DRV8873: Datasheet Clarification

Mark Philip Felipe

Part Number: DRV8873
Other Parts Discussed in Thread: DRV8833

Hi Team, seeking some clarifications.

1. Can DRV8873 control a step motor as DRV8833 (see below block diagram)? Or equivalently is it 2 legs (pair with upper and down MOSFET, see below block diagram) or 4 legs like DRV8833?

DRV8833 (4 x 4 = 16 heaters per board)

DRV8873 (4 x 2 = 8 heaters per board)

2. If 4 legs, can DRV8873 (pin 14, 15, 18 and 19) be controlled for each leg like DRV8833? From the Table 7-23, it looks like being able control each leg respectively

3. Look like both mode EN/IN1 and PH/IN2 as well as mode PWM can apply PWM signals up to 100KHz but how about the independent mode, what type of control signal will be?

4. If question 2 can control each of four outputs individually, how to control each one output with only two control pins as EN/IN1 and PH/IN2? Or control 4 outputs individually through SPI port?

Thank you in advance for your inputs.

-Mark

over 3 years ago

0 Jacob Thompson over 3 years ago

TI__Mastermind 23431 points

Hey Mark,

I think the main confusion here is full-bridge vs half-bridge control. In order to control a stepper motor, you need to be able to drive either phase (A or B) in both directions - forward and reverse. A half-bridge is only ON or OFF, and always ON in one direction (current flows one way). A full bridge allows the driver to control which direction the current flows, resulting in ON(forwards), OFF, and ON(backwards) control. A full bridge is just made up of 2 half bridges (2 H-bridges), but only certain drivers allow you to independently control each H bridge in a full-bridge or in general.

If you look at the parameters of our motor drivers the first one will be "Number of full bridges", and in the case of the DRV8873 it says 1. This is why the datasheet clarifies it can "Drive One DC Motor, One Winding of a Stepper Motor, or Solenoid Loads". The DRV8833 on the other hand has 2 full-bridges, allowing it to control a whole stepper motor.

Heaters I suspect only need a half-bridge for ON/OFF control, though if they have a fan a full-bridge could allow for control of pushing air out or pulling it in by controlling the rotation both directions.

Here's a link about H-bridges: https://training.ti.com/ti-precision-labs-motor-drivers-h-bridge

You mention heaters, so you might find this FAQ on thermoelectric cooling devices useful: https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/967858/faq-what-motor-drivers-should-be-considered-for-driving-thermoelectric-cooling-tec-elements-or-peltier-devices

To answer your questions in short:

1. No, need a full-bridge to control a stepper motor.

2. N/A

3 and 4. I'm not sure on this, could you clarify if needed? Maximum chip voltage absolute max is 40V, recommended max is 38V. Phase/Enable and PWM are two separate modes to control a motor. Here's a FAQ about Phase/Enable vs PWM: https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/963987/faq-what-interface-ph-en-or-pwm-to-use-for-controlling-brushed-dc-motors

Regards,

Jacob Thompson

0 Mark Philip Felipe over 3 years ago in reply to Jacob Thompson

TI__Genius 10840 points

Hi Jacob,

If we setup as independent mode and IN1/IN2 connected with PWM signal. For simplification, PWM has 40% duty cycle. The question is what happens for output pin such as Tdrive = 40 % duty cycle on and Toff = 60 % duty cycle off? How should I setup the Itrip. Is it enabled and with the specific current commend as default 6.5 A?

0 Jacob Thompson over 3 years ago in reply to Mark Philip Felipe

TI__Mastermind 23431 points

Hey Mark,

The OCP will trip when the average current has been over the ITRIP level for longer than tOCP. For this driver, tOCP is 1.35ms, so if you use a PWM frequency of sufficiently high frequency (say 10kHz) then I think it will be the average current for a period of time.

The trip point will depend on the sense resistor you chose as detailed in 8.2.2.2 Motor Current Trip Point.

Here's a brief about OCP and current regulation: https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/850885/faq-overcurrent-protection-ocp-vs-current-regulation

Let me know if that didn't answer your question!

Regards,

Jacob

0 Mark Philip Felipe over 3 years ago in reply to Jacob Thompson

TI__Genius 10840 points

Hi Jacob, seeking further assistance.

If we only have hardware interface device, how could we setup a fixed PWM frequency like 10 KHz? I thought we are very limited because Itrip default is 6.5 A that we cannot change. We can setup the desired SR that will determine Tdrive to reach 6.5A, but we cannot get the desired Toff except its default number at 40us. Overall, it seems to me we cannot setup the desired PWM frequency at all?

0 Jacob Thompson over 3 years ago in reply to Mark Philip Felipe

TI__Mastermind 23431 points

Hey Mark,

Input the desired (10kHz?) PWM signal to the EN/IN1 or PH/IN2 pins. The DRV8873 does not generate a PWM itself, it takes your input PWM and switches things based on it. It supports a max frequency of 100kHz.

Correct that the Hardware version of the device has a fixed tOFF time of 40us, and the fixed Itrip of 6.5A. The tOFF only comes into play when the device current exceeds 6.5A. You can input your desired PWM frequency and as long as its duty cycle allows for the 5us of PWM blanking time (tBLANK). You might also want to see 8.2.1.2 Drive Current and Power Dissipation:

At start-up and fault conditions, the current flowing through the motor is much higher than normal running current; these peak currents and their duration must also be considered. High PWM frequency also results in higher switching losses. Typically, switching the inputs at 100 kHz compared to 10 kHz causes 20% more power loss in heat.

I'm not sure that I fully answered your questions, let me know if there's more I can clarify.

Regards,

Jacob

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DRV8873: Datasheet Clarification