A versatile step-down converter for automotive and industrial applications

March 27, 2014 // By Charlie Zhao
The trend in automobiles and industrial systems is 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 with an Adjustable Output Down to 800mV

The http://www.linear.com/product/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 undervoltage lockout function.

Figure 1. High Efficiency 24V Regulator with Undervoltage Lockout and 300mA Current Limit

24V Regulator with 300mA Output Current Limit and Input Undervoltage Lockout

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

Figure 2. Efficiency of Circuit in Figure 1

Figure 3. Input Voltage Sweep vs Output Voltage Showing Undervoltage Lockout Threshold Level

The RUN pin is programmed for VIN undervoltage lockout threshold levels of 27V rising and 24V falling. Figure 3 shows VOUT vs VIN. This 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 http://www.linear.com/product/LTC3630 can supply up to 500mA of output current, the circuit in Figure 1 is programmed for a maxi­mum of 300mA. An internally generated 5µA bias out of the ISET pin produces a voltage across an ISET resistor, which determines the maximum

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