According to the datasheet:
SLEW Register(Address = 07h)
b7 b6 b5 b4 b3 b2 b1 b0
48mV/us 42mV/us 36mV/us 30mV/us 24mV/us 18mV/us 12mV/us 6mV/us
Current code does not set correct slew rate in some cases:
e.g. Assume ramp_delay is 10000, current code sets slew register to 6mV/us.
Fix the logic to set slew register.