Simple and Easy Power-OK Indicator Detects Under- and Over-Voltage
A Power-OK (POK) indicator lights an LED when the power is on and within a specified range. Also known as an undervoltage/overvoltage (UV/OV) indicator, the indicator notifies portable-equipment users when the battery voltage is too low or the battery is being overcharged.
Though it's not difficult to design a circuit for this purpose using a logic gate, two comparators, three bypass capacitors, an LED, and five resistors, the Figure 1 circuit saves board space while lowering cost.
Figure 1. Residing in a 1.0mm x 1.5mm µDFN package, this IC illuminates the LED as long as the power is OK. When an under- or over-voltage condition appears, it turns off the LED and issues a warning to the µC.
Overvoltage protectors of the MAX4923–MAX4926 family usually connect the power source and load through an external p-channel FET, which is normally ON as long as the power is OK. The devices come with factory-preset voltage thresholds as listed in Table 1.
Table 1. UV and OV Thresholds for MAX4923–MAX4926 Protectors
|Device||Package||OV Threshold (V)||UV Threshold (V)|
|MAX4923ELT+*||6 pin µDFN||7.18||2.44|
|MAX4924ELT+||6 pin µDFN||6.16||2.44|
|MAX4925ELT+||6 pin µDFN||5.65||2.44|
|MAX4926ELT+||6 pin µDFN||4.46||2.44|
These ICs, however, can also serve as simple power-OK indicators. Use the following equation to calculate R in Figure 1: , where (for example) I ≤ 10mA, the typical RON at 25°C is 160Ω, and VIN is the applied source voltage.