Voltage Monitor

Published:2011/8/12 1:28:00 Author:Amy From:SeekIC

Paul Goosens

It is often necessary to monitor the power supply voltage in a piece of equipment. When the device takes its power from a battery, the input voltage can change and provide an indication of how much energy is left. Even a device powered from the mains can benefit from keeping an eye on the (various) power supply voltages in the circuit and take the necessary steps in the event of a fault condition.

If a slightly intelligent reaction is required for this situation, it is obvious to do this with a microcontroller. This requires the microcontroller to periodically measure the voltages with one or more A/D converters and decide whether the values are correct or whether something is the matter.

This naturally costs processor time and makes the firmware a little more compli­cated. This can become a problem, par­ticularly when other functions have to be carried out on a regular basis as well.

Fortunately, chip manufacturer Maxim has a number of ICs in its line-up specifically for this purpose. These ICs all have as task to monitor a number of voltages and when these do not conform to certain requirements the IC generates an inter­rupt, which indicates to the microcon­troller that something has gone wrong.

The MAX1153 and MAX1154 are pro­vided with o 10-bit ADC, while the MAX 1253 and MAX1254 are provided with a 12-bit ADC. All these ICs are capa­ble of monitoring 8 external voltages and 2 internal voltages. These 8 external volt­ages can also be programmed for tem­perature sensors, where an external diode can be used as a simple and cheap sensor.

For each channel, a maximum and mini­mum voltage or temperature can be spec­ified. As soon as the input voltage or tem­perature falls outside this range the inter­rupt output goes low. If desired, a glitch suppressor can be individually configured for each channel. A recursive averaging filter is also available and it is even possi­ble to specify how many successive sam­ples have to be outside the range before the interrupt is generated. The sample-rate of the ADC is also adjustable, to o maximum of 90 ksamples/s.

The SPI bus serves as interface to the con­troller and requires only four signals to communicate.

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