This is a common problem/question. Check down through this 'Air Travel & Getting To Tobago' sub-forum and you will find various topics that cover it, with responses from people who have experience of the issues involved. The most recent appears to be viewtopic.php?f=2&t=9559
In summary, 1 1/2 hours is tight. The problem is that your passage won't be checked right through to Tobago. What I mean by this is that on arrival in Trinidad, you will have to pass through immigration, collect your baggage and leave the arrivals lounge, then make you way to the departures area where you will have to check in with Caribbean Airways for the flight to Tobago. To be honest, unless your flight lands early, rather than late, and you get through within an hour, you could have a problem.