# Setting current on Diodes Inc PAM2863 LED driver IC, with external resistor AND driving different voltage to VSET

I'm looking to use the Diodes Inc PAM2863 DC-DC LED driver in a project.

The datasheet gives a V=IR method for setting the average output current with an external resistor, where Iout = 0.1 / Rs.

However, it then says,

"The above values assume that the VSET pin is floating and at a nominal voltage of VREF (2.5V). Note that Rs = 0.1Ω is the minimum allowed value of sense resistor under these conditions to maintain switch current below the specified maximum value. It is possible to use different values of Rs if the VSET pin is driven from an external voltage."

It doesn't give any other formula for setting the output current with a different voltage on VSET, and my microcontroller will be sending 5V.

How would I adjust the current formula to find the new resistor value?
I need to supply more than 1 amp, so I can't just reduce the signal voltage.

in page 4 of the datasheet, it is mentioned that the maximum voltage on the Vset pin = 2.5V. As with any IC you do not want to apply higher voltages then recommended on any of the pins.

So, try using a voltage divider on the output of your microcontroller.

• Maximum voltage on VSET is 6V. A GPIO pin can be used to enable/disable or dim with a PWM signal. – Misunderstood Sep 23 '18 at 20:43

The text you've quoted from the datasheet doesn't make sense. It would seem to suggest that the maximum output current is actually 1A, since that's the maximum current with RS=0.1, but the datasheet clearly shows that the device is capable of 2A output. I suspect that this is poor wording or an outright error.

However, it's always a bit of a risk to design under the assumption that a datasheet is wrong! I would recommend contacting the manufacturer for clarification. They may or may not be helpful, IC manufacturers exhibit a wide range in responsiveness to individual users.

In any case, the datasheet gives you a formula to determine what RS should be for a given output current with some example values for up to 2A, so you should be able to use that to determine a suitable value for your application and then you don't have to worry about the VSET pin unless you want to dim the LED or turn it on and off, which you can do from a digital pin via a transistor as the datasheet indicates.

• I agree the datasheet is not the easiest to understand. But it does not have any obvious errors. The driver has addition capabilities when the max current is set below 1A. – Misunderstood Sep 23 '18 at 20:46

Just set Rs to the values shown in the datasheet to your target current. When the current is set above 1 A you lose the ability to use VSET as a voltage level dimmer. You can still turn the LEDs off and on and use a PWM signal for dimming.

ISENSE is used to set the maximum output current.

If VSET is left floating (or driven between 2.5V and 6V) the output current will be based mostly on the value of RS where the current can be set from 0 up to 2 A.
I say mostly because Vin also contributes to the actual current as shown below. VSET is a multi-purpose input pin.

1. Enable
2. PWM
3. Voltage Level Dimmer

Between 0 and 2 A, VSET can be used to turn the LEDs off and on (Enable) or dimmed with a PWM signal.

Between 0 and 1 A (Rsmin 0.1Ω ), VSET can also be used to keep the output in continuous conduction (no PWM flickering) and reduce the output current by driving VSET below 2.5V. Here Rs sets the max current when VSET is between 2.5V and the pin's max voltage of 6V. When VSET is driven between 0.3V and 2.5V the output current will be reduced accordingly from the maximum current set by Rs. VSET can be driven between 2.5V and 6V where the effect will be the same anywhere in this range. 