Sharing RExt between multiple MAX6971 LED drivers?

[MsJaye]

Hey there,

I'm designing something using 16 MAX6971 LED drivers, each of which will drive 16 LEDs. The MAX6971 sets the current per LED using an external resistor, RExt. I'm wondering if I can use the same resistor, or possibly a pot, as RExt for all 16 of the MAX6971s, and thus use it as an overall brightness control for the array? (I'll already be using PWM on the array for other purposes, so having a simple overall brightness adjustment would be nice.)

I realise that the value for the shared RExt will probably not be the same as for a single MAX6971 for an equivalent drive current. I'm more wondering if there are reasons this won't work, or would be a bad idea?

Datasheet here: http://datasheets.maximintegrated.com/en/ds/MAX6971.pdf

Thanks,
Jaye.

[fcb]

I'd be careful about paralleling th Rset pin's on multple devices.

Use a current mirror (with 16 outputs...), or perhaps 4x4 output current mirrors instead of the resistors.

[jackbrouno]

What if i only have an resistors

[German_EE]

It doesn't work (I tried it a few years ago). The brightness depends on the current of ONE 6971 running through a resistor. Using a digital potentiometer with either SPI or I2C is one solution especially if you give all potentiometers the same address.

[dannyf]

Without knowing the mechanism Maxim used to program the current via Rset, you probably cannot know for sure.

However, other chips that use a resistor to program have tend to utilize a current drive on the resistor and sense the voltage drop on the resistor to program.

If I were you, I would measure the voltage across the resistor and try to see if it is done via current or voltage.

If the programming is done via voltage, you can probably set a divider and tie all the Rset pins to that divider, via isolation resistors.

If the programming is done via current, you may use current mirrors to do it. This can be done fairly easily and inexpensively, via either discrete parts or ICs (like those 5-20ma transmitters).

But I would start with measuring the voltage / current over a range of resistor values.

[MsJaye]

Without knowing the mechanism Maxim used to program the current via Rset, you probably cannot know for sure.

However, other chips that use a resistor to program have tend to utilize a current drive on the resistor and sense the voltage drop on the resistor to program.

If I were you, I would measure the voltage across the resistor and try to see if it is done via current or voltage.

If the programming is done via voltage, you can probably set a divider and tie all the Rset pins to that divider, via isolation resistors.

If the programming is done via current, you may use current mirrors to do it. This can be done fairly easily and inexpensively, via either discrete parts or ICs (like those 5-20ma transmitters).

But I would start with measuring the voltage / current over a range of resistor values.

Many thanks, this was just the kind of advice I was looking for.

Jaye.