Introduction Negative voltages are used to power an increasing number of LCD displays in automotive infotainment systems. Likewise, in industrial and railroad environments, negative voltage rails are available for instrumentation and monitoring applications. In all cases, the negative rail must be generated from a positive supply, but positive-to-negative ICs are not as readily available as buck controllers. Manufacturers are unlikely to use negative voltages to power the growing number of LCD displays in automotive infotainment systems. Likewise, in industrial and railroad environments, negative voltage rails are available for instrumentation and monitoring applications.

In all cases, the negative rail must be generated from a positive supply, but positive-to-negative ICs are not as readily available as buck controllers. With a tested and qualified inverting converter, it is likely that you already have some approved step-down controllers, such as the LTC3892 dual output controller. To avoid the extra time and cost of testing dedicated inverting converters, the LTC3892 dual-output step-down controller can be used to generate the negative output voltage in a 262 ;uk topology.

Dual output converters: –12 V/3 A and 3.3 V/10 A

The LTC3892 is a dual output controller where one output can be used to provide a positive voltage while the other channel is used to generate a negative voltage, as shown in Figure 1. This solution has an input voltage range of 6 V to 40 V, where VOUT1 equals 3.3 V (10 A) and VOUT2 equals –12 V (3 A). VOUT1 is configured as a pass-through buck converter topology with power chain components Q2, Q3, L1, and output filter capacitors. Setting the output to 3.3 V eliminates the need for a voltage divider on the VFB pin (connected directly to the output) because the LTC3892-2 has a fixed 3.3 V or 5 V output (set by connecting VPRG1 to ground or to INTVCC, respectively) ).

Figure 1. A solution for generating positive and negative voltages. VOUT1 is 3.3 V (10 A), VOUT2 is –12 V (3 A)

VOUT2 is a negative output voltage (relative to GND). Op amp U2 (LT1797) is wired as a differential amplifier to sense this negative voltage and adjust it to the 0.8 V reference of the LTC3892 error amplifier (EA). In this approach, both the LTC3892's EA and this op amp are referenced to system GND, simplifying power supply control and functionality. The seed formula for setting the negative output voltage is:

VOUT2 uses an asynchronous Ćuk topology and includes power chain components Q1, D1, L2 and output filter capacitors. Ćuk topology is extensively covered in other technical literature, so it will not be covered in detail here. The stress on the power link components can be summarized by the following formula:

This solution was evaluated using the DC2727A demo board, Figure 2 shows the VOUT2 efficiency. Also, this approach is available in our LTspice® simulation model of the LTC3892-2.
Figure 2. Efficiency of Negative Output (VOUT2) at 14 V Input

in conclusion

The LTC3892 is a versatile and flexible controller that, while ostensibly designed for synchronous buck conversion, can also be used in Ćuk topologies to generate positive and negative voltages for automotive, industrial and other applications .