LP8860-Q1: Usage with different number of LED/string

Hi James,

LP8861 can support different number of LEDs pers string depending on the application. For the application you described, I calculated Vf of the differenet strings as follows:

String 1 Vf = 4x1.7=6.8V

String 2 Vf=3x2.1=6.3V

String 3 Vf=2x2.5=5V

This configuration can be supported by LP8861 witht the current of 30mA/string but there will be some extra power dissipation in the device because of higher headroom voltage at current sinks for strings 2 and 3. Please see adaptive boost functionality on page 19 pf the data sheet for further explanation of how this works and let me know if there are any further questions.

Regards,

Liaqat

Hello,

TPS61176 is designed to be used in appication where number of LEDs is same in each string with slight variation in forward voltage drop of each string due to LED part to part variation. But it can also be used if different number of LEDs are used in each string as long as the difference between forward voltage drop of the strings stays low. Basically, this device operats as adaptive boost converter wehere, based on the forward voltage of LED string, output voltage is adjsuted to keep desired current flowing through the LEDs but at the same time reduce power consumption in the device by keeping the boost output voltage minimum amount higher than voltage drop of the string with the highest forward voltage drop. So let say three string are used in an application and, at a certain desired current, Vf of string 1 is 10V, that of string 2 is 11V and that of string 3 is 12V. Boost output voltage will be adjsuted to approx. 12.7V i.e. slightly higher than maximum Vf of the string so as not to saturate the current regulator for that string. Becuase of this scheme, current regulator for strings 1 and 2 will burn hgiger power than current regulator for string 3. If different number of LEDs are used in each string, depending on the difference between highest and lowest forward voltage drop, it may or may not be feasable to use this device. If the difference between forward voltage of different strings is too large, the device may dissipate enough excess power so as to hit thermal shutdown.  If you can let me know your aplications parameters (current per string, VF of the strings, number of LEDs in each string, input voltage to boost), I can give you rough idea if it will work or not.

Since this is a switching converter, EMC will always be a concern but it can certainly be mitigated by good layout and board design practices.

Thanks,

Liaqat

Hello,

I reviewed the data sheet for TPS6106x and it is an option. For EMI, I do think that mutiple switching converters may create more issues than a single device but with good board layout, you may still be able achieve good EMI performance.

Regards,

Liaqat

Hello James,

I reviewed  datasheet and LP8557 has six 25mA current sinks and uses input PWM signal for brightness control.

Regards,

Liaqat