What is meant constant voltage control of h-bridge
The LV8075LP has internal analog buffers that drive the motor with a voltage that is 2x the voltage on the VC pin. This is the the reason it is called a constant voltage driver. The Vref pin provides a 1.5 volt reference that can be used as a voltage reference to connect directly to the VC pin or connect to the VC pin through a  voltage divider. To drive the motor with the full supply voltage (full output drive), connect VC to VCC. 

A PWM control scheme can be implemented in two ways:
 The first is it to drive the IN1 or IN2 pin with a PWM signal depending on which direction the motor should turn.
 The second is to used a filtered PWM signal to provide a DC voltage to the VC pin and use the constant voltage driver in the chip to drive the motor. Keeping in mind that the motor voltage is 2x the VC voltage.