MPQ7200 output voltage limited to 18V

DC-DC Power Converters
1

Dear MPS team,
I plan on using the MP7200 to drive 5 white power LEDs in series on a 11 - 15 V DC supply.
Therefore, I need to run buck-boost mode.
There are a few querstions the datasheet does not answer for me. Maybe I overlooked them.

  1. From the block diagram at page 40 of the data sheet it seems there is a overvoltage protection on the SW pin at 18 V, but only in BUCK mode. Why is this so ? The switch is rated 50 volts an din BUCK toplolgy, the input voltage cannot be exceeded. So , WHY ?
    Also it seems this is only active in BUCK mode, not in BUCK-BOOST
    Why is there an 18V OVP and when is it effective ?

  2. Therefore: IF run in Buck-Boost mode, can I generate more than 18V for an LED string ? Effectively the buck-boost application schematic shows a positive-to-negatiev converter with the LED string anode on GND. In this case the chip would see supply + LED string voltage, in my case max. 15V + 16V = 31 volts. No issue for the 50v rated switch, but well above the 18V internal OVP…
    Can the MP7200 generate more than 18V between SW puin and PGND/AGND in buck-boost mode ?

  3. ALSO: I also could connect the anode of the LED string to supply instead of GND. That way, the thing would effectively work as BOOST converter; The floating ground would not need to go to -15VDC but, depending on supply, to at max -4 VDC. This would lower the voltage across the device to 19V DC max. ( Still above 18 ) And improve EMC due to lower voltage swing.
    In that topology, I could then run up to 12 LEDs (nominal 36V) in series.
    Can I connect LED string anode to supply instead of to GND ?

  4. IF connecting LED anode to supply instead of GND: It is important what the low side FET does if the EN/DIM pin is held low; If the LS FET is ON in this case, at 15V supply, the LEDs will light up even with the MP7200 disabled; BUT if theLS FET is OPEN in that case, all would be fine.
    What does the LS FET do if EN/DIM is held low ?

With best regards,
Wolfgang

4

Hi Wolfgang,

Welcome to MPS Technical Forum!

For your four questions, I will answer one by one, if you have any further questions, feel free to reply to me.

  1. The block diagrams are not really circuit diagrams, just functional schematics. So there is a overvoltage protection in BUCK-BOOST mode. For the reason of 18V, you can go to page14, the OVP Threshold vs. Temperature figure will give you the answer. When the threshold is exceeded it is effective.
  2. MP7200 can definitely generate more than 18V between SW and GND in buck-boost mode. You can see the steady state figure at page33. Vsw > 18V.
  3. I would strongly recommend you follow our suggested application circuit. 18V is not a problem and you don’t need to worry about the EMI performance. If you have to change the schematic I can help you to check.

Best,
Fox