ALV tree problem in downloading to excel file.

I have a problem in downloading ALV tree to excel file, since there is no standard functionality in ALV tree tool bar, I found other way.

we can do this by  using standard print preview button , where user can download  to excelfile.

The path is :

go to print preview button

go to  system - list - save -  localfile.

but I want to implement this using a custom push button on the ALV tool bar , I was unable to do this.

I have tried to debug the standard print preview button how exactly it is downloading , but I am not able to figure out how this is done , which function module to call.

I have seen during debug it is using FM LIST_DOWNLOAD and LIST_CONVERT_TO_DAT

I tried these two but i was not successful.

*  CALL FUNCTION 'LIST_DOWNLOAD'

*    EXPORTING

*      list_index          = '0'

*      method              = ' '

*    EXCEPTIONS

*      list_download_error = 1

*      OTHERS              = 2.

but while executing it goes to dump that invalid list index.

can any one help me out .