Microcontrollers, the heart of all modern electronic gadgets, are increasingly powered with sub-5V power supplies. This complicates the control of external loads powered by higher voltages.
The trend towards low voltage supplies is dictated by the fact that modern digital and mixed-signal integrated circuits are CMOS based, and with an ever increasing demand for higher frequencies, the only way you can control the power dissipation is by lowering the supply voltage. As a result, supplies have dropped from 5V to 3.3V and down to 1.8V for many popular, low-power microcontrollers in even the 8-bit range. Typically, 8-bit microcontrollers have a single power supply rail. 32-bit devices may have multiple power supply rails: 3.3V for the physical interface, and 2.5V or lower for internal operation. Some 32-bit devices also have triple power supply rails: 3.3V, 1.8V, and 1.1V for instance.
The reduced I/O supply voltage leads to increased complexity in handling high-side voltage switching. Figure 1 illustrates the conventional solution for controlling a 5V high-side switch, driven here by a 3.3V signal.
Figure 1 Standard method for a high side switching circuit
The low voltage microcontroller switches the NPN transistor T2, which in turn drives the PNP transistor T1. R3 represents the load. The load is grounded, but can also be a floating load with a suitable low-side switch.
This Design Idea proposes an alternative (Figure 2). In place of T2, we make use of Zener diode D1 of appropriate breakdown voltage.
Figure 2 A simpler high side switching circuit using a Zener diode
Traversing the path x→y→z for the transistor in ‘OFF’ and ‘ON’ conditions, the following equations can be used to calculate the value of the breakdown voltage for the Zener diode. A high microcontroller output voltage, added to the Zener breakdown voltage, is enough to turn off T1 as presented in equation (1). When the microcontroller output is set to low, the voltage at node y is low enough to turn the transistor T1 on as presented in equation (2).
For more detail: Zener level-shifter drives high-side switch