MP3438 Vin to Vout gap

DC-DC Power Converters
1

Hi all,

I put this topic in “power converter” as it’s the primary use of the MP3438, even though i’ll use it as a led driver using current sensing.

Context : MP3438 powered by a lithium cell (3.5 to 4.2V), to drive a Cree XHP50 LED, 6V (more like 5 to 6V in my current range). Current ranging from 10mA to 1.5A

The mp3438 being a boost converter, that should roughly work.

The datasheet, page 6, bottom-right graph shows the current limit depending on the output voltage, for 2 different input voltages. For the Vin=3.3V curve, Vout starts arround 4V. For the Vin=6.6V curve, Vout starts arround 9V. Why is the gap so big ? Does that mean the mp3438 can’t power a 8V load from a 6.6V output ?

  1. What would happen when the battery is full at 4.2V and i try to power the led with low current, at 5V ? (Note : the MP3431, often used as a led driver can do that)

  2. For other input values, how can i calculate the minimum reachable boosted output before going into (non boosted) pass through mode ?

Thank you for helping !

2

Hi,

The boost converter will work for your 6.6V input, 8V output.

  1. At a low current, or light load, the boost will work in power saving mode (PSM). This is activated when your inductor current reaches 0A.
  2. Pass-through mode is activated once the input voltage exceeds the set output voltage. Your input voltage will dictate the minimum output voltage.

Best,

Tyler

3

Hi Tyler,

Thank for this answer. I’ve also red that this gap can be calculated using frequency, inductance value etc apparently.