Design Note 543: Introduction
The LTC7138 is a 400mA step-down regulator that can operate over an input voltage range of 4V to 140V, ideal for industrial, telecom, automotive and other applications subject to harsh line transients. Due to the regulator’s hysteretic architecture, no external compensation is required. The output voltage is pin-programmable to 1.8V, 3.3V or 5.0V, or if an external divider is used, the output can be adjusted from 0.8V to VIN. 100% duty cycle operation is possible because of the internal P-channel FET. The LTC7138 is offered in a thermally enhanced high voltage (skipped pins) MSOP package.
Simple 5V/400mA Buck with Wide VIN Range
Figure 1 shows a 5V buck converter with a maximum input voltage of 140V using only four external components. Its output is set to 5V by tying the VPRG1 pin to the SS pin and the VPRG2 pin to ground. It does not require any external compensation.
Figure 1. High Efficiency 5V, 400mA Buck Regulator
Figure 2. Efficiency of the Regulator in Figure 1
Efficiency remains high over a broad range of loads due in part to the LTC7138’s Burst Mode® operation and 12μA no load IQ current. For a 12V input, efficiency peaks at 87% for a load of 10mA and stays above 80% for loads down to 0.4mA. This level of energy conservation makes it ideal for always-on battery operated systems.
24V/800mA Buck Using Two LTC7138s in Parallel
Figure 3 shows two LTC7138s paralleled to provide an output of 24V at 800mA. Parallel operation is implemented by tying the feedback comparator output pin (FBO) of the master to the VFB pin of the slave.
Figure 3. High Efficiency 24V, 800mA Buck Using Two LTC7138s in Parallel
In this setup, the slave follows the master as it enters and exits the burst cycles (see Figure 4).
Figure 4. Parallel Operation of the 24V, 800mA Buck at VIN = 140V, IOUT = 600mA
The output of the 24V buck is set with an external feedback divider. The full load efficiency for this regulator is 93.6% for a 48V input (Figure 5).
Figure 5. Efficiency of the Regulator in Figure 3
32V/400mA Surge Stopper
Figure 6 shows another use for the LTC7138. For inputs of 32V or below, this regulator operates in dropout, where the internal power FET is on continuously.
Figure 6. 32V, 400mA Surge Stopper
When the input exceeds 32V, the LTC7138 switches to keep the output voltage in regulation, as shown in Figure 7.
Figure 7. Output of the Surge Stopper (Figure 6) Is Clamped to 32V During 127V Input Transient
For further protection, an overvoltage lockout can be implemented by tying the OVLO pin to a divider across the input supply. This circuit is suitable for protecting downstream converters or loads in industrial, automotive or avionics systems.
More Features
The LTC7138 provides additional features which makes it suitable for a wide array of applications. These include a RUN pin for an external UVLO, an ILIM pin for programming the current limit or setting up an input side current limit, and soft-start —either internal or external.
Conclusion
The LTC7138 yields low parts count, rugged solutions for wide input voltage applications. It features a 140V maximum input voltage rating, thermally enhanced high voltage MSOP package, Burst Mode operation, low IQ current, pin adjustable output voltage, no external compensation and operation at 100% duty cycle.
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