For systems in which a microcontroller must communicate with peripheral devices operating at different supply voltages, you may need to add a level translator (such as the MAX1840 from Maxim). This device requires two supply voltages (VCC and DVCC) to process the serial digital signals flowing between peripheral and microcontroller. A typical requirement is to translate the logic-high level of an I2C, SPI, or 1-Wire channel from 3.5V to 1.8V.
Such systems often provide a low-dropout (LDO) regulator for each supply voltage, and each LDO also requires two bypass capacitors, one connected to the supply pin and one to the output pin. For portable devices constrained by low cost and limited space, however, we can eliminate the 1.8V LDO and its bypass capacitors by simply substituting a surface-mount green LED (case size 0603) across the level translatorís DVCC and VCC pins (Figure 1).
Figure 1: For low-power loads, a single green LED (D1) replaces a 1.8V LDO regulator in this level-translator circuit.
(Click on image to enlarge)
The VCC pin in this configuration assumes a voltage of ė1.9V, which allows the translator to provide a 1.8V logic-high level to the slave devices. To ensure a stable VCC, the slave devices should operate on low power and draw reasonably constant supply currents. ?