Brief Guideline of Reset IC

Introduction :

Reset IC, as known as Voltage Supervisor or Voltage Monitors, is used to monitor a specific power rail and its output asserts a corresponding signal. In most cases, Reset IC is placed in each subsystem to disable it if the supervised power rail is out of spec.

Key Parameter :

There are several parameters which should be taken into account when Reset IC will be used on our design. For example, Threshold Voltage, Output Topology, Watchdog and so on. I’ll pick up some important parameters to explain them as follows.

Threshold Voltage

Threshold Voltage is a predetermined voltage. If the input voltage of Reset IC rise above or fall below this voltage and then Reset IC will output a signal to enable/disable/reset the device which is connected to Reset IC.

If there is a Reset IC and its Threshold Voltage is typical 2.93V, it’s used to monitor the 3.3V power rail which is main supply of a subsystem. Its output will assert a enable signal to this subsystem when 3.3V power rail rise above its Threshold Voltage. By contrast, if 3.3V power rail fall below its Threshold Voltage, its output will assert a disable signal.

Because Threshold Voltage isn’t a fixed value, please make sure the value of the monitored power rail is above it. Temperature drift of Threshold Voltage should be taken into account,too. If voltage divider is used as the input voltage of Reset IC, please also aware of the accuracy of resistors. Certainly, the value of the monitored power rail can’t over the maximum of the range of input voltage.

Output Topology

There are two fundamental Output topologies, Open Drain and Push Pull. Each of them has its advantage and disadvantage. It depends on where it is used.

Open Drain topology uses a internal N-MOSFET and a external pull-up resistor to output signal. When this N-Channel MOSFET is open, external voltage of pull-up resistor will output via pull-up resistor; when it is closed, output will be pulled down to ground.

Push Pull topology contains one internal N-MOSFET and one internal P-MOSFET. When its P-MOSFET is open and N-MOSFET is closed, output will be pulled down to ground; when P-MOSFET is closed and N-MOSFET is open, VDD will be conducted to output.

Because of external pull-up resistor, Open Drain topology can comply with any logic level of load via its external pull-up resistor rather than the voltage level of its VDD. Based on this characteristic, several Open Drain output can be connected to each other to monitor multiple voltage rails but share one input of a subsystem. However, additional resistor will increase BOM cost and more power consumption.

On the other hand, the voltage level of the output of Push Pull topology is as same as its VDD. So, one output of Push Pull topology can’t be directly connected to another output. Because Push Pull topology doesn’t need external resistor, it can reserved power consumption,BOM cost and PCB space.

In addition, Active Low and Active High are another two different output topologies but I have introduced them in the previous article. Therefore, I won’t waste time on them in this article.

Reset Time Delay

Another important parameter of Reset IC is Reset Time Delay. Reset IC is usually used to ensure correct power-on sequence, so choose the Reset IC with proper Reset Time Delay is necessary. Some particular part of Reset IC can adjust its Reset Time Delay by a external capacitor.

Extra Feature

In addition to the above, some Reset IC have extra feature like Watchdog, Manuel Reset, etc. They are designed to use in some special system.

Watchdog is a function which uses a timer to discover and recover from system malfunction. When system works normally, it should send a periodic signal to clear the internal timer of Reset IC. Once system doesn’t send this signal within timer limit, Reset IC will send a reset signal to system. This mechanism can automatically reboot system as long as system can’t normally send this signal. In general, Watchdog timer of Reset IC is fixed but some of them use a external capacitor to adjust their timer.

Compared with Watchdog, Manuel Reset is very simple. Reset IC receives a input signal from other device and output a reset signal. It is often connected to a Button or main block of whole design.

Conclusion

Reset IC is a basic type of electronic components. Its function isn’t complicated but important. If its design is wrong, product may not boot up or has some unexpected risk.