DRV8412: PWM frequency and off time

Ishiwata,

DRV8412 power circuit is a buck circuit. So, the duty cycle is D=Vout/Vin.

The maximum PWM frequency is 500kHz. PWM frequency is decided by customers according to their output ripple requirement; thermal performance. Please check  https://www.ti.com/lit/an/slva477b/slva477b.pdf "Inductor Selection" section for the output ripple calculation.

According to datasheet description, "For applications with lower than 10-kHz switching frequency and not to trigger BST_UVP protection, a larger bootstrap capacitor can be used (for example, 1-μF capacitor for 800-Hz operation)."

800Hz cycle time is 1.25ms. So, the Off period is short than 1.25ms. Customers can set a higher frequency to reduce the off period to make sure the bootstrap capacitor is charged up.

Would you share the abnormal waveform to understand the startup issue?

Regards,

Wang

Wang,

Thank you for your reply.

Q1: How long is the PWM off time required for a cycle time of 1.25 msec?
Q2: If the bootstrap capacitor is set to 0.1uF, how long is the PWM off time required?
Q3: Is there a formula to derive the bootstrap capacitor and off time for Q2?

Regarding the waveform, there was a comment that I did not want to put it on e2e due to customer's request,
so I would like to send the waveform by private message.

Best Regards,
Ishiwata

Ishiwata,

Q1. No requirement. I may misunderstand your question.

Q2: No requirement.

Q3: No formula.

Do they see any issue or abnormal waveform? If not, they may need to check some training material about the buck converter and bootstrap charge pump circuit.

Regards,

Wang

Wang,

Thank you.

Please tell me the test condition of Ton below.
What are the values for G VDD, P VDD and bootstrap capacitors?

I sent the abnormal waveform to a private message.
Please check there.

Best Regards,
Ishiwata

Ishiwata,

1. Test condition of ton_min: when the output current cannot reach the regulation point (put a high resistance load on the output), DRV8412 will push the duty cycle as much as it can. In this condition, we can find the ton-min.

2. In the datasheet figure 8, each PVDD_x capacitor is 100nF, plus 100uF bulk capacitor; G_VDD_x is 1uF; the bootstrap capacitor is 150nF.

Regards,

Wang

Wang,

Thank you very much.

About 1:
I understand the measurement of Ton_min. Did you confirm Ton_min = 50nsec described in the data sheet by measuring with EVM?
https://www.ti.com/tool/DRV8412DDWEVM
Also, I think that Ton_min = 50nsec described in the data sheet is a value that should be guaranteed.
What are the conditions for the guaranteed value? Or do I understand that it can guarantee all bootstrap capacitor and PWM frequency conditions?

There is a confirmation about the matter that does not work properly at startup.
It is considered that the UVLO, OVP and TSD protection circuits are not working because the Fault is not Low.
Also, G VDD and P VDD were not particularly problematic.
In that case, I think that BST_UVP worked. Will Fault go Low if BST_UVP works?
Also, if BST_UVP works, do you latch it? Or will it automatically recover?
Please tell me the conditions under which BST_UVP operates.

Best Regards,
Ishiwata

Ishiwata,

1. For bootstrap circuit, if the PWM duty cycle allows the low side FET turn-on longer than 50ns, the Ton-min naturally happens. We don't need to guarantee anything. Ton-min is a typical number. I am not sure what do you want to guarantee.

2. It is hard to know what issue they have right now. (why do they think it is Ton-min issue?) Can they get an EVM to confirm the issue with their load?

Regards,

Wang

Wang,

I'm sorry. I will tell you the background. Excuse me.

I received an inquiry from a customer that the driver does not work properly. It was because I was inputting with PWM 100%.
I advised the customer that a Toff period is required Since the DRV8412 is driven by the bootstrap method.
The improvement was achieved by providing a Toff period, but the customer said that it may not work depending on the PWM frequency and duty ratio.
I told customer to adjust the PWM frequency and duty ratio for evaluation as it also depends on the motor used.
However, the customer asked me to give me an indication of the PWM frequency, duty ratio and bootstrap capacitor value, and I asked E2E.

Customers also want to know the cause of the IC not outputting correctly at startup.
The Fault signal also does not go low and latches in a non-operating state.
I think BST_UVP is related to the cause.
Therefore, I would like to know the conditions under which BST_UVP occurs and the conditions for recovery when it occurs, so I contacted you.

Best Regards,
Ishiwata

Ishiwata,

Thank you for more detail info.

If we give 100% PWM duty cycle or the output current cannot reach the regulation point, we could see 50us Ton_min. According to datasheet description, "For applications with lower than 10-kHz switching frequency and not to trigger BST_UVP protection, a larger bootstrap capacitor can be used (for example, 1-μF capacitor for 800-Hz operation)." In this case, the duty cycle is about 1-50ns/1.25ms= 99.996%. I am not sure it can mean anything to the customer.

"the cause of the IC not outputting correctly at startup"

Can they share their schematic? What load do they put on the output? What is their expected waveform?

Regards,

Wang Li

Wang,

Thank you for your support.

I understand the duty ratio for 1μF capacitors.
Please let me know if possible for 0.1uF capacitors.

Schematics and waveforms are sent as private messages for confidential information.

I don't have time, so please tell me about the basic operation of BST_UVP.

Best Regards,
Ishiwata

Wang,

Thank you for your reply.

I'm sorry, I have an additional question.
If a bootstrap capacitor under-voltage protection circuit (BST_UVP) works, will it automatically recover? Or latch?

I think BST_UVP is working and stopped at startup, what are your thoughts?

Best Regards,
Ishiwata