DRV8870: bulk cap_inrush current of drv8870

Hi Danny,

FYI -- An additional reference can be found in Webench: webench.ti.com/.../
Once you reach this site, the ability to create DRV8870/71/72 designs is available.

This blog is also available: e2e.ti.com/.../motor-drive-forum-top-faqs-3-methods-to-prevent-electrical-overstress

Q1. Adequate capacitance of Bulk Cap

1) Could you recommend the adequate capacitance of Bulk Cap when 1A peak of DC motor or Rotary Solenoid ?

>>> The customer knows their system, use case, and layout better than we do, so we do not recommend bulk cap. You can see the bulk cap used on our EVMs.

2) Is there any method(equation) to calculate the capacitance?

>>> You can use this link as a guideline: www.ti.com/.../slta055.pdf

3) If customer use stable main Power from DCDC Regulator (ex, 24VO in the upper schematic), can customer reduce the value of capacitance?

>>> This depends on the distance between the power supply and the bulk capacitance. The bulk capacitors act like a local supply providing current until the power supply can replenish the charge.


Q2. Inrush Current of Bulk Cap

1) On customer’s developing board, there occurred high inrush current due to bulk cap of several motor drivers. Is there any method to reduce it?

>>> Most methods involve reducing the voltage ramp rate or limiting the current. These can be done with transistors that turn on slowly or inductors to limit the current.

2) When customer use several motor driver together in near on board, is it possible to remove some of Bulk Cap?

>>> Yes it is possible, but it should be tested to determine if the system performs properly. Placing footprints for additional bulk and leaving them uninstalled is a good idea.