Question regarding display backlight

[FreshmanNewbie]

I have a display that mentioned LCD supply current and backlight current.

To calculate the overall current required by the display, I hope adding both the above currents should be correct.

However, while checking the LED Forward Current section, I see that for Iled, 0V/0% duty the current is high and for 100%duty, the current is low. Is this a typo? Or am I missing to understand something.

 

[Easy peasy]

For anodes taken to + Vin - if you take the cathodes low ( via suitable resistors ) the the brightness would increase with the amount of time that you hold low.

and vice versa . . .

 

[FreshmanNewbie]

For anodes taken to + Vin - if you take the cathodes low ( via suitable resistors ) the the brightness would increase with the amount of time that you hold low.

and vice versa . . .

Not able to understand. Can you please explain in better way

 

[KlausST]

Hi,

To calculate the overall current required by the display, I hope adding both the above currents should be correct.

How can we know?
* we don´t have the full datasheet
* we don´t know your schematic
* we don´t know what you want to achieve with your math (so I guess - and that´s all I can do - your math may be correct, but it makes no sense)

Example:
You may have a battery specified with 12V and you may have an incandescent lamp with 12V.
Then the math: 12V + 12V = 24V is correct ... but it makes no sense, because most probably there nowhere is 24V in this circuit.

I see that for Iled, 0V/0% duty the current is high and for 100%duty, the current is low. Is this a typo?

I´m sure, if you read the datasheet thoroughly .. it will tell you.
With the snippet of datasheet you show .... we can only guess. But I´m not doing electronics design by "guessing".

Klaus

 

[betwixt]

My guess is the LCD has a driver transistor in series with the LEDs so the signal is inverted. As mentioned, it could also be low side driven but if it was, the implication is all the LED current has to be carried by the PWM driver and I would expect a note on the data sheet to explain that.

Brian.

 

[Easy peasy]

For anodes taken to + Vin - if you take the cathodes low ( via suitable resistors ) the the brightness would increase with the amount of time that you hold low.

and vice versa . . .

I'm terribly sorry - but if you can't understand the above you are in a bit of a pickle.

Assume an LED with the cathode (K ) grounded, and say a pwm drive to the top ( via suitable resistor ) the brightness will increase as the pwm increases

Now, if we connect the anode ( A ) to a positive power source and use a pwm drive to the cathode (K) then the brightness will increase as the pwm goes to low values, e.g. 10% ( as this is 10% high, 90% low - where the current flows ) At 100% ( pwm as high as the volt source ) there will be no light at all.

One drive scheme is the inverse of the other - it all depends whether the part is intended for the LED's to be pwm driven from the top or bottom.

 

[FreshmanNewbie]

I'm terribly sorry - but if you can't understand the above you are in a bit of a pickle.

Assume an LED with the cathode (K ) grounded, and say a pwm drive to the top ( via suitable resistor ) the brightness will increase as the pwm increases

Now, if we connect the anode ( A ) to a positive power source and use a pwm drive to the cathode (K) then the brightness will increase as the pwm goes to low values, e.g. 10% ( as this is 10% high, 90% low - where the current flows ) At 100% ( pwm as high as the volt source ) there will be no light at all.

One drive scheme is the inverse of the other - it all depends whether the part is intended for the LED's to be pwm driven from the top or bottom.

Thank you very much for the clarification.

 

[KlausST]

Hi,

I'm terribly sorry - but if you can't understand the above you are in a bit of a pickle.

I have to clarify a bit:

the brightness would increase with the amount of time that you hold low.

1) It´s not that the brightness increases with time.
--> it´s the average brightness, that increases (from the snippet we don´t know if there is any electric filter involved)

2) it´s not related to the LOW time (only)
--> it´s related to the duty cycle, thus it also is related to HIGH time.

Duty cycle is defined as : duty_cycle = active_time / (active_time + inactive_time)
Since in this application LOW means ACTIVE... it is valid to say
here: duty_cycle = LOW_time / (LOW_time + HIGH_time) = LOW_time / period_time

So the average brightness of the backlight will be the same for
* 30% duty cycle on 1kHz PWM
* 30% duty cycle on 10kHz PWM
* 30% duty cycle on any valid PWM frequency.
although the LOW_time at 10kHz PWM is much less than at 1kHz PWM

At least this is true for the most (low_active) PWM applications.
I´m sure the full datasheet will clarify al in more detail.

the brightness will increase as the pwm increases

PWM means pulse width modulation
(it does not mean "duty cycle")
it is defined by
* LOW level (0V according snippet of datasheet)
* HIGH level (3.3V according .. datasheet)
* duty cycle (or HIGH time AND LOW time) (0% to 100% according datasheet)
* frequency (or period time) (1kHz to 10kHz according datasheet)

@EasyPeasy:
This is not meant to by picky. It is meant for the OP to learn the correct use of terminology.
For sure, if I or any other experienced designer reads your post .. we all know what you mean.
But this unexperienced member may be confused by sloppy wording.

Klaus

 

[Easy peasy]

there is an element of English here, brightness = average or perceived brightness = brightness

in any constant freq pwm, increased low time = increased time the signal is low - to assume variable freq is non sequitur

PWM and duty cycle go hand in hand, increased pwm = increased ON time in common engineering parlance.