Hi,
I want to use the MP1488 in a ligthing solution. I wonder how I can calculate the needed minimum input voltage. The Spec is 2.5 - 6V but the typical Application is calculated for 4.5-6V VIN.
I’m planning to drive 6 or 9 LEDs at 100mA each from a LiPo battery. Should i reduce the LEDs per strand or how can I achieve stable operation between 3.0 and 4.2V Vin?
Thanks in advance
David
Hi David,
If you are planning to drive 6 or 9 LEDs that each require 100mA, then in the worst-case scenario, your output current will be ~1A. The MP1488 has an internal current limit of 800mA so you could run into issues if you plan on using 9 LEDs with only one MP1488. You can get around this issue if you use multiple MP1588’s in parallel, but a better solution would be the MP3309A which supports your Vin range and LED load requirements. If you do decide to use the MP3309A, we recommend configuring a higher switching frequency to keep peak-peak output current ripple low, and to size the inductor following the datasheet equations.
About calculating minimum input voltage: it depends on your required output power and how much current you want to pass through your inductor. Let me know if you need additional help.
Best,
Luke
Not to rain on any parades but the datasheet is useless in determining that power that this thing can deliver. The simple back of the envelope calculation you would take the switch current limit multiply it by the input voltage and then assume 100% efficiency and that would set your output power. In this case the switch current limit is 800mA so at 3.0V in and 800mA your maximum power is 2.4W. Sadly that current is only called out as a typical, maybe the worst case is 600mA . There is basically no way to know worst case what this chip is capable of. To know that they need to specify a min switch current limit,
Ok maybe this was a bit unclear. I hadn’t a special LED in mind but now i settled on 6x 0.2W (60mA ~3V).
So that’s 120mA total when using two strings or 180mA total with three strings.
But in any case I wonder if I can calculate the minimum Input voltage for whatever application.
So again… application is driving 6 0.2W WLEDs from a single LiPo Battery.
Thanks for your input. You are right, there is some essential information missing in the datasheet and some passages are akward
A 2.2µF output capacitor is sufficient for most applications but
up to 1µF may be used.
It’s not so much about maximum power but the minimum input voltage for a certain output power.
So In the typical application there a 9 strings with three WLEDs each. Total current is 180mA. Thats 20mA per string and 9-10V output voltage → 1.6-1.8W
In this application the VInputMin is 4V (albeit in the typical application on the front page it’s 4.5V)
So maybe there is a better alternative if I want to drive WLEDs from a single LiPo cell.
OK so now we have the output power as either 1.2W or 1.8W, doesn’t matter
So lets pick 1.2W on the output. Circuit is 90% efficient now you need 1.32W on the input.
switch current limit is 0.8A ( or less which is my beef!) So the minimum voltage is 1.32W/0.8A= 1.65V
Which is absurd But at Vmin of 3V 0.8A gives you 2.4W of capability for a 1.2W load that is gonna work
For the 1.8w load it gets a little uncertain. We are getting closer to the limit, and have to fret a bit.
First off the simple minded calculation was assuming an infinite boost inductor, i.e no current ripple.
A real inductor will have typically ( be chosen for) 40% current ripple so +20% over the average is the peak current . That drops your peak power capability from 2.4W to 1.92W vs a 1.8W load.
We also now have to get serious about what the real switch current limit is.
The datasheet provides no clues, it is hopeless a mere guess. Only the typical switch peak current is listed, I presume you want to make more than 10 of these and would like them to work the same so sooner or later you are going to buy one with a lower than typical current limit ( hence lower than typical power capability) what is that number?? It isn’t given, your guess is as good as mine.
The other point is that it looks like this chip uses slope compensation, which is a technique that decreases the switch current limit as the duty cycle goes up. So now you have to go to the datasheet and find how much the switch current limit decreases at the operating point ( set by Vin and V lamp)
There is no such graph.
Long story short is probably will work at 3V with 6 LEDs, it is really unclear if it will work with 9 LEDs.
I would point out, the failure will be “soft” The LEDs will still light up, just not at the power/current that you designed for.