Q: What good solution can produce a micro DC power supply voltage of only a few hundred millivolts?

A: Simply connect a clean applied positive voltage to the feedback resistor of the DC-DC converter.

In the past few years, the power supply voltage of electronic components has been declining due to the continuous reduction of the geometric size of digital circuits such as microcontroller, CPU and DSP. There are also some applications requiring low supply voltage in the measurement field.

For many years, linear regulators and switching regulators have adopted a feedback voltage of about 1.2V. This voltage is generated by the band gap circuit in the DC-DC Converter IC, which determines the minimum voltage that can be set using an external resistive voltage divider. So far, most modern regulator ICs can produce output voltages of 0.8V, 0.6V or even 0.5V. The internal reference voltage source is also designed in this way, so a lower voltage can be obtained. Figure 1 shows this type of switching regulator ltc3822, which generates a feedback voltage of 0.6V with a reference voltage of 0.6V.

Figure 1. Ltc3822dc-dc converter capable of generating output voltage of 0.6V or higher.

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

Using some techniques, you can also make a switch or linear regulator produce a voltage lower than the feedback voltage. This can be achieved by using the circuit shown in Fig. 2. Connect the resistance voltage divider with an applied bias positive voltage to adjust the output voltage. The voltage can be generated by a low voltage drop regulator (LDO) or a reference voltage source. In this way, the resistance voltage divider constitutes a voltage divider, and the flow direction of current IFB is opposite to the conventional situation in Fig. 1. In Fig. 2, the current flows from the external reference voltage source to the output voltage through the resistance divider.

Equation 1 shows the relationship between the feedback voltage (VFB) of the IC, the required output voltage (VOUT), the applied positive DC bias voltage (voffset), and the resistors R1 and R2 of the resistive voltage divider.

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

Figure 2. Adjusting the circuit in Figure 1 can produce an output voltage lower than 0.6V.

This design concept is generally suitable for generating a voltage lower than the rated minimum voltage of a switching regulator or a linear regulator. However, it should be noted that the applied reference voltage source should be started and operated before the DC-DC converter is turned on. If the auxiliary voltage is 0V or has high resistance, the DC-DC converter may generate too high voltage and damage the load circuit.

In the worst case, that is, when the switching regulator is not turned on but the auxiliary voltage has been applied, the current IFB flowing through the resistance voltage divider will charge the output capacitor to make its voltage higher than the set voltage. This happens when the load has very high impedance. Therefore, it may be necessary to set a minimum load to avoid this situation.

The accuracy of the auxiliary voltage (1V in Figure 2) of the resistance voltage divider will directly affect the accuracy of the generated power supply voltage. Therefore, a particularly clean low ripple voltage should be used.

In addition, not all voltage converters are suitable for such operations. For example, the measurement range of the current detection amplifier in a DC-DC converter may only provide an operating range at a higher voltage. It should also be noted that a very low voltage is generated at a higher input voltage and a low duty cycle is required. Here, it may be very helpful to select a switching regulator IC with short minimum on time and operate at low switching frequency.

Figure 3. Simulation tools (such as ADI’s LTSpice) can be used ®） Perform an initial test on the circuit.

If you want to run a linear regulator or switching regulator at an output voltage lower than that specified by the IC manufacturer, it is very useful to use simulation tools (such as ADI’s LTSpice) for initial inspection. Figure 3 shows a circuit composed of ltc3822, using an additional voltage source as the bias of the feedback path. In this circuit, a 200mV output voltage is generated. According to the data book, ltc3822 is suitable for generating a minimum output voltage of 0.6V. In the circuit, the auxiliary voltage source (voltage source V2 in Fig. 3) can be realized by LDO regulator or reference voltage source. Using the techniques described in this paper, the circuit can be fully tested and even produce a lower output voltage.

LTC3822

● no current detection resistor is required

● synchronous drive of full n-channel MOSFET

● can provide high current output

● constant frequency current mode operation for excellent voltage and load transient response

● VIN: 2.75V to 4.5V

● ± 1% 0.6V reference

● low differential pressure operation: 99% duty cycle

● optional frequency (300kHz / 550kHz / 750khz)

● internal soft start circuit

● optional maximum peak current detection threshold

● digital run control pin

● output overvoltage protection

● micro power shutdown mode: IQ = 7.5 μ A

● slim heat resistance enhanced leadless (3mmx3mm) DFN package or 10 pin MSOP package

Editing: hfy