Table 21. Design Parameters

8.2.2.1 Gate Drive Average Current

The gate drive supply (VCP) of the DRV8305 is capable of delivering up to 30 mA (RMS) of current to the external power MOSFETs. The charge pump directly supplies the high-side N-channel MOSFETs and a 10-V LDO powered from VCP supplies the low-side N-channel MOSFETs. The designer can determine the approximate RMS load on the gate drive supply through the following equation.

Example: 36 nC (Q_G) × 6 (N_SW) × 45 kHz (f_SW) = 9.72 mA

Note that this is only a first-order approximation.

8.2.2.2 MOSFET Slew Rates

The rise and fall times of the external power MOSFET can be adjusted through the use of the DRV8305 IDRIVE setting. A higher IDRIVE setting will charge the MOSFET gate more rapidly where a lower IDRIVE setting will charge the MOSFET gate more slowly. System testing requires fine tuning to the desired slew rate, but a rough first-order approximation can be calculated as shown in the following.

Example: 9 nC (Q_GD) / 50 mA (IDRIVEP) = 180 ns

8.2.2.3 Overcurrent Protection

The DRV8305 provides overcurrent protection for the external power MOSFETs through the use of VDS monitors for both the high-side and low-side MOSFETs. These are intended for protecting the MOSFET in overcurrent conditions and are not for precise current regulation.

The overcurrent protection works by monitoring the VDS voltage drop of the external MOSFETs and comparing it against the internal VDS_LEVEL set through the SPI registers. The high-side VDS is measured across the VDRAIN and SH_X pins. The low-side VDS is measured across the SH_X and SL_X pins. If the VDS voltage exceeds the VDS_LEVEL value, the DRV8305 will take action according to the VDS_MODE register.

The overcurrent trip level can be determined with the MOSFET R_DS(on) and the VDS_LEVEL setting.

Example: 0.197 V (VDS_LVL) / 4.1 mΩ (R_DS(ON)) = 48 A

8.2.2.4 Current Sense Amplifiers

The DRV8305 provides three bidirectional low-side current shunt amplifiers. These can be used to sense the current flowing through each half-bridge. If individual half-bridge sensing is not required, a single current shunt amplifier can be used to measure the sum of the half-bridge current. Use this simple procedure to correctly configure the current shunt amplifiers.

1. Determine the peak current that the motor will demand (IMAX). This demand depends on the motor parameters and the application requirements. IMAX in this example is 14 A.
2. Determine the available voltage output range for the current shunt amplifiers. This will be the ± voltage around the amplifier bias voltage (VBIAS). In this case VBIAS = 1.65 V and a valid output voltage is 0 to 3.3 V. This gives an output range of ±1.65 V.
3. Determine the sense resistor value and amplifier gain settings. The sense resistor value and amplifier gain have common tradeoffs. The larger the sense resistor value, the better the resolution of the half-bridge current. This comes at the cost of additional power dissipated from the sense resistor. A larger gain value allows for the use of a smaller resolution, but at the cost of increased noise in the output signal and a longer settling time. This example uses a 5-mΩ sense resistor and the minimum gain setting of the DRV8305 (10 V/V). These values allow the current shunt amplifiers to measure ±33 A across the sense resistor.