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| Low Voltage Detection & Power Cut IC |
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| JDW:
I am working on a design that uses a 3.3V fingerprint sensor with onboard UART and ARM MCU. I experienced the memory getting suddenly wiped a few times for reasons unknown, so I contacted the sensor manufacturer and they replied that when the voltage falls between 2.0V and 2.5V the memory might be erased. They suggest I add my own circuitry to power off the sensor module when voltage falls below 2.7V. To me, that seems like an oversight on their part for not having that critically important voltage detection and power-down built-in, but since I've already written the code for this module, I'd like to continue using it. Therefore, I am curious if there is a low-cost, single IC solution to this problem? In other words, I am looking for a single IC that can detect when the voltage rail fails from a nominal 3.3V to below 2.7V and then it will cut 3.3V power to the sensor (or perhaps cut the GND side). The Sensor draws about 75mA in its full power state (and microamps when sleeping) so the switch would need to handle that current. If there is no single chip IC solution, what discrete circuit would you propose? Thank you. |
| pigrew:
One of the better ways to detect an undervoltage is a power supervisor IC. It outputs a signal (usually open drain) when the voltage is low. As an example, there is a Rohm BD4828G-TR which triggers at 2.8 V. These chips have some hysteresis, and the threshold isn't exact (within a few percent normally), so you need to dig into the datasheets. There are thousands of variations available on DigiKey, active low/high, different voltages, adjustable hysteresis, etc.... With this sort of chip, you would need to pair it with some sort of power switch which could be as simple as a high-side pFET (if you find an active high reset).... Your voltage is quite close to the minimum voltage of a Li-Ion battery. You may be able to find an undervoltage protection IC designed for batteries, which would have an integrated switch. There also exist various over-voltage/over-current/under-voltage protect switches (like 5V ones for USB) that also do this. Maybe you'd want to use a "load switch" IC (e.g. MIC2091-1YM5-TR) instead of a single FET... might end up being lower power. Also, these are available with inverting or non-inverting inputs. EDIT: Another idea, can you use the supervisor IC to send the reset/shutdown signal to the sensor, instead of cutting its power? |
| David Hess:
Microprocessor supervisory ICs often include that function as part of their reset or power good output signals. Typically they would be combined with a regulator which supports disabling its output because open loop transistor switching of a logic supply can be tricky. Some regulators support or can be configured to do this without an external IC. |
| JDW:
Thank you for the suggestions. I'm still trying to figure out why the sensor's onboard Flash memory IC containing learned fingerprints is suddenly getting erased. Again, the manufacturer told me such could occur if voltage falls between 2.7V and 2.0V, but it seems they have the circuit on their PCB to accommodate such, as shown below. Furthermore, when you kill power to the device, the voltage will always fall below 2.7V and to 2.0V as it proceeds to 0V, so perhaps I am totally misunderstanding what they are saying, or perhaps they don't know what they are talking about. Any thoughts? |
| David Hess:
--- Quote from: JDW on August 26, 2019, 06:24:53 am ---I'm still trying to figure out why the sensor's onboard Flash memory IC containing learned fingerprints is suddenly getting erased. Again, the manufacturer told me such could occur if voltage falls between 2.7V and 2.0V, but it seems they have the circuit on their PCB to accommodate such, as shown below. Furthermore, when you kill power to the device, the voltage will always fall below 2.7V and to 2.0V as it proceeds to 0V, so perhaps I am totally misunderstanding what they are saying, or perhaps they don't know what they are talking about. Any thoughts? --- End quote --- The problem is when the device is operated outside of its specified voltage range and this includes low voltages and when the voltage momentarily dips below the minimum specified operating voltage which is a particularly dangerous condition leading to undefined behavior. Flash memory is especially a problem if a power dip or power loss occurs during an erase or programming operation but the same applies to other memory types. The solution is to detect the possibility of operation out of bounds and immediately reset or shut down. Operations which cannot be interrupted need to have backup power, sometimes with capacitors, to finish. Sometimes the supply voltage is increased closer to the maximum giving more time to detect a low voltage condition and this is reflected in voltage supervisors which may come in different versions to support slightly different trip voltages. |
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