Enabling a MIC2290 Boost Converter with a P-MOSFET

Question

This circuit aims to power a low-power (0.4W) 24V LED using a MIC2290 boost converter from a 5V regulator. A uC should enable and disable the LED output. In a previous version of this circuit, I have used the MIC2289 without a MOSFET with the EN pin connected to a uC. Unfortunately this was a mistake, as when disabled the output voltage just followed the input voltage of the boost converter. I have seen this in my prototype PCB and from researching the internet it seems like this is a mistake that others have made before me.

Looking at this answer, I have made the following schematic based on the MIC2290 to drive one low-power 24V LED. Since the MIC2289 is a current-mode LED Driver, I would like to use it as well as this in my design to drive a string of four high-power UV LEDs at 500mA. Here are my questions:

1. Why not place the MOSFET before the VIN instead of between the VIN and the inductor?
2. How does one select the capacitor value for C1 in the above circuit (not the C1 capacitor in the datasheet for the MIC2290)?
3. Is there possibly a better IC for my application that I have missed? I have considered the TPS6104 as well. However the MIC2289 IC has high power density and low power loss in the sense resistor so it would be ideal to power my 4x UV LEDs.

I have attached an image of the typical application circuit for the MIC2290 in case my schematic is not so clear.

Answer 1

There is nothing wrong with your approach and as @winny says, you can purchase boost converters with disconnect. However, you must pay attention to the switch current rating for your design to reach 24 volts at 400 mA.

For a boost converter:

So you are running at about an 80% duty cycle at a 24 volt output. This means your output is on 20% of the time. If you want your average current to be 400 milliamps on the output, and it is on only 20% of the time, then the instantaneous current has to be at least five times the average current or 2 amperes in your case. So you will need a converter that will support a 2 amp switch current. Also keep in mind that the P-channel FET and the battery will see over 2 amperes of current at your 80% duty cycle.

The 0.1 uF capacitor is fine, mounted as close as possible to your converter. It is only for high frequency associated with switching. It won't do anything to smooth out the current from the battery at this power level. As in all layouts, keep the high current traces short and wide.

Good luck!

Answer 2

I have seen examples where the LED current is broken at the bottom of the LED. Two examples are shown in red "X". It is simple to drive a MOSFET that is sitting on ground. With a 24V LED what is wrong with having 5V on it?
Does it light?