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I've read a couple of application notes where it is stated that measuring current to use that value in the feedback loop helps stability because sensing before the RC net takes a pole away (see http://cds.linear.com/docs/en/application-note/AN140fa.pdf page 11, figure 13). I get that. What I don't understand completely is how do you know which current value you need to have if you only know the voltage value you what to stabilize at the output but you don't know how much you are loading the SMPS.

Thanks for your answers

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  • \$\begingroup\$ "The current loop can be closed by sensing the inductor current through a sensing resistor, the inductor DCR voltage drop, or the MOSFET conduction voltage drop." Sounds like they have it covered to me. \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 6 '15 at 17:12
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Have a look at figure 13:

enter image description here

There are two control loops.

In the inner loop, the voltage across RSEN is compared with a target value to determine when to switch the PWM circuit. This is the current sense circuit.

In the outer loop, the output voltage is compared with a reference, and the target for the inner loop is adjusted to achieve the desired output voltage.

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  • \$\begingroup\$ Do you agree that Current Control converters have tighter control over the load than Voltage Control? \$\endgroup\$ – KyranF Jul 6 '15 at 17:29
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If the converter in question were running with a fixed load, you would be able to come up with some correlation between the current through the inductor (or the FET) and the voltage seen at the output. In reality, we do not really know what the loading condition is going to be, so feedback control is necessary. There will not be a particular current target we are shooting for, but rather the controller will sense that the output voltage is not where it needs to be, and respond with more or less current as required to stabilize the output.

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