应用笔记 4545

Flexible Overvoltage/Undervoltage Detector Monitors Negative and Positive Voltages

By: Franco Contadini

摘要 : Combining a window comparator (MAX6764) with a positive-voltage monitor (MAX6887/MAX6888) enables the circuit to monitor a negative voltage as well, as is often required in telecom systems.

A similar version of this article appeared in the January 1, 2008 issue of PD magazine.

Multivoltage supply supervisors (such as the MAX6887) provide several voltage-detector inputs for positive voltages, each with factory-set thresholds for undervoltage and overvoltage. The acitve-low RESET output asserts when any input drops below its undervoltage threshold, or when you assert the manual reset (MR). The acitve-low OV output asserts when any input exceeds its overvoltage threshold. These capabilities are useful, but telecom applications often require that you monitor a negative supply voltage for the RF circuitry as well.

To monitor negative voltage you can make use of the MAX6887 adjustable-input option (Figure 1), in which a level-shifting circuit connects one side of the resistive divider to a positive level and the other side to the negative voltage. This approach, however, produces inverted output logic. If, for example, you monitor -6V with thresholds at -6.5V and -5.5V, the circuit asserts acitve-low UV when VIN = -6.5V and acitve-low OV when VIN = -5.5V.

Figure 1. MAX6887 adjustable input option.
Figure 1. MAX6887 adjustable input option.

The circuit of Figure 2 overcomes this limitation by adding a simple window-detector IC (Figure 3) to monitor the negative supply. The detector's acitve-low UV output connects to the multivoltage supervisor's acitve-low OV output, and the detector's acitve-low OV output connects to the supervisor's acitve-low RESET output. Thus, the acitve-low RESET output goes low when the negative voltage decreases to -5.5V, and the acitve-low OV output asserts low when the negative voltage increases to -6.5V. Three resistors (R1–R3) set the under- and over-voltage thresholds UV and OV. R1 connects to a positive reference voltage, and R3 connects to the monitored negative voltage.

Figure 2. This IC (in SOT23 package) is a simple window comparator that monitors a supply voltage with separate under/overvoltage outputs.
Figure 2. This IC (in SOT23 package) is a simple window comparator that monitors a supply voltage with separate under/overvoltage outputs.

Figure 3. By combining the circuits of Figures 1 and 2, one pair of terminals warns of under- or over-voltage for multiple positive voltages and one negative voltage.
Figure 3. By combining the circuits of Figures 1 and 2, one pair of terminals warns of under- or over-voltage for multiple positive voltages and one negative voltage.

If your system doesn't include a positive reference voltage, you can use the supervisor's 2.55V BP output. To maximize DC accuracy, the sum of R1 + R2 + R3 should draw only a few microamps from the BP output. Using the principle of superposition, you can then calculate the voltages at UVIN and OVIN for any given set of resistor values as follows:
Equation 1.
where VM is the monitored negative supply voltage.

Operation of the Figure 3 circuit is illustrated in the scope shot of Figure 4, in which the yellow trace (CH1) represents the monitored negative voltage VM as it ranges from 0V to -7V. Other traces are:

R1 (black) = UVIN
CH2 (blue) = MAX6764 acitve-low UV
CH3 (green) = OVIN
CH4 (pink) = MAX6764 acitve-low OV

Figure 4. These waveforms illustrate operation of the Figure 3 circuit.
Figure 4. These waveforms illustrate operation of the Figure 3 circuit.

The nominal value for the monitored negative voltage is -6V. Both acitve-low OV and acitve-low UV outputs have a 10kΩ pullup to 5V, and the VCC terminals of both ICs connect to a 5V supply. The MAX6764 Output acitve-low UV (MAX6887 acitve-low OV Output) goes low at VM = -6.55V (and goes high at VM = -6.52V). The MAX6764 output acitve-low OV (MAX6887 acitve-low RESET Output) goes low at VM = -5.53V (and goes high at VM = -5.55V).
下一步
EE-Mail 订阅EE-Mail,接收关于您感兴趣的新文档的自动通知。
Download 下载,PDF格式
© , Maxim Integrated Products, Inc.
The content on this webpage is protected by copyright laws of the United States and of foreign countries. For requests to copy this content, contact us.
APP 4545:
应用笔记 4545,AN4545, AN 4545, APP4545, Appnote4545, Appnote 4545