Speaking from the Nato summit in Wales, the Prime Minister also insisted the Yes campaigners did not have a monopoly on patriotism and that people could be proud Scots but still believe fervently in staying within the family of the UK.
He stressed he would not resign as PM if defeated, stressing that this month's vote was "purely and simply" about Scotland's place within the UK and not about his future as the nation's leader.
Mr Cameron, who has said he is a Unionist "head, heart and soul", strongly denied the Nationalists' repeated charge that the No campaign had been grindingly negative
"I don't think it's been negative or complacent," declared the PM. "The No campaign is working very hard to make an important set of arguments.
"Certainly, my role in the campaign has been to make a wholly positive case, very much saying to people in Scotland - look, it is your choice but be in doubt the rest of the UK - England, Wales and Northern Ireland - we want you to stay.
"My argument throughout, and a very large part of the No campaign's argument, is not saying - look, Scotland can't be independent; of course, it could be independent but we would all be better off by staying together."
Asked if he would put more heart into the campaign and would say 'Scotland we love you, stay with us', he replied: "I have absolutely said that in terms and will say so on my next campaign visit."
While the Conservative leader - regarded as "toxic" to the campaign by many in the No camp - is not coming to Scotland this week, he is expected to do so next week.
Mr Cameron argued being a proud Scot and a proud Brit were not mutually exclusive.
"In fact, I would argue you can be far prouder of your Scottishness, your Scottish identity, your belief in Scottish history and traditions and still believe you can succeed in the UK. It's not true that the only patriotic option is to vote Yes; you can be a proud patriotic Scot but believe in this family of nations we have in the United Kingdom."
Asked if he would resign if there were a Yes vote, showing he had failed to keep alive the 300-year-old Union, he replied: "It's very important to say No to that emphatically for this reason; what is at stake here is not this prime minister or that prime minister or this party leader or that party leader, what is at stake is the future of Scotland. It is for the Scottish people to decide - do you want to separate yourself from the UK or do you want to stay in the UK?"
When it was pointed out that the people of the rest of the UK might feel hard done by if he failed in his mission to keep the country together, he said: "It's very important for people in Scotland to realise the consequence of their vote is purely and simply about Scotland and its place in the UK. We shouldn't tie up into this vote the future of Alex Salmond, the future of me or anyone else."
He added: "In this case, this is about the future of Scotland. It's a desperately important question. I care passionately about it. It would break my heart if Scotland were to leave the UK but I absolutely believe it's right to give people in Scotland the choice...I dearly hope the Scottish people will vote to stay in our family of nations."