Simple and Easy Power-OK Indicator Detects Under- and Over-Voltage

A similar version of this article appeared in the May 1, 2008 issue of PET.

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

*Future product—contact factory for availability.

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 R_ON at 25°C is 160Ω, and V_IN is the applied source voltage.