Use Smart High-Side Drivers to Keep Things from Exploding
Working for a semiconductor company is a lot of fun. I’m happy to say I’ve worked for five of them, not counting my consulting work. Everything is leading-edge or bleeding-edge – it’s a very edgy business. The 1990s were especially edgy as that was the wild west of the semiconductor business, when boundaries were broken every week and breakthroughs in semiconductor physics seemed like magic.
I’m known for keeping a quote log in those days. This is a written record of some of the silliest – or strangest – quotes or dialogs ever heard in the business world (“What time is the 10 o’clock meeting?”). Semiconductor professionals say some of the strangest things.
Like the time an analog semiconductor design engineer stood up in a meeting with a major automotive customer and announced, very slowly, “I have got three things to say. The first, is not important. The second, is company private so I cannot tell you. And the third, I forgot,” – then he sat down. You can imagine the look on the faces around the table.
Semiconductors smarten up; people sometimes don’t
Sometimes the most memorable moments come from discussions with product people at the factory. I was program manager for a neat little high-side driver. These new smart semiconductors were just becoming popular because they saved so much design effort for the engineer. High-side drivers are smart little things that can also protect against circuit and device faults, including short-circuit detection.
It was during the wild west era that I was on the phone with a European QA test engineer. I’d learned to respect the test engineers as some of the brightest people in any company. They are crucial when the first engineering samples of a chip are available because the QA engineers decided what the part can and cannot do. The conversation with the QA engineer went like this:
Bill: “So, what’s the status of our newest high-side driver?”
QA: “Well, there is a problem with the short-circuit protection.”
Bill: “Uh, what problem?”
QA: (long pause) “The part, it explodes.”
QA: “But don’t worry, we have a test to see if a part is good.”
Bill: “Wait, what test, what are you…?”
QA: (proudly) “We short circuit the part. If it does not explode, it is good!!!”
Name of company and product withheld to protect the guilty (you know who you are). I later learned they would short each high-side driver on the test rig, and if it exploded the QA engineers would all cheer, then take a sip of an adult beverage. (Drinking and testing are still not advisable.)
Now every time someone mentions a high-side driver, I smile.
Modern high-side drivers are even smarter, and besides short-circuit protection they can detect faults like a broken connection between the load and ground, and potential thermal faults.
Figure 1: The STMicroelectronics ISO8200BQ comes in a TFQFPN32 package with a nice fat VCC pad that can handle 45 volts at 5.6 amps. This allows the octal high-side driver to supply up to 0.7 amps to each of the eight channels. (Image source: STMicroelectronics)
STMicroelectronics has always had an impressive range of high-side drivers that interface easily to most microcontrollers. For example, the ISO8200BQ is an octal high-side driver with more smarts than some of people I worked with in the 90s. Each of the eight channels has a rated RDS(ON) of 0.11 Ω at 25°C and can handle 0.7 amps regardless of whether the load is inductive, capacitive, or resistive. Yes, it has short-circuit protection, so it won’t explode. Each channel also has over-temperature protection, so it won’t overheat.
The ISO8200BQ interfaces easily to the 8-bit port of most microcontrollers for independent control of each channel and requires only a few external components. The ISO8200BQ has a global output enable OUT_EN. When pulled low, this disables all the outputs. This increases the safety of the application by allowing the microcontroller to turn off all the loads, useful in an industrial situation to prevent things from exploding.
The device also has an active low LOAD (LOAD\) signal that latches all the data on the input pins IN[1:8] into the channel logic buffers. When the LOAD\ signal is low, data passes into the eight channel logic buffers. When the LOAD\ goes high, all the data is latched into the buffers. The high-side driver outputs OUT[1:8] match the state of the eight channel logic buffers when the active-low SYNC (SYNC\) signal goes low, and the driver states are latched when SYNC\ goes high.
If both LOAD\ and SYNC\ are held low all the time, the ISO8200BQ behaves like a common high-side driver, and signals on IN[1:8] are immediately reflected on OUT[1:8]
Galvanic isolation makes everything a whole lot safer by electrically separating the 5 volt logic side of the device from the high voltage 48 volt side of the device. This increases the reliability and safety of the circuit.
Now it’s easy to drive any kind of load, and if it does not explode, it is (still) good!