Input lower than set output for LM2576

[HGP]

Hi,

I was wondering if it is okay to feed a voltage lower than the set output voltage for a switching regulator like LM2576.

I'm trying to control the output at two terminals (18V and 12V) depending on the temperature sensed. The 18V output 1 is controlled through a PWM that regulates the voltage to 9V when a certain temperature is achieved. I want the 12V terminal to also switch to 9V (or slightly lower) at the same temperature. To do this, I wanted to avoid using a second PWM controlled regulator and was wondering if I could directly feed the first PWM output to an LM2576-12. During normal operation, it's input would be the 18V output of the first output which it will then convert to 12V. When the switching temperature is achieved, output 1 will go to 9V. Is it okay to feed this to the LM2576-12 to obtain nearly 9V at output 2.

 

[HGP]

To clarify the scenario.....this is what I'm trying to get.

Input Vdc = 24V. When the sensed temperature is greater than the set point, o/p 1 = 18V, o/p 2 = 12V. When the sensed temperature reaches the set point, o/p 1 = 9V, o/p 2 = 9V.

I have a DC-DC converter doing the 24 to 18/9V through PWM control based on the sensed temperature. I was thinking if it is possible to avoid a second converter (which does 24 to 12/9) and instead use a 2576-12 which is fed with o/p 1. Is it okay to feed 9V to the 2576-12 to get the 9V at o/p 2 (I'm guessing it would actually be around 7-8V) for a long time?

 

[duke37]

Most convertors are either boost or buck although you can get some which will do both.
The LM2576-12 is a buck convertor for dropping the voltage. Feed the second convertor from the power supply and change the feedback resistor for the lower voltage when necessary.

Is this right?
Condition 1
Input 24V o/p 18V and 12V
Condition 2
Input 24V o/p 9V and 9V

Split the feedback resistor and connect the junction through a resistor to ground when the higher voltage is required.

 

[HGP]

Hi duke37. Thanks for the reply! Yes, that would work with a minimal addition of components.

My query is fundamentally regarding the 2576. The intended application of LM2576-12 is that it will, at any voltage between 12.7 and 40, give an output of 12V without the use of any feedback resistors (fixed output voltage application). I was thinking that if the input to it is less than 12, the output should be the same as the input minus a small drop for the internal switching transistor. But since the datasheets don't mention its behaviour at lower than the intended voltage, I was wondering if it is okay to do so.

 

[duke37]

I cannot answer your question. You can get an adjustable 2576 which will run off your 24V supply. It is better not to use components in a way which is not specified.

 

[HGP]

Yeah, I suppose an adjustable 2576 would be the way to go.

I was curious about the 2576-12 because if you see the block diagram of the module, it is a simple buck converter with an error amplifier. For a lower voltage, the switch might just continuously end up in the ON state, like a buck converter with 100% duty cycle implying losses at the transistor forward voltage and the inductor resistive drop being the voltage drops. The thing I am not sure about is will the inductor core saturate and maybe not work as well when it actually has to to the 18 to 12 conversion? Or are there any other issues?

 

[BobK]

The inductor core may be saturated, but it would not matter if it is not switching.

Bob

 

[HGP]

Hi Bob. Hopefully, this doesn't affect the inductor when it is switching for the 18 to 12 conversion. Do you see any other issues with feeding a lower voltage to the 2576?