Shimizu-san, if the DRV8870 VREF input is grounded, the ITRIP formula goes to zero, and the device won't drive much current.
A good solution here is to use the DRV8872 instead. It has an internal "3.5V VREF", and instead of having a VREF pin there's an open-drain fault status output. nFAULT can be left unconnected if you don't need to monitor it.
Best regards,
RE