Adding an outpiut current clamp to a LTC7803 based Buck?

[cupoftea]

Hi,
We wish to add a current clamp (opamp error amplifier) to the output of the LTC7803 in Buck (24Vin, 15Vout, 3Aout)

https://www.analog.com/media/en/technical-documentation/data-sheets/ltc7803.pdf

Would it be better to have the external current clamp error amplifier pull down the SS pin or the ITH pin.?
The datasheet suggests that any "fiddling" with the "SS/TRACK" pin after start-up will have no effect.


Pg 33 of the LTC1871 datasheet possibly suggests that the ITH pin can be pulled down by external circuitry....

https://www.analog.com/media/en/technical-documentation/data-sheets/1871fe.pdf

...but analog.com fall short of saying its allowed.

 

[stenzer]

Hi,

I haven't checked the two datasheets in detail, but have you considered to use the RUN pin for complete shutdown, or manipulating the output/feedback voltage to decrease the peak current?

What is your aimed function / target response?

BR

 

[cupoftea]

Thanks yes, i appreciate the foldback feature...but with the tolerances, you could still get an overload of 150% and that woudlnt even kick in.
RUN can shut it down yes. But we want clamping to say 3.2A.

 

[stenzer]

Hi,

But we want clamping to say 3.2A.

isn't that the case if the sense resistor has been chosen properly, to limit the peak current to 3.2 A? Is there an actual use case to allow more current e.g. for a short time?

BR

 

[cupoftea]

Thanks, due to the "room" that has to be left for the slope compensation ramp, and the fact that the sense resistor must be undersized, to account for the sense voltage tolerance, the Buck made for 3A output will give significantly more than 3A when overloaded, indeed, significantly more than 3.2A, ....this is why we need the clamp.

 

[stenzer]

Have you considered to increase the switching frequency in case your aimed clamping current is exceeded? The question would be what's the response time if changing the programmable frequency at the FREQ pin.

What is your opearting switching frequency and MODE of operation?

 

[cupoftea]

Thanks, its 150kHz and CCM.
Yes its true that if an overload were on the output.....the output current could be regulated down to say 3.2A by reducing the switching frequency.....though i wouldnt be sure if this was the way to go , instead of as already mentioned, playing with the ITH pin or the SS/Track pin.

Ayk, An opamp (acting as an error amplifier) with diode and resistor could be put into the FREQ pin, anode toward pin, but the opamp would then likely need a slightly negative rail in order to be able to pull more current through the resistor, whilst getting into regulation.

 

[D.A.(Tony)Stewart]

The ITH threshold and ranges are defined for each mode.

Why can't they be used to solve your problem?

 

[Eden60]

Hi,

To add a current clamp to the LTC7803 Buck regulator, consider pulling down the ITH pin based on the LTC1871 datasheet suggestion. However, be cautious as the datasheet doesn't explicitly allow it. Since manipulating the "SS/TRACK" pin has no effect, consulting Analog Devices or testing might be needed for a clear approach.
https://www.edaboard.com/threads/adding-an-outpiut-current-clamp-to-a-ltc7803-based-buck.4generative ai09482/

I hope this will help you.

 

[D.A.(Tony)Stewart]

Understanding this tells you it performs

 

[tarun2790]

Adding an output current clamp to an LTC7803-based buck converter can be a useful feature to protect both the converter and the load from excessive current conditions. The LTC7803 is a synchronous step-down DC-DC converter controller that can operate at high frequencies and support a wide input voltage range. Here's a general approach to add an output current clamp:

1. **Current Sensing Circuit**: Implement a current sensing circuit that measures the output current flowing through the buck converter. This can be done using a shunt resistor in series with the load and a differential amplifier to measure the voltage drop across the shunt resistor. The output voltage of the differential amplifier will be proportional to the output current.

2. **Comparator**: Use a comparator to compare the sensed output current with a preset threshold. The threshold can be set to the maximum allowable output current for your application.

3. **Feedback Loop**: Integrate the comparator output into the feedback loop of the LTC7803 controller. When the output current exceeds the threshold, the comparator will trigger the feedback loop to adjust the duty cycle of the buck converter, thereby reducing the output voltage and limiting the output current.

4. **Protection Logic**: Implement protection logic to ensure that the buck converter responds appropriately to overcurrent conditions. This may include shutting down the converter or entering a hiccup mode to protect against excessive current draw.

5. **Testing and Calibration**: Once the current clamp circuit is implemented, it's important to thoroughly test and calibrate the system to ensure proper operation under various load conditions. This may involve adjusting the threshold of the comparator and fine-tuning the feedback loop parameters.

6. **Considerations**: Keep in mind factors such as response time, hysteresis, and accuracy when designing the current clamp circuit. Additionally, ensure that the added components do not introduce significant losses or affect the efficiency of the buck converter.

By adding an output current clamp to your LTC7803-based buck converter, you can provide an additional layer of protection against overcurrent conditions, enhancing the reliability and safety of your power supply system. what is splunk used for