Controlling Load Switch Enables - Page 1
Forum section: electronics / projects

author: bhj99
author_rank: Contributor
timestamp: June 25, 2023, 02:30:00 pm
content: Hello, I plan to use the PCAL6524 to support GPIO expansion for a design. I plan to use two of the I/O ports as outputs to control the active high switch control enable inputs of the TPS22966 dual channel load switch device (these enables cannot be left floating). Essentially, I would like to be able to shut off a channel of the load switch at some point in my application.From the PCAL6524 datasheet, at power-on, the I/Os are configured as inputs. In section 6.6 of the DS, it states "When an I/O is configured as an input, FETs Q1 and Q2 are off, which creates a high-impedance input. The input voltage may be raised above VDD(P) to a maximum of5.5 V." So, are the ports NOT pulled-up to Vcc initially via the internal resistors to prevent floating inputs/ensure a define logic state prior to being able to be configured as input or output from the system master? I ask this in order to determine if I need external resistors or can I rely on the PCAL6524? If external pull-ups are installed, am I still able to turn off channels of the load switch independently through software using the PCAL6524?I have schematic snippets attached for reference. Appreciate any help.

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 25, 2023, 02:50:43 pm
content: I think it's pretty obvious what HiZ input means. Being HiZ and floating is the expected behavior of a GPIO in input mode. Them being indeterminate isn't a problem to the expander itself since they're configured by internal registers through I2C.If you don't want it to float at any condition then sure you can add a pull up/down. Your expander should be able to "overpower" the pull up/down provided you sized the resistors correctly w.r.t expander's max current sink/source.

author: bhj99
author_rank: Contributor
timestamp: June 25, 2023, 03:10:07 pm
content: Thanks for the confirmation, just did not want to miss obvious information on the GPIO expander.Regarding your comment on overpowering the external pull-ups, the expander datasheet states that device Port P outputs have 25 mA sink capabilities. The programmable pull-up/down resistors are 100 kohm typically. I currently have 10 Kohm pull-ups to 3.3V (so 0.33 mA) near each of the load switch enable pins; do you think the sizing is sufficient or should I reconsider?

author: ajb
author_rank: Super Contributor
timestamp: June 25, 2023, 07:59:40 pm
content: Quote from: bhj99 on June 25, 2023, 03:10:07 pmAs you've noted, there's only 330uA flowing through the resistors with 3.3V across them, and the input to the load switch likely adds a miniscule amount of current--so miniscule it can normally be ignored--so that should be no trouble at all for an output that can sink 25mA. If you want to confirm this, you'd need to look at the output V/I characteristics of the expander and the input threshold of the load switch. The PCAL6524 datasheet has conveniently extensive output V/I graphs (often you only get one or two 'VOL@ X mA' entries in a table), so you can use these to determine the exact output voltage at the expected load current. As long as that's below the input threshold of your load switch under your expected operating conditions then you're good.