Shopping for high side smart switches. Why are control inputs active low? - Page 1
Forum section: electronics / projects

author: e100
author_rank: Frequent Contributor
timestamp: December 05, 2020, 02:41:07 pm
content: Thus requiring a transistor or MOSFET if you intend to drive them from a microcontroller.Is active low a universal thing for smart switches, or is it just that the ones I was looking at (Infineon and International Rectifier) are intended for the automotive market and that is the standard for automotive?Examples:http://www.farnell.com/datasheets/1651319.pdfhttp://www.farnell.com/datasheets/2332433.pdf

author: VooDust
author_rank: Regular Contributor
timestamp: December 05, 2020, 03:42:01 pm
content: Uhmm... no?Switch off: Pull the switch control line high via a resistor and set your MCU pin to high impedance.Switch on: Pull the control line low by setting your MCU pin to low, i. e. sinking current.Specify your resistor adequatly so the MCU is able to drive the line low (higher resistor value = better but check control input leakage current to be sure).Edit: maybe not a good idea. You could use a resistor divider to ground to not put too much voltage on the MCU pin.

author: VooDust
author_rank: Regular Contributor
timestamp: December 05, 2020, 03:48:28 pm
content: Regarding your actual question, no I don't know why active low is "a thing", probably has to do with how transistor circuits eventually make up your chip. Maybe there's some advantages.Btw, active high does not solve your problem, what if Vhigh = 0.8*Vin and Vin were 18V?Here's an active high switch that I recently used, it's not smart though:https://www.digikey.ch/product-detail/en/TPS27081ADDCR/296-34970-1-ND/3671576/?itemSeq=343628594If you look at the schematic in the datasheet, it is essentially a high side switch driven with an additional mosfet - just like you described.

author: e100
author_rank: Frequent Contributor
timestamp: December 05, 2020, 04:35:05 pm
content: Quote from: VooDust on December 05, 2020, 03:42:01 pmIf I'm reading the datasheet correctly for a device such as BTS50055, the control pin can output a voltage close to the supply voltage.Page 8http://www.farnell.com/datasheets/1651319.pdfQuote:"When the device is switched off (IIN =0) the voltage between IN and GND reaches almost Vbb. Use a mechanical switch, a bipolar or MOS transistor with appropriate breakdown voltage as driver. VZ,IN = 66V (typ)"Which means directly connecting a microcontroller output is not going to be possible.

author: e100
author_rank: Frequent Contributor
timestamp: December 05, 2020, 04:46:07 pm
content: Quote from: VooDust on December 05, 2020, 03:48:28 pmIn my naive thinking I incorrectly assumed that 'smart switches' would be controlled using 3.3 or 5v logic level inputs.

author: TimFox
author_rank: Super Contributor
timestamp: December 05, 2020, 05:22:57 pm
content: In general, active-low is a thing because it allows the driver or input switch to be a single-pole to ground or common. For example, an external spst switch that does not need a “power” connection. This has a long history, but may not be appropriate for some applications.

author: phil from seattle
author_rank: Super Contributor
timestamp: December 05, 2020, 05:53:54 pm
content: Active low, in general, is an open collector/open drain kind of thing going back a long way. The BTS50055 datasheet doesn't really specify limit for Vin. But on page 8 the ESD diagram says that the voltage on IN can reach Vbb so I would assume using a small mosfet to control via digital logic. I'd be tempted to use an optoisolator for anything noisy. BTW, I find the terminology in that datasheet to be odd. Vin is the voltage at the control input. Drive voltage is Vbb. Plus, in a world filled with microcontrollers, there is nothing in that datasheet about using one to control it, seems odd.

author: MarkL
author_rank: Unknown
timestamp: December 05, 2020, 11:52:20 pm
content: Quote from: e100 on December 05, 2020, 04:46:07 pmThey exist. You just need to dig through the datasheets.For example:Infineon ISP752R, 6V to 52V, 200mohm typ., 1.3A nom.Infineon BTS5030-1EJA, 5V to 28V, 60mohm typ., 5A nom.STM VN7003ALH, 4V to 28V, 3.5mohm typ., 80A min. SC limitAll are compatible with 3.3V or 5V logic, and all have logic high=ON.

author: Datman
author_rank: Regular Contributor
timestamp: December 06, 2020, 12:16:28 am
content: >> I incorrectly assumed that 'smart switches' would be controlled using 3.3 or 5v logic level inputs.You said it: 3.3V or 5V?... But driving at 0V, it is the same!