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.
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