DRV8307: DRV8307 to change direction, the Motor speed is different ?

Part Number:DRV8307

Hello ! everyone, I have some problem of DRV8307 

According the DRV8307 EVM Schematic, I design the circuit to drive the BLDC Motor, like the picture below:

I use msp430g2553 I/O pin to connect to ENABLE,DIR,BRAKE pin,

and generate a 20KHz, duty 25% PWM to DRV8307 PWM pin.

start the motor, ENABLE = LOW, BRAKE = LOW,

stop the motor, ENABLE = HIGH, BRAKE = HIGH,

DIR =  HIGH, The motor  rotates Clockwise OK, the motor speed is OK

DIR =  LOW, The motor  rotates counterclockwise OK,but the motor speed is slow.

I read the datasheet, everytime start the Motor, Increase the PWM duty 1% to 25% slowly

But the problem is still the same.,Does anyone help me to figure out it ?  Thanks