Youhao Xi

For applications using USB Type-C Power Delivery (PD) and wireless charging, the output voltage (V_OUT) from the charger can fluctuate higher or lower than the input voltage. Four-switch buck-boost regulators are popular in these applications because adjusting their feedback signal can dynamically change the V_OUT.

A buck-boost regulator’s output voltage can be adjusted either by varying the error amplifier’s reference voltage (V_REF) or by varying the feedback voltage. If monolithic pulse-width modulation (PWM) four-switch buck-boost controllers such as TI’s LM34936 and LM5176 do not provide the ability to access V_REF, however, varying the feedback voltage becomes the only way.

To help you design systems using buck-boost controllers with dynamic output voltages, in this series I will discuss a few options for using the feedback signal to adjust the output voltage. The first installment will focus on using switched resistors, while the second installment presents a different approach that requires fewer components and signal lines.

Understanding the V_OUT setting and feedback signal

Figure 1 shows a typical V_OUT setting for the controller and error amplifier. By looking up the V_REF for the error amplifier in the data sheet, Equation 1 calculates V_OUT by setting the values of R₁ and R₂:

Equation 1.

Figure 1 Feedback Circuit and v_OUT Setting

Using switched resistors to adjust V_OUT

Figure 2 shows a simple option to dynamically adjust V_OUT between two voltage levels by incorporating a switch, S₁. Assume that S₁ is an ideal switch, and that its impedance is infinity when off and 0 Ω when on. When switch S₁ is turned off, R₃ is not in the picture of the V_OUT setting, and is simply the same as given in Equation 1. To avoid confusion in this discussion, let’s assume that the voltage is V_OUT1, as expressed by Equation 2:

Equation 2.

If S₁ is turned on, it will place R₃ in parallel with R₁. The new output voltage, V_OUT2, will satisfy Equation 3:

Equation 3.

where

Equation 4.

This results in V_OUT2 being greater than V_OUT1. By solving Equation 2 and Equation 3, you can determine the resistor values. By toggling S₁, you can switch V_OUT between V_OUT1 and V_OUT2.

Figure 2 V_OUT Adjustment with a Switched Resistor

Preventing a false OVP trigger

Some PWM controllers like the LM34936 and LM5176 have built-in output overvoltage protection (OVP), implemented by monitoring the feedback (FB) pin voltage. If the FB pin sees >10% above V_REF, the controller riggers OVP and stops switching until after the FB voltage falls below the hysteresis of the OVP threshold. Because of this feature, any abrupt resistor switching should be prevented, since turning off S₁ suddenly will cause the FB voltage to jump up instantaneously and create a false OVP event. The solution is to switch S₁ gradually, where R₅ and C₁ delay the on/off command of S₁ to engage or disengage R₃ gradually.

There are a couple of factors involved in selecting the resistor-capacitor (RC) delay time constant; the RC needs to match up with the loop response time, as well as the transition time between V_OUT levels specified by the application, in order to enable proper transition between the two voltage outputs.

Multivoltage programming

Adding switched resistor branches can program additional V_OUT levels. Figure 3 shows an approach employing three switched resistor branches to set the four V_OUT levels for USB Type-C PD applications.

Figure 3 Three Switched Resistor Branches for Four v_OUT Settings

Table 1 summarizes the programming schemes for the four different voltages.

Table 1: Programming switched resistors for USB Type-C PD

As shown in Figure 4, it is important to make sure that switching to different resistors won’t trigger a false OVP event in the feedback loop. Incorporating an RC for each of the switches will help avoid an OVP event while switching to the proper voltage rail at the appropriate time.

Figure 4 Gradually Switching the Resistor to Avoid Triggering an OVP Event During Dynamic v_OUT Adjustment

Conclusion

By incorporating switches and resistors into the feedback loop, buck-boost converters can dynamically adjust the V_OUT for USD Type-C PD and wireless charging applications. This approach is straightforward, simple and easy to implement. Controlling the speed of the switches is a requirement when using four-switch buck-boost controllers.

For a different approach using fewer components and signal lines, see the second installment of this series.

Additional resources

• See TI’s USB Type-C PD reference designs at  www.ti.com/product/LM5175/toolssoftware
• Check out the LM34936 and LM5176 controller data sheets.