How can we generate extremely low voltages of only a few hundred millivolts? This article will tell you~

Over the past few years, the supply voltage of electronic components has continued to decrease due to the continuous reduction in the geometric size of digital circuits such as microcontrollers, CPUs, DSPs, etc. There are also some applications in the measurement field that require low supply voltages.

For many years, linear and switching regulators have used a feedback voltage of about 1.2 V. This voltage is generated by a bandgap circuit in the DC-DC converter IC, which determines the lowest voltage that can be set using an external resistor divider. By now, most modern regulator ICs can generate output voltages of 0.8 V, 0.6 V, or even 0.5 V. The internal reference voltage source is also designed in this way, so lower voltages can be obtained. Figure 1 shows this type of switching regulator, the LTC3822, which generates a feedback voltage of 0.6 V from a 0.6 V reference voltage.

Figure 1. The LTC3822 DC-DC converter can generate low output voltages of 0.6 V or higher.

However, if a supply voltage lower than 0.6 V is required, the circuit shown in Figure 1 needs to be modified or it cannot be used.

With some tricks, you can also make a switching or linear regulator generate a voltage lower than the feedback voltage. This can be done by using the circuit shown in Figure 2. Connect the resistor divider to an externally applied bias positive voltage to regulate the output voltage. This voltage can be generated by a low dropout regulator (LDO) or a reference voltage source. In this way, the resistor divider forms a voltage divider, and the direction of current I _FB flows is opposite to the normal situation in Figure 1. In Figure 2, the current flows from the external reference voltage source through the resistor divider to the output voltage.

Equation 1 shows the relationship between the IC's feedback voltage (V _FB ), the desired output voltage (V _OUT ), the applied positive DC bias voltage (V _{OFFSET ), and the resistors R1 and R2 of the resistor divider.}

For the resistor divider value selection, it is recommended that the sum of R1 and R2 be between 100 kΩ and 500 kΩ. This makes the bias current low enough for power efficiency, but high enough to prevent excessive noise coupling into the sensitive feedback path.

Figure 2. The circuit of Figure 1 can be modified to produce output voltages below 0.6 V.

This design concept is generally applicable to generating voltages below the minimum rated voltage of the switching regulator or linear regulator. However, a few points should be noted: The external reference voltage source should be started and running before the DC-DC converter is turned on. If this auxiliary voltage is 0 V or has high resistance, the DC-DC converter may generate excessive voltage and damage the load circuit.

In the worst case, when the switching regulator is not yet turned on but the auxiliary voltage is already applied, the current IFB flowing through the resistor divider _will charge the output capacitor to a voltage higher than the set voltage. This can happen when the load has a very high impedance. So it may be necessary to set a minimum load to avoid this situation.

The accuracy of the resistor divider's auxiliary voltage (1 V in Figure 2) directly affects the accuracy of the resulting supply voltage. Therefore, a particularly clean, low-ripple voltage should be used.

Furthermore, not all voltage converters are suitable for this type of operation. For example, the measurement range of the current sense amplifier in a DC-DC converter may only provide an operating range at higher voltages. It should also be noted that generating very low voltages at higher input voltages also requires a low duty cycle. Here, choosing a switching regulator IC with a short minimum on-time and operating at a low switching frequency can be very helpful.

Figure 3. Initial testing of the circuit can be performed using simulation tools such as ADI's LTspice ^{® ​​.}

If you want to run a linear or switching regulator at an output voltage lower than that specified by the IC manufacturer, it is very useful to do an initial check using a simulation tool such as ADI’s LTspice. Figure 3 shows a circuit constructed with an LTC3822 using an additional voltage source as a bias for the feedback path. In this circuit, a 200 mV output voltage is generated. According to the data sheet, the LTC3822 is suitable for generating output voltages as low as 0.6 V. In the circuit, the auxiliary voltage source (source V2 in Figure 3) can be implemented with an LDO regulator or a reference voltage source. Using the techniques described in this article, the circuit can be fully tested and even lower output voltages may be generated.