TPS56637: is a switching regulator of this type backdriveable?

PS - I noticed that the LM61460 datasheet warns:

"The input voltage must not be allowed to fall below the output voltage. In this scenario, such as a shorted input test, the output capacitors discharge through the internal parasitic diode found between the VIN and SW pins of the device. During this condition, the current can become uncontrolled, possibly causing damage to the device. If this scenario is considered likely, then a Schottky diode between the input supply and the output must be used."

This seems like a possible problem with the setup I described, when the power supplied to a LM61460 is absent, but the secondary (USB) source present farther downstream in the circuit.  Is that correct?  How do I correctly configure the Schottky diode that is mentioned here?

The TPS56637 datasheet simply says:

"The TPS56637 is designed to operate from input supply voltage in the range of 4.5 V to 28 V. Buck converters require the input voltage to be higher than the output voltage for proper operation."

Does this mean that it also cannot handle the possibility of backdriving, and that I really need reverse current protection in either case?

PPS - I noticed, in section 8.4.3.2.1 of the LM61460 datasheet, that "Diode emulation prevents reverse current though the inductor...", and that diode emulation is related to a Discontinuous Conduction Mode (DCM), which apparently the TPS56637 can also operate in.  Does this mean that one or both of these devices, when properly configured, can safely be attached to a line that might be otherwise powered with a different 5V source, without providing separate reverse current protection?

(ooops, didn't intend to hit the "This resolved my issue" button yet - not clear that I can undo that?  hope you still see this...)

Hi Gerold - Regarding the application:  we are building a motor control HAT for a Raspberry Pi (RPi).

In normal operation, our HAT will be powered by a substantial 2S to 6S LiPo, or 7V to 28V wall wart, which will drive other power busses (for driving motors and servos) in addition to providing 5V power to both the RPi, and certain other components on our HAT.

The RPi itself also has a USB input for a 5V/3A power supply.  For normal operation as described above, the 5V line on the HAT must connect to the 5V line on the RPi through the 40-pin header. [One could actually even code the MCU on our HAT using 5V provided by the RPi as a convenient power supply - however, the motors and servos would be unpowered in this mode...]

However, we have to be prepared for the possibility that the user might accidentally connect BOTH the LiPo to our HAT as well as the 5V/3A power supply to the connected RPi.  Further discussion in the last paragraph at the link below, which states "If a HAT back-powers a Pi and uses a power source that does not try to sink current (and will safely stop/pause regulation if its input voltage is higher than its regulation voltage) it is OK to not include a ZVD on a HAT. If you are unsure or don't know then please include the ZVD!"

Thanks in advance for any further guidance you can provide.  - Tom

github.com/.../designguide.md

It seems my question here (about whether or not reverse current into a switching regulator is a concern) is close to that recently posed at the link below.  I'm not sure the answer provided at that link (implementing schottky diodes, with 0.4v drop, for protection against reverse current) is that great for me - too much heat would be dissipated by such a schottky at the 4A to 6A current on the 5V line in my application.  I'd probably prefer a low Rds(on) mosfet and a LM74700 controller, but I really don't want to commit the board space for that it is not actually necessary?

https://electronics.stackexchange.com/questions/518343/a-voltage-backdrive-on-dc-dc-converters

Hi Thomas

   Let us use this as the active thread .

Regards,

Gerold

Hi Thomas,

When another 5V power source is forced at output side of TPS56637 and its voltage is higher than the output voltage of IC, the results are related to the mode of IC.

If TPS56637 is set as FCCM mode, the power will be transmitted from output side to input side. If your 7V-28V input source doesn't have sink ability, that will cause the input voltage rise and abnormal operation of IC.

If TPS56637 with 7V-28V input voltage is set as Echo mode, it will prevent the bi-directional power. When the 5V power source is forced at the output side of TPS56637, buck will stop switching. If the output voltage becomes higher than 125% of the target voltage of IC, the overvoltage protection will be triggered. That means if the the 5V voltage doesn't exceed the OV value, IC can work with the other 5V power source.

The above analysis is suitable for the situation that the 5V power source is added during the operation. If the 5V power source is added before the startup of IC, the IC will initiate switching and start ramping up only after the internal reference voltage becomes greater than the feedback. 

Regards,

Andrew Xiong

Hi Thomas

   Does this answer your question?  If yes, we will go ahead and close this ticket.

Regards,

Gerold

Dear Andrew -

Thank you so much for your thoughts.  It seems, for the case in which 5V power (from a different power source) might be applied to the output of the TPS56637, you suggest operation in Eco mode, not in FCCM mode.  That's easily done (by tying the MODE pin to GND).

My concern is your last sentence: that is, what happens when

(a) the 7V-28V input is NOT applied to the  TPS56637, but 5V is applied (from the different power source) to its output, then

(b) someone later plugs in the 7V-28V input to the TPS56637, with 5V already applied (from the different power source) at its output.

Is condition (a) ok from the perspective of the  TPS56637?  Is the startup of the  TPS56637 under the condition described in (b) ok?

thanks so much for the advice,

Tom

Gerold - not quite. please see my follow-up question in this thread just now...  - Tom

Hi Yuchang, Andrew, and Gerold -

I asked this same question to the LM61460 support engineers on e2e, and one of them responded quite simply:

"The reverse current will only occur if Vin < Vout. If Vin is not actively pulled low, it won't be an issue (the input caps will just stay at Vout - Vdiode).

If Vin is actively pulled down, then a diode in series with the input blocks the reverse current."

It is indeed my case that, when Vin (attached to the Vreg) is unplugged from the LiPo, then Vin just floats - it is NOT actively pulled low.  From the simple answer above (for the LM61450), then, it seems the situation that I expressed concerned about will in fact not be a problem, and thus no diode (and the losses associated with a diode) is required (for the LM61450).

The answers in the present thread, for the TPS56637, are more detailed/nuanced, but also seem to arrive at the same conclusion.  Setting the TPS56637 IC in Eco mode and to output 5.1V, based on the responses received thus far, I do not anticipate a problem to arise if about 5V (from a secondary USB power supply) is presented on the output, whether or not Vin on the TPS56637 is plugged into the LiPo.  

If anyone disagrees, please do set me straight via a return message.  Otherwise I'm concluding that this design situation is safe.  :)  Thanks all for the help!