Microcontrollable adjustable and switchable constant current source for driving LED's

[calltronics]

Hi,
I am looking for a suitable solution for driving LED strings with constant current.
As depicted below I need to do some precise timed switching on/off of the LED’s from a microcontroller.
Pretty standard microcontroller (Ti) with DIO, AD and DA.

Could anyone help in pointing me in the best direction for the switchable constant current source circuit solution.
As depicted in the attached diagram?

• Range = (10mA to 150mA)
• The microprocessor needs to adjust the constant current to enable a set current dependent on which of the various LED strings are attached.
• The constant current requires on/off switching from a digital port on Micro, not fast (0.1sec on/off).

Anyone got any good starting suggestions?

 

[calltronics]

As you have a micro you could setup a 4 bit current source to give you 0 to 160mA in 10mA stages ( 1 more bit will give 0 to 320mA). With only (max) 3.5V to drop fairly small transistors could be used (~300mW max dissipation) - more if 5 bit.

mike.

Thanks for that, never thought of this approach. Have you any good links for more information?

 

[rjenkinsgb]

If you can restrict the current to 100mA using series-parallel LEDS - and especially if you could integrate the current sense resistor into the fitting - something like this [the B suffix version] would be ideal:

https://www.richtek.com/assets/product_file/RT9284A-E=RT9284B-E/DS9284AB_E-06.pdf

It boosts output voltage to the LEDs so the load current can be kept low by using series / parallel combinations; and it's switch mode so efficiency is much higher than with an analog constant current control.

If you can't add the sense resistor, select one for the maximum current, eg. 100mA, add a resistor between the sense resistor and IC sense input and another from an MCU DAC or filtered PWM output to the IC sense pin.

With appropriate resistor values, that can then allow a programmable positive bias in to the sense pin, so any lower current can be set as the IC current limit.

The separate PWN input would allow 0-100% dimming within that range.

 

[danadak]

Wireless or wired???? - not sure why you ask this concidering the question and diagram.

Just curious about whether or not there is user interface for controlling brightness.

Classic circuit to do current control, a V to I converter (non PWM, Vset would be on
chip DAC, could use S/H for multi channel approach) -

Better would be PWM and use a sampling R (shunt) in string to measure peak
current to calculate the avg current and adjust as necessary.

This is single chip approach 4 strings shown, could expand to 8 easily. Many other onchip
resources available for other tasks. See right hand window for resources used/left. Could add
a button(s) to use to control individual string brightness ( current ).

Basically current is measured by SAR of 2 ohm R in emitter of NPN and PWM duty cycle adjusted to
control average current in string. Button could be mechanical or Capsense (onchip capability).

Other onchip resources -

Regards, Dana.

 

[calltronics]

As you have a micro you could setup a 4 bit current source to give you 0 to 160mA in 10mA stages ( 1 more bit will give 0 to 320mA). With only (max) 3.5V to drop fairly small transistors could be used (~300mW max dissipation) - more if 5 bit.

mike.

Interesting, can you send me more details? or point me in the right direction?

 

[Pommie]

Not the ideal solution, if you look at something like the LT3092 (SOT223) and use 4 set to 10, 20, 40 & 80mA with transistors to switch them on/off. The diagram on page 18 - "Pulsed current source - load to Vin" then that will do what you require.

Hopefully, there's a more suitable device available. Quoted prices ATM are ridiculous.
If you can find something like the AL5809 with the right current limits then they would be perfect.

Mike.
Note, 160mA should be 150mA. 0 to 15 representing 0-150mA.

 

[Les Jones]

A variation on Pommie's suggestion would be to use a cheap ebay step up voltage regulator modified as a constant current source. As you seem to be able to use 4 digital I/O pins to select the required current you could use 4 mosfets to select four binary related current sense resistors. This is a link to the way I modified a step up voltage regulator to be a constant current source. The four current sense resistors and their switching mosfets would replace R3 in my circuit.

Les.