ADP5133 VOUT1/2 Pin

Hi John,

Whether using fixed or adjust (variable output) versions, always  tie the Output Voltage Sensing pins to VOUT or positive of the output capacitor and cannot be left floating.

Hi Fil

Thanks for the reply, my question was regarding the VOUT pin, can this pin be left floating on the variable output devices?  I see no reason why i cannot be left floating.  The Functional Block Diagram suggests that the VOUT pin on the variable output device is only used for discharging the external capacitors in the event of a fault or the output being disabled.

Hi John,

I'm referring to the same pin: they are VOUTx and named Output Voltage Sensing pins. Pin serves two purposes: one is for output discharge and the other is to sense the output voltage. Accurate output voltage information is used for slope compensation for stable feedback loop response.

Hi Fil

Thank you for your advise so far. 

Sensing the output voltage is done via the feedback pin on the variable voltage part and not the VOUT pin. 

For the fixed voltage output version VOUT is used and the Feedback pin must not be connected.  

I have built a PCB using the variable voltage version of the ADP5133, using external resistors to set the output voltage using the FB pin and not connecting the VOUT pin.  The output is stable over the operating voltage and load ranges.   When disabling the chip, the output takes a little longer to reduce to 0V when the VOUT pin is not connected.  I have also measure the VOUT pin when it was floating, the pin measure 0V as would be expected if the internal feedback resistor were not fitted. 

So the only item left from your concerns is Slope Compensation for stable feedback and loop response.  The slope compensation voltage sense input is connected to both FBn and VOUTn pins in the functional block diagram of the ADP5133. 

If the functional block diagram is correct, I really do think that leaving the VOUT pin disconnected on the variable voltage version is fine.

If you still believe that the VOUT pin must be connected please could you explain your reasoning whilst referencing the functional block diagram in the datasheet.

The “Voltage Sensing” term is perhaps a little confusing, but it is actually sensing the output voltage, but not for the reason that you think. The FBx pin is the normal voltage sensing that is used as part of the voltage regulation loop, i.e. to control the regulation level specifically. For the regulation, the loop doesn’t actually need to know the absolute voltage on OUTx.  It only cares about the voltage on FBx compared to the internal REF voltage.

But there is another stability concern, related to this topology of current mode buck DC-DC, which is slope compensation.  If you recall, a current mode DC-DC can be unstable (or technically bi-stable) if the duty cycle is greater than 50%.   We adjust for this internally with slope compensation, where we add some artificial “slope” into the error amp output signal.  To calculate the right amount of slope, we need to know the ratio of VOUT and VIN.   So there is a circuit inside which needs to sense VOUT so that it can compare it to a sensed VIN and set the slope accordingly.   If we don’t have VOUTx to sense, then we don’t have this measure, because we don’t actually know (internally) what the external resistor divider ratio is (between VOUTx and FBx). So we are sensing VOUTx purely so that we can get the slope comp correct.

Depending on your input and output voltages, you may be operating less than 50% duty so you are not seeing any issue.

Hi Fil

Thank you for your time and patience. Case closed.

John

Hi Fil

Thank you for your time and patience. Case closed.

John