LM36922H: Full-scale ILED Setting under Enabling via PWM

Hi Hideyuki-san,

Both strings will be 25 ma when PWM is 100% duty cycle. The LM36922H does not have external ISET resistor to set max ILED current, it is fixed at 25 mA/string maximum.

Let me know if you need anything else.

Best regards,
Randy

Dear, Randy-san. 

Thank you so much for your teaching. It is quite helpful for me. 

I would like to make sure several things in advance. 

Sorry again, Please give your advice,. 

1. Boost fSW. 

Under external PWM mode, How much is Boost fSW? 1MHz?

2. Dimming Ratio. 

I wonder 1:500 dimming ratio could be achievable under Boost fSW=1MHz $

external PWM(fPWM) = 2kHz? 

Best Regards, 

H. Sakai

Hello Hideyuki-san,

The default setting for Boost fSW is 1 MHz and 4.7 uH inductor.

The dimming ratio does not vary with Boost fSW, it is dependent on the PWM input supplied to LM36922H and LM36922H PWM Sample Rate.

External PWM frequency 2 kHz will be a problem because the default PWM Sample Rate is 24 MHz, is it possible to increase fPWM to 5 or 10 kHz?

Best regards,

Randy

Dear, Randy-san. 

Thank you so much for your reply. 

The customer would like to realize over 1 : 500 Dimming ratio. 

It is possible to change fPWM to 5kHz. 

1. How much will be estimated as Dimming ratio under fPWM=5kHz? 

    1 : 200? 

2. Application Report "SNVA768" described as below. 

   I wonder fPWM=2kHZ is really impossible under 24MHz PWM Sample rate? 

Best Regards, 

H. Sakai

Hello Hideyuki-san,

The PWM resolution will still be 11-bit at 5 kHz pmw input frequency so the dimming ratio is dependent on the PWM source.
I am concerned that the PWM input frequency could go below 2 kHz which would result in timeout and turn off the current sinks.
As long as the PWM frequency does not go below 1.85 kHz it will be okay.

Best regards,
Randy

Dear, Randy-san.
Thank you so much for your reply.

I understood your mentioning.

I tried to understand to read out the datasheet.
When fPWM=5kHz, the signal will converted to 11 bit according Table 3.
Another important setting will be PWM Input Hysteresis according Table 4.
Looks the default will be 4 LSBs.
So that, I suppose 127 steps(1 : 128) will be the limit of Dimming ratio under fPWM=5kHz.

I wonder my understanding is correct?

Best Regards,
H. Sakai

Hello Hideyuki-san,

The LM36922H pwm hysteresis does not affect the sampling of the pwm input. It is used to determing when to update the pwm duty cycle in the event of a change in direction (i.e. ramp up to ramp down).  Please refer to attached file for additial detail.

Let me know if you need any clarification on this feature.

Best regards,

RandyLM36922H pwm hystersis.pdf