TPS92640: TPS92640 500:1 Analog Dimming

Dear Sir,

We want to control the LED by adjust IADJ voltage linearly.  After testing the TPS92641EVM  , it will show blinking when the IADJ voltage around 50mV~100mV. can this situation be fixed? thanks.

Hello,

Can you let me know what your operating conditions are when you see this blinking? Do you have any scope shots of the LED current and the SW node? There are cases where you can hit minimum ON time when analog dimming and I'm wondering if that is the cause, but it shouldn't cause blinking. Scope shots would be helpful to diagnose this.

Thanks,

Clint

Dear Sir,

This is our application circuit figure 1 below. So far we use EVM for testing. We have tried to add 100uF capacitor in the output then the blinking can be solved. but due to size limit, I need to find out another way to figure out blinking issue.  please help give me a suggestion, thanks.

PS: the LED current is too small (below 100uA) , we can't capture it . please see figure 2 below SW waveform

SW waveform

Ok, I see.

The switching waveform that would be useful is at the low dimming levels where you are seeing the blinking, and your current should not be nearly that low for 50mV to 100mV on IADJ. But I think I know what is going on here and why capacitance fixes it.

When you dim to such a low level eventually you will be in discontinuous conduction mode (inductor current falls to zero each cycle). This can allow a small amount of capacitance to discharge an the voltage on the VOUT pin will be very low and unstable. It can even simply drop into a minimum on time/maximum off time type of situation and give you less current that you expect. For this reason you need more output capacitance for deep analog dimming to keep the VOUT voltage stable. You should not need 100uF, but you will probably want something like a 10uF ceramic on the output rather than just 0.1uF that comes on the EVM.

Regards,

Clint

Dear Clint,
but it doesn't work when we try 10uF capacitor in the output. still have any recommendation? and according to the datasheet desciption that  Driver IC maintain CCM working by very low current.

Can you explain more about this ? your reply will be highly appreciated, thanks.

Ah yes,

That is correct. To get linear and proper dimming the output capacitor needs to be large enough to supply the negative inductor current for the entire low side switch ON time. In your case 10uF may not be enough. If you need to dim to such low levels correctly you will need to add enough output capacitance to maintain the current flow. The easiest way is to keep adding Cout until it works properly and hopefully it is less than 100uF.

Regards,

Clint