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We have designed a couple of power supplies (±7.0VDC/0.70A) based on the LM25575 device. The power supplies has been based on output from the Webench tool. The schematics is attached for information.
It seems that the Rramp resistor value doesn't have any influence at the design, because have tried to have only one of the resistors mounted, but is does change anything. The Webench tool also reply with the same value, whether we type e.g. 0.7A or 0.2A. Any comments about the function of this resistor?
During verification it has turned out that the negative one has a different current limit than the positive one. The positive one shuts down at approximately 2.1A which corresponds to the datasheet value, while the negative one shuts down at 1.3A. Can't explain this, so would appreciate some feedback about root cause?
Thanks in advance for feedback.
First I'll answer the current limit question.
When you are using the part in the inverting configuration, the high side switch current of the device is equal to the output current + the input current. This is higher than the current the switch would see in the standard buck configuration. This will cause the part to hit current limit at a lower output current to protect the device.
Here is a link to an appnote that describes the effect.
2) What does Rramp do? The resistor increase the slope of the ramp that is added to the emulated current ramp. If your system is already stable then the additional ramp has the effect of moving the part closer to voltage mode. Therefore if your design does not need the ramp for stability it doesnot need to be added.
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In reply to Marc Davis-Marsh:
Agree with Marc. The output current is not the same in buck configuration and inverting buck-boost configuration.
In buck, inductor current always follows to the load. In buck-boost, inductor current only flows to the load when the LS FET is on. So only part of the inductor current contribute to the DC current seen at the load side. With the same peak current limit set at the inductor current, lower DC current seen at the output of inverting buck-boost configuration is expected.
The Rramp set the slope compensation value for the peak current mode control. Peak current mode control is inherently unstable when duty cycle > 50%. The slope compensation is always used when Vout is high, e.g. duty cycle is high. The amount of the slope compensation needed is only related to the Vout, Vin, L. Not load level. That's why with different setting of load current, the Webench calculation for Rramp is the same.
Hope this helps.
TI Wide Vin Buck Converter & Controller Applications Engineer
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