SPVA023 Application note

SPVA023 July 2025 LM25184 , LM2611 , LM27761 , LM5158 , LMR33640 , TLV171 , TPS63700

2.4 Generate Negative Power Supply Using A Cuk Regulator

A Cuk regulator also can generate a negative power supply from a positive input voltage power supply. The topology of the Cuk regulator is shown in Figure 2-8 .

Figure 2-8 Cuk Converter Topology

In the first stage, switch Q1 is switched on, diode D is switched off. Current of inductor L1 increases linearity, couple capacitor C discharges to output capacitor C_out and loaded by inductor L2. The current of inductor L2 increases linearity since C is large enough. In the second stage, switch Q1 switches off and diode D is switched on. Couple capacitor C is charged by inductor L1 and input power supply. The voltage of couple capacitor C increases. Inductor L2 charges output capacitor Cout and load by diode D. Equation 8 shows the duty and stress voltage of Q1 and diode (D) for Cuk converter. Customers can select MOSFET and diode according to Equation 8, Equation 9, Equation 10, Equation 11, and Equation 12 .

Equation 8.

D = \frac{|V_{o u t p u t}| + V_{F}}{|V_{o u t p u t}| + V_{i n p u t} + V_{F}}

Equation 9.

V_{Q 1} = V_{i n p u t} + |V_{o u t p u t}| + V_{f} + \frac{V_{C_r i p p l e}}{2}

Equation 10.

V_{D} = V_{i n p u t} + |V_{o u t p u t}| + V_{f} + \frac{V_{C_r i p p l e}}{2}

Equation 11.

I_{L 1_a v g} = \frac{|V_{o u t p u t}| \times I_{o u t p u t}}{V_{i n p u t} \times η}

Equation 12.

I_{L 2_a v g} = I_{o u t p u t}

LM2611 is a current mode Cuk regulator that can operate from 2.7V-14V with 1.4MHz switching frequency Customers can use a previously mentioned equation to estimate the minimum output voltage under a specific input voltage. The output voltage can be configured by Equation 13

Equation 13.

V_{o u t p u t} = - (1 + \frac{R_{T}}{R_{B}}) \times V_{R E F} = - 1.23 \times (1 + \frac{R_{T}}{R_{B}})

Customera also can use a boost regulator to build a Cuk converter since there are less Cuk converters in the market to meet miscellaneous applications from customers. However, designers must add an external inverter amplifier to converter the negative output to positive feedback voltage since the feedback of most boost converter requires a positive input feedback voltage. As an example, designers can use LM5158 and TLV171 to build an Cuk converter, as Figure 2-9. The output voltage is shown in Equation 14

Equation 14.

V_{o u t p u t} = - \frac{R_{f}}{R_{1}} \times V_{R E F}

Figure 2-9 Use Boost Converter to Build a Cuk Converter