关键词: temperature sensor, system monitor, temperature sensing, 2-wire, ADC, voltage monitoring, voltage monitor, temp sensing, analog digital converter, voltage detector 関連製品

# MAX6652 System Monitor Combines Voltage Monitoring with Temperature Sensing

This application note shows four examples of how to implement the MAX6652 into a design.

The MAX6652 is a system-monitor chip with a 2-wire digital interface. It automatically monitors system temperature and four voltages, measuring each with an 8-bit analog-to-digital converter (ADC) once every 300ms (max). Temperature and voltage measurements are continuously compared against programmable thresholds: upper and lower thresholds for voltage and a single over-temperature threshold for temperature. When temperature is too high or when a voltage falls outside of the allowed window, an interrupt is generated that notifies the system controller that a problem exists.

The sensitivities of the voltage monitor inputs have been scaled so that inputs of 12V, 5V, 3.3V, and 2.5V will yield ADC output codes of 192 (or 3/4 of full scale). When the voltages being monitored are the same as these nominal values, it is easy to set up alarm thresholds for, say, 5% above and 5% below the nominal voltage levels.

The MAX6652 can also be used to monitor input voltages that are significantly different from the nominal values, provided that the correct detection thresholds are chosen. A few examples will illustrate how this is done.

## Example 1: Nominal Case.

VCC = 5V; monitored voltages are 12V, 5V, 3.3V, and 2.5V.
In this example, if each voltage is at the nominal value and the ADC has no error, the ADC will produce an output code of 192 for each input. More accurately, the nominal input voltage is the threshold at which the code changes from 191 to 192. Therefore, when we calculate an ideal code, we always round down to the nearest integer value. For alarms 5% above and below the nominal input voltages, we have (rounded down to the nearest integer values):

high limit = 192 × 1.05 = 201
and
low limit = 192 × 0.95 = 182
The total unadjusted error of the ADC is 1% of full scale (maximum) or 2.56LSBs, so consider this potential error when choosing voltage limits. Figure 1. Circuit of Example 1. All input voltages are at the nominal values.

Table 1. Output Codes and ±5% Limits for the MAX6652 Used to Monitor the Nominal Power-Supply Voltages
 Monitored Voltage MAX6652 Input Nominal Output Code -5% Limit +5% Limit 12 12VIN 192 182 201 5 VCC (5VIN) 192 182 201 3.3 3.3VIN 192 182 201 2.5 2.5VIN 192 182 201

## Example 2: VCC = 3.3V.

Monitored voltages are 12V, 5V, 3.3V, and 2.5V.
This example is similar to the first one, except that the MAX6652 operates from a 3.3V supply instead of the nominal 5V supply. Measuring the 3.3V supply is simple. First calculate the code that corresponds to 3.3V at the VCC pin (rounded down to the nearest integer value):

192 × 3.3V/5V = 126
Then calculate the codes for the high and low limits, again rounded down to the nearest integer values. For alarms 5% above and below the 3.3V nominal power-supply voltage, we have the following:
high limit = 192 × 3.3V/5V × 1.05 = 133
and
low limit = 192 × 3.3V/5V × 0.95 = 120

Monitoring 5V is more difficult in this example. The 5V can't be applied directly to the 3.3VIN input, because that input is scaled for a maximum input voltage of 4.383V. A resistive voltage divider, as shown in Figure 2, attenuates the 5V supply so that it can be measured at the 3.3VIN input. Using a 39kΩ and a 20kΩ resistor, 5V applied to the divider results in a voltage at the 3.3VIN pin of:

VIN = 5V × 39kΩ/59kΩ = 3.305V

This produces a code equal to 192, so the ±5% thresholds are at 182 and 201.

The limits for the 12V and 2.5V inputs are the same as in the first example. Figure 2. Circuit of Example 2. The same as Example 1, but the MAX6652 operates from a 3.3V supply, which necessitates attenuating the 5V supply so that it can be monitored at the 3.3VIN input.

Table 2. Output Codes and ±5% Limits for the MAX6652, Monitoring the Nominal Power-Supply Voltages but with a 3.3V Supply Voltage for the MAX6652
 Monitored Voltage MAX6652 Input Nominal Output Code -5% Limit +5% Limit 12 12VIN 192 182 201 5 3.3VIN (attenuated) 192 182 201 3.3 VCC (5VIN) 126 120 133 2.5 2.5VIN 192 182 201

## Example 3: Low-voltage system.

VCC = 3.3V; monitored voltages are 5V, 3.3V, 2.5V, and 1.8V.
The 3.3V supply voltage can be monitored as in Example 2.

The 12VIN pin is available for monitoring the 5V supply. With 5V applied to this input, the output code will be as follows:

192 × 5/12 = 80
For alarms 5% above and below the 5V nominal power-supply voltage, we have the following:
high limit = 192 × 5V/12V × 1.05 = 84
and
low limit = 192 × 5V/12V × 0.95 = 76
The 3.3VIN pin will be used for monitoring the 1.8V supply voltage. With 1.8V applied to this input, the output code (rounded down) will be as follows:
192 × 1.8/3.3 = 104
For alarms 5% above and below the 1.8V nominal power-supply voltage, we have (rounded down):
high limit = 192 × 1.8V/3.3V × 1.05 = 109
and
low limit = 192 × 1.8V/3.3V × 0.95 = 99
The 2.5V input limits are the same as in the first two examples. Figure 3. Circuit of Example 3. This circuit is used in a low-voltage system to monitor 5V, 3.3V, 2.5V, and 1.8V. The MAX6652 operates from a 3.3V supply. The 12VIN input monitors the 5V supply, and the 3.3VIN input monitors the 1.8V supply.

Table 3. Output Codes and ±5% Limits for the MAX6652 in a Low-Voltage System, Monitoring 5V, 3.3V, 2.5V, and 1.8V
 Monitored Voltage MAX6652 Input Nominal Output Code -5% Limit +5% Limit 5 12VIN 80 76 84 3.3 VCC (5VIN) 126 120 133 2.5 2.5VIN 192 182 201 1.8 3.3VIN 104 99 109

## Example 4: Alternative low-voltage system.

VCC = 3.3V; monitored voltages are 5V, 3.3V, 2.5V, and 1.8V.
This is an alternative to Example 3. In this example, the1.8V supply is monitored at the 2.5VIN input instead of at the 3.3VIN input, and the 2.5V supply is monitored at the 3.3VIN input. This improves the measurement resolution for the 1.8V supply, while making the resolution for the 2.5V supply somewhat worse relative to Example 3.

The 3.3V supply voltage is monitored as in Examples 2 and 3.

The 12VIN pin monitors the 5V supply as in Example 3.

The 3.3VIN pin will be used for monitoring the 2.5V supply voltage. With 2.5V applied to this input, the output code (rounded down) will be as follows:

192 × 2.5/3.3 = 145
For alarms 5% above and below the 2.5V nominal power-supply voltage, we have (rounded down):
high limit = 192 × 2.5V/3.3V × 1.05 = 152
and
low limit = 192 × 2.5V/3.3V × 0.95 = 138

The 2.5VIN pin will be used for monitoring the 1.8V supply voltage. With 1.8V applied to this input, the output code (rounded down) will be as follows:

192 × 1.8/2.5 = 138
For alarms 5% above and below the 2.5V nominal power-supply voltage, we have (rounded down):
high limit = 192 × 1.8/2.5 × 1.05 = 145
and
low limit = 192 × 1.8/2.5 × 0.95 = 131 Figure 4. Circuit of Example 4. This circuit is an alternative to the low-voltage circuit in Example 3. Monitored voltages are still 5V, 3.3V, 2.5V, and 1.8V, but the inputs to the 3.3VIN and 2.5VIN pins are swapped to give better resolution when measuring the 1.8V supply.

Table 4. Output Codes and ±5% Limits for the MAX6652 in the Alternative Low-Voltage System, Monitoring 5V, 3.3V, 2.5V, and 1.8V
Note that the 1.8V input voltage produces a larger ADC code, resulting in a higher-resolution measurement.
 Monitored Voltage MAX6652 Input Nominal Output Code -5% Limit +5% Limit 5 12VIN 80 76 84 3.3 VCC (5VIN) 126 120 133 2.5 3.3VIN 145 138 152 1.8 2.5VIN 138 131 145