LP8555: Conversion ratio VOUT/VIN problem

Kouassi KRA

Prodigy 70 points

Part Number: LP8555

Hello,

According to LP8555 datasheet page 6, VOUT/VIN (MIN) = 1,3.

In my application at minimum current ILED=5 mA, i have :

VIN (MAX)=5,5V and VOUT(MIN)=Vf(LED) * Number of LED = 1,764*4 = 7,056V => VOUT/VIN =1,28 <1,3 .

I plan to use an 18 ohm resistor in series with the LEDs to deal with this.

However, is it possible to solve this problem by increasing the headroom voltage ? if so, which register should I change ?

over 4 years ago

0 Yuting Shao over 4 years ago

TI__Genius 13001 points

Hi Kouassi,

LP8555 supports configuring the headroom voltage. VHR0 registor (address 79h) bit[2:0] control the minimum headroom voltage from 50mV to 260mV.

BR,

Robin

0 Kouassi KRA over 4 years ago in reply to Yuting Shao

Prodigy 70 points

Hi,

Thanks for your response, but at page 48 section 9.2.1.2 it's states that VHRO registor (address 79h) bit[2:0] is the maximum voltage headroom at minimum current. Is it a mistake ?

0 Yuting Shao over 4 years ago in reply to Kouassi KRA

TI__Genius 13001 points

Hi Kouassi,

Yes, this is a mistake and the right description should be "LED driver minimum headroom voltage at minimum current".

BR,

Robin

0 Kouassi KRA over 3 years ago in reply to Yuting Shao

Prodigy 70 points

Hi,

What about VHR0_SLOPE ? Is the definition "maximum headroom voltage at maximum current" section 9.2.1.2 correct?

If so, why the worst case Headroom is 2V (section 8.5.12) while VHRO_SLOPE(MAX)=300 mV (section 8.5.21) ?

Regards,

Kouassi

0 Yuting Shao over 3 years ago in reply to Kouassi KRA

TI__Genius 13001 points

Hi,

Section 9.2.1.2 definition is right.

300mV is just the max. adaptive headroom voltage instead of the max. headroom voltage.

In fact, max. headroom voltage can up to be 31V and 2V in section 8.5.12 is just considered a certain amount allowance in that typical application.

BR,

Robin

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LP8555: Conversion ratio VOUT/VIN problem