Maximum LED current of LP8556

Hi Takahiro,
Yes, LP8556 can really support 50mA per channel, but it was depend on the minimum input voltage and LED counts per channel. So can you tell me the application condition? Thanks.
Sean

Thank you for your support!

We use LP8556 under the following conditions.

Vin=3.0~5.5 or 24V
Vout=24.5V
(VF=6.15V*4series)
CH=6ch
LED current=50mA

Please tell us the recommended IC if we can't use LP8556.

Best regards,

Takahiro Nishizawa

Hi,
For your application, the input range 3.0-5.5 can not support this application, because it will trigger the current limit. for Vin=24V ,it's ok to support your application. But we recommend less than 20V for the VDD pin, for the EN/VDDIO you need provide another low power rail(3.3V) for this device.
Sean

Thank you for your answer!

Do you know recommended LED driver under the following conditions?

Vin=3.0~5.5V
Vout=24.5V(VF=6.15*4series)
LED current=50mA per ch
CH=6ch

best regards,

Tsakahiro Nishizawa

Hi ,
The minimum 3V input voltage is two low for maximum output current, most backlight driver has internal 1.8A current limit. So maybe external FET backlight driver is more suitable. how about LP8863. You can have a review about the datasheet. It should support those spec.
Sean

thank you for your answer!


We understand your answer.
Do you have any information about LP8863?
For example, dataseet.

We checked LP8863-Q1.
Do you have non-Q1 product about LP8863?



And New question about LP8556
Can we use LP8556 under the following conditions?

Vin=24V
Vout=24.5V(VF=6.15*4series)
LED current=50mA per ch
CH=6ch
Duty is no problem? 24V→24.5V

Best regards,

Takahiro Nishizawa

Hi
For LP8863, you can search on www.ti.com, it only has the Q verision. There are lots of information about this device.
For LP8556, it suggest to with less tan 20V for the input voltage, for 24V, it may have some risk. i don't recommend you to do this way. Thanks.
Sean

Thank you for your answer!


Our customer demands 75Hz about PWM frequency.
Do you know recommended LED driver?

Vin=3.0~5.0V
Vout=24.5V(VF=6.15*4 series)
LED current=40~50mA per ch
CH=6ch
PWM dimming=75Hz PWM frequency
It is used for backlight.


PWM frequency of LP8863-Q1 is 152Hz.
It is too high.


Best regards,


Takahiro Nishizawa

Hi,
LP8556 can support direct PWM dimming, it can support 75Hz dimming frequency. But it has internal current limit（1.8A for BGA Package, 2.1A for QFN Package). for input voltage, you need to increase the power supply to support 6ch*50mA application.
Sean

Thank you for your answer!

Is there no product that meets the following conditions?

Vin=3.0~5.0V
Vout=24.5V(VF=6.15*4 series)
LED current=40~50mA per ch
CH=6ch

And please tell us the maximum LED current per ch about LP8556.
we think it is about 30mA.

Vin=3.0~5.0V
Vout=24.5V(VF=6.15*4 series)
CH=6ch
Maximum LED current per ch=?


Best regards,


Takahiro Nishizawa

Hi Takahiro,
I'm sorry that we don't have the device that can support this specs;
For LP8556, for minimum input=3V , the maximum current is about 30mA(less than 30mA). That depends on the inductor , switching frequency and some other things.
Sean

Thank you for your answer!

We have new questions.

Question 1.
Please tell us the inductance value under the following conditions.
And do you know the formula about inductance value?
In datasheet p.48, 9.2.2.1 Recommended Inductance for the Boost Power Stage,
LED Current is 20mA. we want to know the current value under other conditions.

Vin=3.0~5.0V
Vout=24.5V(VF=6.15V*4 series)
LED current=25mA perch

Question 2.
Please tell us the current value when short-circuited.

Question 3.
The PWM frequency assumes Input = Output.
Are there any problems when switching frequency is 1250kHz

Question 4.
In datasheet, PWM frequency is 75Hz~25000Hz.
Can we use LP8556 by the following method?

Input frequency=75Hz
Output frequrency=75Hz
(Input frequency=Output frequency)


Best regards,


Takahiro Nishizawa

Hi ,
I'm sorry we don't have the device can support your application for this low input voltage and high output power with internal FET.
For LP8556, it should be support about 30mA for your specs, but it also depends on the switching freqiency and inductor and some other consideration. Thanks.
Sean

Thank you for your answer!

Please tell us your answer about these questions of LP8556.

Vin=3.0~5.0V
Vout=24.5V(VF=6.15V*4 series)
LED current=25mA per ch

Question 2.
Please tell us the current value when short-circuited.

Question 3.
The PWM frequency assumes Input = Output.
Are there any problems when switching frequency is 1250kHz

Question 4.
In datasheet, PWM frequency is 75Hz~25000Hz.
Can we use LP8556 by the following method?

Input frequency=75Hz
Output frequrency=75Hz
(Input frequency=Output frequency)


Best regards,


Takahiro Nishizawa

Hi
For question2, you need to test it with EVM, I don't have the data now.
For question 3,I don't understand your question, can you give more detail description?
For question4, yes, LP8556 support direct dimming, please refer to the datasheet.
Sean

Thank you for your answer!

About question 3,
Switching frequency is 1250kHz.
PWM dimming frequency is 75Hz.
Are there any problem?
Does the IC operate normally?

About 4,
We can use LP8556 that input PWM frequency is 75Hz and output PWM frequency is 75Hz(input=output).
Is this understanding correct?

Best regards,


Takahiro Nishizawa

Hi,
Yes, LP8556 support direct PWM dimming mode,it can work normally with 75Hz.
Sean

Thank you for your answer!

I understand that these are no problems.

Switching frequency is 1250kHz
PWM dimming frequency is 75Hz(input=output).

Best regards,

Takahiro Nishizawa

Thank you for your answer!

Please tell us the formula about inductance value.

In datasheet p.48, 9.2.2.1 Recommended Inductance for the Boost Power Stage,
It is written that current is 20mA.
We want to calculate under other conditions(Vin and current and so on)

Best regards,


Takahiro Nishizawa

Hi,
Please refer to this application notes:www.ti.com/.../snva763.pdf
Sean

Thank you for your answer!

In this document, the formula of p.2~,
We need to check the efficiency.
www.ti.com/.../snva763.pdf

Please tell us the efficiency about LP8556.

Vin=3.0~5.0V
Vout=24.5V(Vf=6.15*4)
ch=3ch
LED current=50mA per ch


Will the contents of this document apply to LP8556?


In datasheet of LP8556 p.8,
curret limit, value of IBOOST_LIM_2X = 1 is typical only.
Do you know minimum and maximum value of current limit?


Best regards,


Takahiro Nishizawa

Thank you for your answer!

I have questions about the formula of this document p.2~.
www.ti.com/.../snva763.pdf


Question 1.

We need to check the efficiency by your document.
But we can't check the efficiency.
We can't check the inductance value if we don't know the efficiency.
Please tell us the efficiency.
Do you know the method to calculate the recommended inductance value?

Vin=3.0~5.0V
Vout=24.5V(VF=6.15V*4 series)
LED current=50mA per ch
CH=3ch

We want to check the inductance value under the above conditions.



Question 2.

Does this document also apply to LP8556?
www.ti.com/.../snva763.pdf


Best regards,


Takahiro Nishizawa

Hi,
This application note suit all most boost+current sink topology. For your calculation, just use 85% as the efficiency to choose the inductor.
Sean

Thank you for your answer!

I understand that efficiency is 85%.

We try to use 4.7μH.
Are there any problems?

Vin=3.0~5.0V
Vout=24.5V(VF=6.15V*4 series)
LED current=50mA per ch
CH=3ch