Buck converter automotive and industrial grade solutions

This paper describes the trend in automotive and industrial systems to replace mechanical components with electronic components, resulting in a multiplication of the number of microcontrollers, signal processors, sensors and other electronic devices used.

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introduction

The trend in automotive and industrial systems is to replace mechanical features with electronic components, resulting in a multiplication of the number of microcontrollers, signal processors, sensors and other electronic devices used. The problem here is that 24V automotive electrical systems and industrial equipment use relatively high voltages for motors and solenoids, while microcontrollers and other electronic components require much lower voltages. Therefore, there is an obvious need for a compact high efficiency buck converter that requires very low voltages from high input voltages.

65V input, 500mA DC/DC converter with adjustable output down to 800mV

The LTC3630 is a general-purpose Burst Mode synchronous step-down DC/DC converter that includes a preset output voltage that can be selected by pin. Alternatively, a feedback resistor can be used to set the output as low as 800mV. An adjustable output or input current limit of 50mA to 500mA can be set with a single resistor. The hysteresis characteristics of this topology provide inherent short-circuit protection. This provides higher output current by connecting multiple LTC3630s in parallel and connecting the FBO of the master to the VFB pin of a slave. The device has an adjustable soft start. A precision RUN pin threshold voltage can be used to implement undervoltage lockout.

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 conversion application circuit that demonstrates the characteristics of the LTC3630, including undervoltage lockout and output current limiting. The work efficiency is shown in Figure 2.

Figure 2: Efficiency of the circuit shown in Figure 1.

Figure 3: Output voltage waveform when the input voltage changes (displays the undervoltage lockout threshold level)

Figure 4: Output current and output voltage waveform when the resistive load changes (output current limit is 300mA)

The RUN pin is set for the 27V (rising) and 24V (falling) VIN undervoltage lockout threshold levels. Figure 3 shows the relationship between VOUT and VIN. This feature ensures that VOUT is only conditioned when it has sufficient input voltage.

The 24V output voltage can be set using one of the 800mV 1% reference or preset voltage. The circuit uses a 5V preset option and a feedback resistor to set the output voltage. This improves the noise immunity of the circuit and allows the use of feedback resistors with lower resistance values.

Although the LTC3630 can deliver up to 500mA of output current, the circuit in Figure 1 is set for a maximum of 300mA. The 5?A bias current output from the ISET pin and internally generates a voltage across the ISET resistor that determines the maximum output current. Figure 4 shows the output voltage when a resistive load drops from approximately 100 Ω to as low as 8 Ω while the output current remains close to the 300 mA programmed value. In addition, the hysteresis topology used in this DC/DC converter can also provide inherent short circuit protection.

Input current limit

Figure 5 shows another useful feature of the LTC3630. In this 5V circuit, the current limit is set by a resistive divider connected between VIN and ISET, which produces a voltage that tracks VIN on the ISET pin. This allows VIN to control the output current to determine the input current.

Figure 5: 5V Regulator with 55mA Input Current Limit

As the voltage on the ISET pin increases, the current limit of the converter will increase. Figure 6 shows the steady-state input current versus input voltage and available output current (before the output voltage begins to fall outside the regulation range). When using the parameter values ​​shown in Figure 5, the input current is limited to approximately 55 mA over an input voltage range of 10V to 60V.

Figure 6: Input voltage vs. load current and input current (using the input current limit circuit shown in Figure 5)

in conclusion

The LTC3630 offers a combination of features for high efficiency, high voltage applications. Its wide output voltage range, adjustable current capability and inherent short-circuit resistance make the DC/DC converter easy to handle demanding applications.