Mike Shriver, Linear Technology
Design Note 543
The LTC7138 is a 400 mA step-down regulator that can operate over an input voltage range of 4 V to 140 V, 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 1.8 V, 3.3 V or 5.0 V, or if an external divider is used, the output can be adjusted from 0.8 V 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 5 V/400 mA Buck with Wide VIN Range
Figure 1 shows a 5 V buck converter with a maximum input voltage of 140 V using only four external components. Its output is set to 5 V by tying the VPRG1 pin to the SS pin and the VPRG2 pin to ground. It does not require any external compensation.
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| Figure 1. | High Efficiency 5 V, 400 mA Buck Regulator. |
Efficiency remains high over a broad range of loads due in part to the LTC7138’s Burst Mode® operation and 12 μA no load IQ current (Figure 2). For a 12 V input, efficiency peaks at 87% for a load of 10 mA and stays above 80% for loads down to 0.4 mA. This level of energy conservation makes it ideal for always-on battery operated systems.
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| Figure 2. | Efficiency of the Regulator in Figure 1. |
24 V/800 mA Buck Using Two LTC7138s in Parallel
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| Figure 3. | High Efficiency 24 V, 800 mA Buck Using Two LTC7138s in Parallel. |
Figure 3 shows two LTC7138s paralleled to provide an output of 24 V at 800 mA. 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 24 V buck is set with an external feedback divider. The full load efficiency for this regulator is 93.6% for a 48 V input (Figure 5).
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| Figure 4. | Parallel Operation of the 24 V, 800 mA Buck at VIN = 140 V, IOUT = 600 mA. |
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| Figure 5. | Efficiency of the Regulator in Figure 3. |
32 V/400 mA Surge Stopper
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| Figure 6. | 32 V, 400 mA Surge Stopper. |
Figure 6 shows another use for the LTC7138. For inputs of 32 V or below, this regulator operates in dropout, where the internal power FET is on continuously. When the input exceeds 32 V, the LTC7138 switches to keep the output voltage in regulation, as shown in Figure 7. 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.
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| Figure 7. | Output of the Surge Stopper (Figure 6) Is Clamped to 32 V During 127 V Input Transient. |
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 ISET 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 140 V 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.
