Conceptual framework can be done if you are following specific type of methods or techniques for your research where there are already number of methods or techniques are available. you should also emphasis on the method or technique you are using stating your reasons for opting that particularly.

Suryani - do are not forced to do anything. A theoretical (tested) framework assists - and a conceptual (non-tested) framework can also potentially assist. It depends on so many things. The attached chapter chapter might assist.

I agree with Dean. The following works might be helpful if you want to explain the philosophical foundation that informs mixed methods as a research approach:

Article Paradigms Lost and Pragmatism Regained: Methodological Impli...

http://onlinelibrary.wiley.com/doi/10.1002/ev.20054/abstract

http://journals.sagepub.com/doi/abs/10.1177/1558689815575861?journalCode=mmra

I don't think you need a formal conceptual framework, but I would recommend having a clear justification for your design. In particular, a convergent parallel design usually starts with parallel tracks, where the two methods pursue distinct but related purposes, followed by a convergent phase where the two sets of results are compared and integrated.