LM74900-Q1: Load dump TMR Timer setting

Chul Kim

Part Number: LM74900-Q1
Other Parts Discussed in Thread: LM74900

Hi.
I have a question about LM74900-Q1.

It is used under the following conditions of LM74900-Q1.

An application that receives battery voltage from a vehicle.

Overcurrent Limit = 11A / duration = 1ms
short Current = 20A / 5us
It is a situation where the battery voltage of a normal vehicle is received.

- I wonder how long the Overcurrent duration of the Load dump (LM74900) is generally set to in automotive application.

We will set the general value first and then proceed with the board test later to find the optimal value.

it can be set the TMR terminal by adding the appropriate capacitor between the GND.

over 2 years ago

0 Praveen GD over 2 years ago

TI__Guru 57737 points

Hi Chul Kim,

The timer setting/duration can vary from application to application.

This setting depends on the maximum DC load current which you expect in your system and the maximum transient load current magnitude and duration that is acceptable.

Your idea of starting with a 1ms timer and later tuning it after testing can be done in case you do not have knowledge of maximum transient load current magnitude and duration in your system.

0 Chul Kim over 2 years ago in reply to Praveen GD

Intellectual 260 points

Hi Praveen GD

Thank you for your repley

The input current of the DCDC converter(TPS655288-Q1) connected after the Load dump (LM74900) is the waveform C above (Boost mode),

and the duration is 2ms.

In this case, how much TMR Time is a good choice?

Any advice would be appreciated.

Thank YOU.

0 Praveen GD over 2 years ago in reply to Chul Kim

TI__Guru 57737 points

Hi Chul,

Yes, in case you know that the peak current trough the LM74900-Q1 FETs lasts for 2ms, you can set the timer to 2ms.

0 Chul Kim over 2 years ago in reply to Praveen GD

Intellectual 260 points

Praveen Thank You.!!

I don't understand well, can you explain it in more detail?
Why should I set it to 2ms?

0 Praveen GD over 2 years ago in reply to Chul Kim

TI__Guru 57737 points

Hi Chul,

If the max transient current that you expect to be flowing through LM74900-Q1 FETs is for 2ms, then you can set the timer duration for 2ms or a little more than 2ms. Let's take an example for better understanding,

Let's say in your system, the maximum DC load current is 10A
The peak current drawn by the load is 15A for 2ms. So,
- You do not expect any current to through the FETs of LM7490-Q1 above 10A for more than 2ms.
In this case, you can set the over current threshold at say 12A (for some error margin from 10A) and timer duration at 2.2ms.
- The LM7490-Q1 controller will turn OFF the FET if detects a current > 12A for > timer duration which is 2.2ms
- By doing so you are protecting your system from fault conditions which can draw current >10A for >2ms time duration.

0 Chul Kim over 2 years ago in reply to Praveen GD

Intellectual 260 points

I have a question about the last part of your post.
"The LM7490-Q1 controller will turn OFF the FET if detects a current >12A for > timer duration which is 2.2ms
By doing so you are protecting your system from fault conditions which can draw current >10A for >2ms time duration."

If the overcurrent detection is more than 2.2ms for 12A (slightly higher than 10A), isn't it already more than 10A of current flowing for more than 2ms?

Shouldn't the overcurrent detection be within 2ms of 12A current to protect the system?

0 Praveen GD over 2 years ago in reply to Chul Kim

TI__Guru 57737 points

Hi Chul,

the current limit threshold will have accuracy error because of which you will have to set the current limit threshold at slightly higher value so that the minimum current limit threshold is greater than the maximum load current.

The same is applicable to the timer duration as well. If you want to allow a 2ms transient current to flow, the timer duration has to be set higher than 2ms such that the minimum timer duration is greater than the allowable transient current duration of 2ms.

the min and max threshold values of OCP and TMR thresholds will be updated in the RTM datasheet when the device is released for production.

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Bulgarian	Hindi	Portuguese
Catalan	Hmong Daw	Romanian
Chinese Simplified	Hungarian	Russian
Chinese Traditional	Indonesian	Slovak
Czech	Italian	Slovenian
Danish	Japanese	Spanish
Dutch	Klingon	Swedish
English	Korean	Thai
Estonian	Latvian	Turkish
Finnish	Lithuanian	Ukrainian
French	Malay	Urdu
German	Maltese	Vietnamese
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Haitian Creole	Persian

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LM74900-Q1: Load dump TMR Timer setting