Inquiry About Rext Range for PCA9956B

[mskim90]

Dear NXP Support Team,

I am currently designing a circuit using the PCA9956B LED driver and would like to clarify the recommended range for the external resistor Rext​ used to set the LED output current.

Specifically, I am unable to find the minimum and maximum values for Rext​ in the PCA9956B datasheet. Based on similar products like the PCA9955B, the range appears to be 640Ω to 100kΩ, but I would like to confirm if this applies to the PCA9956B as well.

Could you please provide guidance on the appropriate Rext​ range for this device? Additionally, if there are any supplementary design guidelines for optimizing current settings, I would greatly appreciate your input.

Thank you for your support, and I look forward to your response.

Best regards,

[guoweisun]

HI mskim90

You can get the full data or related information on datasheet page23:

7.3.13.1 Adjusting output current

[mskim90]

I am currently using Rext value as 6.8KΩ. Is it okay to use this resistor value as 52KΩ?

[guoweisun]

The open/short circuit source voltage is influenced by the external resistor Rext
and may falsely trigger the open condition if Rext is bigger than 3.6 k. Use 3 k or
smaller Rext to guarantee no false open triggers; if smaller analog currents are required
with Rext larger than 3 k then don't use the open/short detection.

[mskim90]

I saw this article in the data sheet. But it is not clear whether this Rext value is up to 10K ohm or 52K ohm.

[guoweisun]

1: If use  open/short detection suggest low 3K.

2: If don't case ittem1,you can use below two equations to calculate the Rext for give the number of LED current and IREFx setting value,the range IREFx is from 1 to 255

[mskim90]

I want to control the Io_LED_MAX value to a very small value less than 1mA.
So the resulting value is Rext 53K.

I am asking because there is no Rext Max value in the data sheet.
Is it okay to use it like this?

[guoweisun]

Don't suggest to use such value, for example you can select suitable resistor which less than 3K ,then setting the IREFx value to getting 1mA current .

[mskim90]

In the data sheet I have, "Adjusting output current" can be found on page 32, but the values ​​for Rext and min/max are not clearly stated.