High Voltage, High Efficiency Positive-to-Negative Converter, Do You Need It?
“The LTC3896 is a highly integrated controller designed for positive-to-negative conversion. Solutions based on this controller have high efficiency and very low quiescent current, which is important for battery-operated systems. In addition, the device is highly versatile with programmable frequency, a wide input voltage range of 150V, and an output voltage to C60V. It simplifies the design of automotive and industrial power supplies with control signals referenced to host ground.
Positive-to-negative conversion (inverting output) is widely used in LCD devices, OLED displays, audio amplifiers, industrial machinery, semiconductor manufacturing process control equipment, measurement tools, test systems, LED drivers, and battery chargers. Many of these applications require high power stages and extended input voltage range, two inherent characteristics of the LTC3896 positive-to-negative controller. This controller is particularly suitable for automotive due to its ability to handle very high input and output voltages (up to 150V total voltage stress), its ability to drive standard grade MOSFETs, and its low operating quiescent current of 40µA (10µA in shutdown) application.
Wide 7V~72V input to C12V/5A output
Figure 1 shows an LTC3896-based positive-to-negative converter. The solution provides C12V/5A output from a 7V to 72V input voltage range. In the automotive market, the LTC3896’s ability to handle high input voltages eliminates the need for bulky and expensive voltage suppressors, while the low minimum input voltage keeps sensitive systems operating even during cold crank conditions.
Figure 1: The C12V output converter (U1) delivers 5A to the load over a 7V to 72V input voltage range. Note that the control signals RUN, PGOOD and PLLIN are referenced to GND
The converter’s power chain consists of MOSFETs Q1, Q2 and Inductor L1. The output filter is based on ceramic capacitor COX. EXTV of U1CC pin is connected to GND and a 12V potential is generated on this pin (relative to VOUTC). If a power good signal is required, an external voltage source referenced to GND should be used. The LTC3896’s control and interface signals, including RUN, PGOOD, and PLLIN, are referenced to system GND to eliminate the need for level shifters in processor-controlled systems.
in product manual  Guidelines for calculating voltage and current stress on components around the LTC3896 are detailed in . For a basic evaluation, the following equations can be used to calculate the duty cycle (D), the average inductor current (IL) and MOSFET voltage stress (VDS):
Demonstration circuit DC2447A  The generality of the LTC3896 is illustrated. Designers can test the controller’s many features, including synchronization to an external clock, the ability to use an external linear regulator to reduce thermal stress on the IC under high output voltage conditions, and the ability to generate C5V or C3.3V outputs simple solution.
Figure 2 shows the efficiency at various input voltages. Figure 3 shows the thermal image of the converter in operation.
Figure 2: Efficiency curves for the circuit shown in Figure 1. The output voltage is C12V and the maximum load current is 5A.
Figure 3: Thermal image of DC2447A was taken at 36V input voltage, C12V/5A output.
Given on the right is an assembly diagram of the demo board.
The LTC3896 is a highly integrated controller designed for positive-to-negative conversion. Solutions based on this controller have high efficiency and very low quiescent current, which is important for battery-operated systems. In addition, the device is highly versatile with programmable frequency, a wide input voltage range of 150V, and an output voltage to C60V. It simplifies the design of automotive and industrial power supplies with control signals referenced to host ground.