topic Re: Inquiry About Rext Range for PCA9956B in Digital Signal Controllers

Inquiry About Rext Range for PCA9956B

mskim90 — Wed, 08 Jan 2025 02:42:48 GMT

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,

Re: Inquiry About Rext Range for PCA9956B

guoweisun — Wed, 08 Jan 2025 02:54:33 GMT

HI mskim90

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

7.3.13.1 Adjusting output current

Re: Inquiry About Rext Range for PCA9956B

mskim90 — Wed, 08 Jan 2025 04:27:12 GMT

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.

Re: Inquiry About Rext Range for PCA9956B

mskim90 — Wed, 08 Jan 2025 04:38:59 GMT

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

Re: Inquiry About Rext Range for PCA9956B

guoweisun — Wed, 08 Jan 2025 04:54:42 GMT

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.

Re: Inquiry About Rext Range for PCA9956B

mskim90 — Wed, 08 Jan 2025 05:14:56 GMT

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.

Re: Inquiry About Rext Range for PCA9956B

guoweisun — Wed, 08 Jan 2025 05:20:38 GMT

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

Re: Inquiry About Rext Range for PCA9956B

mskim90 — Wed, 08 Jan 2025 07:00:26 GMT

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?

Re: Inquiry About Rext Range for PCA9956B

guoweisun — Wed, 08 Jan 2025 08:00:46 GMT

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 .