DRV8704 Datasheet, PDF (25/38 Pages) Texas Instruments – Dual H-Bridge PWM Gate Driver

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DRV8704
SLVSD29 – OCTOBER 2015
Example:
If the gate-to-drain charge is 5.9 nC, and the desired rise time is around 20 to 100 ns:
IDRIVE₁ = 6.7 nC / 20 ns = 335 mA
IDRIVE₂ = 6.7 nC / 100 ns = 67 mA
Select IDRIVE between 67 and 335 mA.
We select IDRIVEP as 200-mA source and IDRIVEP as 400-mA sink.
We select TDRIVEN and TDRIVEP as 525 ns.
8.2.2.3 Current Chopping Configuration
The chopping current is set based on the sense resistor value, shunt amplifier gain set by the ISGAIN register,
and the TORQUE register setting. The following is used to calculate the current:
I_CHOP
2.75 V u TORQUE
256 u ISGAIN u R_ISENSE
(10)
Example:
If the desired chopping current is 5.5 A:
Set R_SENSE = 100 mΩ.
Set ISGAIN to the 5 V/V setting.
The TORQUE register can be (decimal) 255.
8.2.2.4 Decay Modes
The DRV8704 supports several different decay modes: slow decay, fast decay, mixed decay, and automatic
mixed decay. The current through the motor windings is regulated using an adjustable fixed-time-off scheme.
This means that after any drive phase, when a motor winding current has hit the current chopping threshold
(I_TRIP), the DRV8704 will place the winding in one of the decay modes for TOFF. After TOFF, a new drive phase
starts.
8.2.2.5 Sense Resistor
For optimal performance, it is important for the sense resistor to be:
• Surface-mount
• Low inductance
• Rated for high enough power
• Placed closely to the motor driver
The power dissipated by the sense resistor equals I_RMS ² × R. For example, if peak motor current is 3 A, RMS
motor current is 2 A, and a 0.05-Ω sense resistor is used, the resistor will dissipate 2 A² × 0.05 Ω = 0.2 W. The
power quickly increases with higher current levels.
Resistors typically have a rated power within some ambient temperature range, along with a derated power curve
for high ambient temperatures. When a PCB is shared with other components generating heat, margin should be
added. It is always best to measure the actual sense resistor temperature in a final system, along with the power
MOSFETs, as those are often the hottest components.
Because power resistors are larger and more expensive than standard resistors, it is common practice to use
multiple standard resistors in parallel, between the sense node and ground. This distributes the current and heat
dissipation.
Copyright © 2015, Texas Instruments Incorporated
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