Step-down converter for automotive, industrial apps

In automobiles and industrial systems, there is a trend to replace mechanical functions with electronics, thus multiplying the number of microcontrollers, signal processors, sensors and other electronic devices throughout. The issue is that 24V truck electrical systems, future 48V split-voltage level systems and industrial equipment use relatively high voltages for motors and solenoids while the microcontrollers and other electronics require much lower voltages. As a result, there is a clear need for compact, high efficiency step-down converters that can produce very low voltages from the high input voltages.

65V input, 500mA DC/DC converter
LTC3630 is a versatile Burst Mode synchronous step-down DC/DC converter that includes three pin-selectable preset output voltages. Alternatively, the output can be set via feedback resistors down to 800mV. An adjustable output or input current limit from 50mA to 500mA can be set via a single resistor. The hysteretic nature of this topology provides inherent short-circuit protection. Higher output currents are possible by paralleling multiple LTC3630s together and connecting the FBO of the master device to the VFB pin of a slave device. An adjustable soft-start is included. A precision RUN pin threshold voltage can be used for an under-voltage lockout function.

Figure 1: 24V regulator with 300mA output current limit and input under-voltage lockout.

Figure 1 shows a 48V to 24V application that showcases several of the LTC3630's features, including the under-voltage lockout and output current limit. Operational efficiencies are shown in figure 2.

Figure 2: Efficiency of circuit in figure 1.

The RUN pin is programmed for VIN under-voltage lockout threshold levels of 27V rising and 24V falling. VOUT VIN feature assures that VOUT is in regulation only when sufficient input voltage is available.

The 24V output voltage can be programmed using the 800mV 1% reference or one of the preset voltages. This circuit uses the 5V preset option along with feedback resistors to program the output voltage. This increases circuit noise immunity and allows lower value feedback resistors to be used.

Although the LTC3630 can supply up to 500mA of output current, the circuit in figure 1 is programmed for a maximum of 300mA. An internally generated 5A bias out of the ISET pin produces a voltage across an ISET resistor, which determines the maximum output current. Figure 3 shows the output voltage as a resistive load is varied from approximately 100 down to 8 while maintaining the output current near the programmed value of 300mA. In addition, the hysteretic topology used in this DC/DC converter provides inherent short-circuit protection.

Figure 3: Resistive load sweep vs output current.

Input current limit
LTC3630 has another useful feature. In 5V circuit, the current limit is set by a resistive divider from VIN to ISET, which produces a voltage on the ISET pin that tracks VIN. This allows VIN to control output current which determines input current. An increased voltage on ISET increases the converters current limit.

Conclusion
The LTC3630 offers a mixture of features useful in high efficiency, high voltage applications. Its wide output voltage range, adjustable current capabilities and inherent short-circuit tolerant operation makes this DC/DC converter an easy fit in demanding applications.

About the author
Charlie Zhao is Applications Engineer at Linear Technology Corp.

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