Different viewport widths for landscape and portrait

My website is more or less optimized for mobile devices, however it needs a minimum width of 480 px. Using

<meta name="viewport" content="width=480">

works well, but only in portrait mode. If I switch to landscape, it uses 480 px too, which makes everything too big. Is there a way with maybe a script to switch to a higher width if landscape (mobile users only) is used? Thank you.