DRV8305_16 Datasheet, PDF (46/59 Pages) Texas Instruments – Three Phase Gate Driver

DRV8305
SLVSCX2B – AUGUST 2015 – REVISED FEBRUARY 2016
8.2.1 Design Requirements
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Table 20. Design Parameters
DESIGN PARAMETER
Supply voltage
Motor winding resistance
Motor winding inductance
Motor poles
Motor rated RPM
Number of MOSFETs switching
Switching frequency
IDRIVEP
IDRIVEN
MOSFET Q_G
MOSFET Q_GD
MOSFET R_DS(on)
Target full-scale current
Sense resistor
V_DS trip level
Amplifier bias
Amplifier gain
REFERENCE
PVDD
M_R
M_L
M_P
M_RPM
N_SW
f_SW
I_DRIVEP
I_DRIVEN
Q_g
Q_GD
R_DS(on)
I_MAX
R_SENSE
V_{DS_LVL}
V_BIAS
Gain
VALUE
12 V
0.5 Ω
0.28 mH
16 poles
2000 RPM
6
45 kHz
50 mA
60 mA
36 nC
9 nC
4.1 mΩ
30 A
0.005 Ω
0.197 V
1.65 V
10 V/V
8.2.2 Detailed Design Procedure
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.
Gate Drive RMS Current = MOSFET Q_g × Number of Switching MOSFETs × Switching Frequency
(2)
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.
MOSFET Slew Rate = MOSFET Q_GD / IDRIVE Setting
(3)
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.
Overcurrent Trip = VDS Level (VDS_LVL) / MOSFET R_DS(on) (R_DS(on))
(4)
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