new language interfaces, with an emphasis on Western Europe, India, South Africa and Japan. It also will increase investment in its mobile technology platform, expand offerings for both publishers and advertisers and grow the AdMob sales and development teams in the United States.

AdMob also announced its publisher network now includes more than 6,000 partners worldwide. New publishers in 2008 include MySpace and MovieTickets.com.

“AdMob is well capitalized and has achieved cash flow positive results, giving them a unique opportunity in this difficult economic climate to aggressively capture market share,” said Jim Goetz, partner, Sequoia Capital, in a press release. “They have some of the same qualities that we saw in Google, Yahoo!, YouTube and Apple, including their exceptional team and powerful technology platform.”