On the surface it is a controller, but in fact it...

Latest update time：2021-07-11

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LTC3892

Negative voltages are used to power the growing number of LCD displays in automotive infotainment systems. Similarly, in industrial and rail environments, negative rails are needed 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 step-down controllers. Manufacturers are unlikely to have a tested and qualified negative output converter, but they are likely to have some approved step-down controllers, such as the LTC3892 dual output controller. To avoid the additional time and cost of testing a dedicated negative output converter, the LTC3892 dual output step-down controller can be used to generate the negative output voltage in a C`uk topology.

Dual output converter: –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. The input voltage range of this solution is 6V to 40V, with V _OUT1 equal to 3.3V (10A) and V _OUT2 equal to –12V (3A). V _OUT1 is configured as a pass-through buck converter topology with power train components Q2, Q3, L1, and output filter capacitors. Setting the output to 3.3V does not require 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 grounding VPRG1 or connecting it to INTV _CC , respectively ).

Figure 1. A solution for generating positive and negative voltages. V _OUT1 is 3.3V at 10 A and V _OUT2 is –12V at 3A.

V _OUT2 is a negative output voltage (with respect to GND). Op amp U2 (LT1797 wired as a differential amplifier) senses this negative voltage and scales it to a 0.8V reference for the LTC3892 error amplifier (EA). In this approach, both the LTC3892 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:

V _OUT2 uses an asynchronous C`uk topology and includes the power train components Q1, D1, L2 and the output filter capacitor. The C`uk topology is widely covered in other technical literature, so it will not be described in detail here. The stress on the power train components can be summarized by the following formula:

This solution was evaluated using the DC2727A demo board, and the V _OUT2 efficiency is shown in Figure 2. Alternatively, this approach is also available in our LTspice ^{®
simulation model for the LTC3892-2.}

_{Figure 2. Efficiency of the negative output (V OUT2} ) at 14V input .

in conclusion

The LTC3892 is a versatile and flexible controller that, while ostensibly designed for synchronous step-down conversion, can also be used in a C`uk topology to generate positive and negative voltages for automotive, industrial and other applications.

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