August 29, 2019
| By: Jim Harrison
Guest Blogger, Lincoln Technology Communications
I was asked recently, what kind of applications need a voltage supervisor? A silly question. All apps need a supervisor. Certainly, any design with a processor—which is almost every design. That processor can start acting very strangely when its power supply falls below spec. All engineers abhor the thought of the catastrophic design failure this can cause, and in some cases, it can be a serious safety issue.
Even if the design does not have an MCU, the EE does not want their circuit to fall into a state of silliness if their power supply sags but does not go away. Come to think of it, we humans should have a voltage supervisor.
There are a number of different features worth looking into for these devices. Mouser lists 15,789 “supervisory circuits” and Avnet offers 16,904 in its catalog. But do not fear, these numbers are greatly exaggerated by the many, many versions of the same basic device. There are a few important variations to consider.
So, What Specs Do We Look For?
The first specification to consider might be the number of channels. For many portable or IoT designs a single channel is all that’s needed for a single supply. On the other side of things, there are single chips with 12 channels. There are even ones that monitor 32 power rails—for what application, I really can’t imagine. Some of these multi-channel ICs have a single reset output and some have an output for each channel.
If the design has two power rails, it could be best to use two single-channel devices. They are inexpensive, extremely small, and can greatly simplify PCB layout, which will also minimize noise pickup of long traces. On the other hand, if the power circuits are adjacent to one another in the layout, a dual, triple, or quad supervisor would be the most effective.
The most obvious specification is the reset voltage, or threshold. This is determined by the nominal supply voltage minus the supply tolerance—usually 5% or 10%—plus the supervisor tolerance and a small guard band. So, a 3.3V supply dipping to its 5% tolerance would be at 3.135V. If the accuracy of the supervisor is ±1%, we’ll add that in and then put in a 1% guard band. Then the reset will be 3.07V nominal. It’s important to test to be certain every device in the design works properly at this low voltage.
Figure 1. The MAX16140 4-bump WLP Package.
Supervisors are available with threshold voltages as low as 0.4V. At these low voltages one must be especially careful to avoid coupled noise problems. Noise can be coupled into PCB traces from adjacent transformers or RF circuits—or from external sources. The video below, on “High-Frequency Noise Rejection in Voltage Supervisory IC,” shows how a supervisory IC can facilitate safe and reliable system operation.
Many supervisors are factory set and offer lots of trip threshold voltages via a part number suffix. The single channel MAX16140, for example, offers 32 choices from 1.70V to 3.25V. Some supervisors are adjustable or trimmable. The set-point accuracy is a key specification. The MAX16140 has ±1% accuracy at room temperature and ±1.5% over -40° to 125°C. Some devices are as poor as 3.5%.
Operational Features to Consider
With all these devices, generally, a reset output is asserted when the monitored voltage at the supervisor’s input falls below a factory-trimmed threshold. The reset is maintained for a specifiable minimum timeout period after voltage at the input returns above the threshold. This delay timeout is usually around 200ms, but can range from 5µs to 2s or more. This timeout will occur due to a fault condition and during power-up of the system.
That single channel MAX16140 (Figure 1), for example, has timeout delay period part number options for eight values from 217µs to 2,000ms. This chip comes in a tiny 0.78mm x 0.78mm x 0.5mm, 4-bump WLP package. One of the big advantages of this particular device is its extremely low supply current, which makes it great for battery-powered designs. Nominal supply current is just 370na. It also has a debounced reset push-button input that can be set to active low or high or edge-triggered.
On most supervisors there is a choice of output types. I prefer an active-high, open-drain output that will go to a pull-up resistor on the MCU reset input. One can choose push-pull or open-drain output configurations in many supervisor ICs.
Additional Supervisory Options
Another example IC is the MAX16134 triple windowed voltage supervisor that keeps a watch out for both under and over voltage conditions (Figure 2). It has three independent open collector outputs and features ±1% threshold accuracy over temperature. The chip’s SOT23-8 package takes little space, and 17 combinations of the three monitored supply voltages are available.
Figure 2. Simplified block diagram for MAX16132/33/34/35 voltage supervisors.
Many of these ICs offer a hardware watchdog timer, normally controlled by one output bit of the MCU. A good example is the MAX16155. This single-channel IC offers the watchdog plus super low power at only 400na typical (900na maximum over -40ºC to 125ºC) and a 1.2V to 5.5V operating supply range. It also has a logic input that will disable the watchdog function. The IC’s threshold accuracy is ±2.5%.
Some applications require battery-backup support for external RAM so that in the event of a brownout, the RAM data is protected while the MCU is in reset. An example is the MAX6364 (Figure 3). The chip’s reset output goes high when VCC is below the reset threshold and for at least 150ms after VCC rises above VTH. And, when VCC falls below the reset threshold, BATT in is connected to OUT to power the memory—as long as VBATT is at least 20mV greater than VCC. The OUT current can be 20ma continuous.
Figure 3. Low-Power Supervisory Circuit with Battery Backup.
More complex processors/MCUs, and those with Wi-Fi/networking, often require precise sequencing of the supply rails on power up. You can easily do that with ICs like the MAX16042 triple sequencing/supervisory chip which has independent open-drain outputs along with a push-pull reset output. It will enable one supply after another with a time delay during power up and disable all if any fall beneath their threshold. Power control can be done with the enable pin on DC/DC converters or with power MOSFETs in series with two of the three supplies.
The wide variety of voltage supervisors can be daunting, but engineers should not be intimidated. The understanding of a few key aspects will put you on the right track for these extremely important devices.