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LP5891: VLEDR and VLEDG/B

Kurumi Hasegawa
Expert 6811 points
Part Number: LP5891
Other Parts Discussed in Thread: , , LP5890EVM

Hi team,

 

The datasheet has example setup for Four Devices Stackable Connection, which has VLEDR and VLEDG/B separated. Would you please share why those are separated?

 

Table 8-1 shows higher example value for VG/B than VR. So I assume required voltage for VG/B (VF) is higher than VR. But LP5891 has constant current source for VB/G/R.

Is it OK to connect same voltage line for VB/G/R with series resistors? In that case resistor on VR line will have higher value.

 

The EVM has VR=5V or 3.3V selectable, with VG=5V and VB=5V. It does not have resistors to limit current.

Best regards,

Kurumi

  • 0
    TI__Mastermind 19655 points

    Hi Kurumi,

    • Generally, Red/Green/Blue LEDs have different forward voltages. That is the reason why we design separate power supply pins (VR/VG/VB) for Red/Green/Blue LED, because the user can supply different-color LEDs with appropriate voltage supplies respectively and therefore reduce the voltage drop on the current sources. This will reduce the power loss on the current sources and therefore improve system power efficiency.
    • Generally, forward voltage of green and blue LEDs are higher than that of red LEDs. The green and blue LEDs have the similar forward voltage, and they can be supplied by the same power rail. The red LED can be supplied by a power rail with lower voltage and can share the power rail with VCC if it can meet all requirements. This is what we do in Section 8.2.1 Design Requirements from the datasheet.
    • Do not connect series resistors for VR/VG/VB. That may lead to unstable power supplies and cause large power loss. As is stated in the datasheet, the ripple of the LED supply voltage must be less than 5% of their nominal value.
    • It is ok to connect the same power rail (e.g., 3.8V) for VR/VG/VB as long as the excessive power loss on the current sources won't cause any thermal problem.
    • For LP5890EVM / LP5891EVM / LP5891Q1EVM, we design the jumper J7 to let the user select different power rail for VR. Therefore, they can evaluate the device under different power supply scheme.
    • You can refer to section 9 Power Supply Recommendations from the datasheet and [Q5] from this [FAQ].

    Best Regards,

    Steven