• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 62 subscribers
  • Views 525 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LMZ14203EXT: Voltage protection of Vout

Julhio
Intellectual 310 points
Part Number: LMZ14203EXT
Other Parts Discussed in Thread: LMZ14203

Hello

I have a situation that in the initialization of the circuit will be placed 5V voltage on the output of LMZ14203EXT, with no voltage at the input. The intention is use a startup voltage for the circuits fed by the Vout of the LMZ14203EXT.

The equivalent circuit and the sequence of operation (1, 2 and 3) is bellow:

Will the LMZ14203EXT suffer any damage on this situation ?

Thanks,

Julhio.

  • 0
    TI__Guru 50120 points

    Hi Julhio,

    Looking at the feedback resistor divider it looks like you are set to regulate at 5.5V output. Your application sounds like you have an prebiased startup condition. The LMZ14203EXT will properly start up into a prebiased output mentioned in the datasheet(Section7.3.5). Since you will go from 5V external on Vout to 5.5V regulated Vout, this shouldn't be a problem.

    The issue only occurs if the external voltage(5V) is higher than the set regulated voltage from the LMZ14203  since this will result in back current flowing into the VOUT pin. To prevent this you would normally put a blocking diode on VOUT such that it blocks current flowing back into the VOUT pin that could potentially overstress the LOW-side switching FET. 

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    Intellectual 310 points

    Thanks for the answer !

    My intention was use a blocking diode on Vout, but I had a layout issue and the board is actually on the above-mentioned situation.

    You said that could potentially overstress the low-side FET, but I thought that the upper side FET that could be the problem, because if this is a MOSFET, the current should flow through the internal MOSFET diode.

    So, without the blocking diode, on situation 1 (without input voltage), I really could have a problem, right ? Even if it is by a short time, about one second ?

    Thank very much !

  • 0 in reply to Julhio
    TI__Guru 50120 points

    Hi Julhio,

    The reverse current will try to find it's way to ground. In this case if you are in an operating condition where the inductor current is reversed (5Vext to lower Vout regulation voltage) it will try to find its way through the low-side FET and stress the part. This is further noted in an app note (SNVA736 in Section 3.1 Current Limit). This is the reason why having negative current limit option is important to prevent excessive reverse current. Otherwise, you would need an external blocking diode to prevent current from ever sinking into the Vout pin. In your schematic case, I think the diode orientation should be reversed as to prevent current from backflowing into the device during your external power supply startup sequencing.

    Regards,

    Jimmy