TLC5940 based current source - suggestions sought

[boylesg]

Fairly easy if the LED supply voltage is 5V - same as the TLC5490 supply. Just use a PNP transistor.

But it doesn't work if 12V is on the emitter of the PNP transistor - the PNP will always be turned on since the 12V on the emitter is always greater than the max 5V that can be applied to the base through the TLC5490 channel?

 

[FvM]

Not clear what you want to achieve with transistor.

the PNP will always be turned on since the 12V on the emitter is always greater than the max 5V that can be applied to the base through the TLC5490 channel?

??? TLC5940 output channel compliance is 18V rather than 5V.

 

[boylesg]

Not clear what you want to achieve with transistor.



??? TLC5940 output channel compliance is 18V rather than 5V.

VO Output voltage range
V(OUT0) to V(OUT15) –0.3V to 18V
EEPROM program range V(VPRG) –0.3V to 24V

According to the datasheet you can use 0.3V to 18V on the output channels, so 5V through the LEDs should be fine.


VIH High-level input voltage 0.8 VCC VCC V

Also according to the data sheet HIGH on an input channel can only ever be Vcc=5V, which is fine except when you want to make the TLC5940 a LED current source via PNP transistors with a voltage greater than Vcc=5V.

Would be a lot more flexible if they made VIH equal to the LED supply voltage.

 

[FvM]

Sorry, I don't understand what you want to achieve.

Why "make the TLC5940 a LED current source via PNP transistors"?
Why drive more than 5V to any of the TLC5940 digital inputs?

Can you sketch a schematic of the intended configuration?