RT8059 step-down DC/DC converter - can't get it to be stable, annyone with experience?

[spec]

Hi Spec,

What type/model of LEDs can you suggest with those parameters... I have been experimenting with some warm white 5050 types, they have three LEDs per chip, approx 10mA for each LED, high brightness (19 lumens per LED)
https://www.superbrightleds.com/moreinfo/surface-mount-smd/warm-white-5050-smd-led-120-degree-viewing-angle-6000-mcd/317/#/tab/Specifications

Hy Andy,

Here is a list of high brightness LEDSs (none are white though) which are specified at 2mA and some have a forward voltage (VF) of around 2.5V:

The reason that I want the regulator is to keep the supply to the LEDs constant... if I forget the regulator and simply go with series resistors to limit current, then the current (and LED brightness) with change significantly as the li-ion battery voltage drops over its range from 4.2V to ~3.4V.

That is a good point but, without a regulator, the change in LED current would be less than +- 30% and, as the human eye's response is logarithmic, the brightness would not be too noticeable.

Also, I need to be able to switch the LED on and off... using a transistor to do this will lose 0.6V, but the RT8059 would only lose 0.2V (and the RT8059 has a handy enable pin to switch the device on and off).

A bipolar transistor could be configured as a switch to drop less than 100mV but a MOSFET would drop 0V in practical terms.

Essentially what I'm trying to do here, is drive a LED(s) from a consistent source that is powered by a li-ion battery (whose voltage isn't consistent).

I think you are aiming at a constant voltage source through a resistor, but a LED is best driven by a constant current to define the brightness.

I hadn't heard of a ceramic inductor before either! But here they are... (this is just an example, not where I bought them)
https://www.coilcraft.com/0603cs.cfm

Hmm, interesting: have to investigate ceramic inductors.

spec

 

[Andy_C_]

Hi Spec,

I think you are aiming at a constant voltage source through a resistor, but a LED is best driven by a constant current to define the brightness.

I think this would be a better solution for driving the LEDs then... what do you think?
https://www.maximintegrated.com/en/products/power/led-drivers/MAX1916.html

It is a "high-performance alternative to the simple ballast resistors used in conventional white LED designs".


Andy.

 

[spec]

Hi Spec,



I think this would be a better solution for driving the LEDs then... what do you think?
https://www.maximintegrated.com/en/products/power/led-drivers/MAX1916.html

It is a "high-performance alternative to the simple ballast resistors used in conventional white LED designs".


Andy.

Nice: may be good for another application I am looking at too; something else I have learned from this post.

spec

(PS: I haven't analyzed the MAX1916 in detail yet)

 

[Les Jones]

Hi spec,
Phosphor bronze is springy similar to beryllium copper. I too was interested in chemistry as a teenager.

Hi Andy,
For driving a number of LEDs I would connect them in series and us a step up regulator. I would configure its as constant current source by connecting the + end of the current sense resistor to the comparitor input on the regulator chip. A chip with a low reference voltage is best. I have done it using a modified stepup converter bought on ebay. This is how I did it.

Les.

 

[spec]

...

I hadn't heard of a ceramic inductor before either! But here they are... (this is just an example, not where I bought them)
https://www.coilcraft.com/0603cs.cfm ...

Hy again Andy,

I have solved the mystery of 'ceramic' inductors: Ceramic (alumina) is a physically stable material with a low loss, but it has no significant magnetic properties. So a ceramic capacitor is a bit of a misnomer and, electrically, a ceramic capacitors can be likened to air cored inductors which are primarily designed for high to microwave frequencies. As such they would not be optimum for most inverters.

The other three main kinds of inductor cores do have magnetic properties:

(1) Iron (generic name) laminated

(2) Iron powdered

(3) Ferite

My understanding is that, on balance, Ferite cores (with an air gap) are generally best for modern high frequency converters. For your application the inductor must support a current of at least 1.5A, and perhaps go for 2A to allow a safety margin.

spec

Vishay inductor 101: https://www.vishay.com/docs/49782/49782.pdf