Robust 140V Step-Down Regulator for Industrial, Telecom and Automotive Environments

May 18, 2016 // By Mike Shriver, Linear Technology
Industrial, automotive, telecom and other applications are subject to harsh line transients. A 400mA step-down regulator, the LTC7138, can operate over an input voltage range of 4V to 140V, ideal for these demanding applications.

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 V IN. 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 V IN Range

High Efficiency 5V, 400mA Buck Regulator


Efficiency of the Regulator in Figure 1

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 V PRG1 pin to the SS pin and the V PRG2 pin to ground. It does not require any external compensation. 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

High Efficiency 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. In this setup, the slave follows the master as it enters and exits the burst cycles (see Figure 4). 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).

Parallel Operation of the 24V, 800mA Buck at VIN = 140V, IOUT = 600mA

Efficiency of the Regulator in Figure 3
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