Robust 140V VIN, 400mA Step-Down Regulator for Industrial, Telecom and Automotive Environments Design Note 543 Mike Shriver The LTC ®7138 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 pinprogrammable 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. 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. 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). 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. When the input exceeds 32V, 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. L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 100 90 L: SUMIDA CDRH105RNP-151NC VIN 5V TO 140V C IN 1µF 250V VIN SW LTC7138 ANODE RUN SS VPRG1 VFB OVLO VPRG2 GND D1 VOUT 5V COUT 400mA 22µF EFFICIENCY (%) 80 L1 150µH 70 60 50 40 30 10 0 DN543 F01 Figure 1. High Efficiency 5V, 400mA Buck Regulator 10/15/543 VIN = 12V VIN = 48V VIN = 100V VIN = 140V 20 0.1 1 10 ILOAD (mA) 100 1k DN543 F02 Figure 2. Efficiency of the Regulator in Figure 1 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. 100 90 L1 220µH SW LTC7138 (MASTER) RUN ANODE VOUT 24V 800mA VIN CIN1 1µF 250V X7R OVLO VFB SS VPRG1 VPRG2 ISET FBO D1 732k 80 EFFICIENCY (%) VIN 24V TO 140V COUT 10µF 6x 200k GND 70 60 50 40 20 10 0 L2 220µH SW LTC7138 (SLAVE) VFB RUN VIN CIN2 1µF 250V X7R OVLO ANODE VPRG2 VPRG1 ISET FBO SS VIN = 36V VIN = 48V VIN = 100V VIN = 140V 30 D2 L: COILCRAFT MSS1278-224KLB 1 0.1 100 1k DN543 F05 Figure 5. Efficiency of the Regulator in Figure 3 L1 100µH VIN 4V TO 140V VIN RUN FBO SS VPRG1 VPRG2 CIN 1µF 250V X7R L1/L2: COILCRAFT MSS1278-224KLB D1/D2: DIODES INC SBR1U200P1-7 COUT : TAIYO YUDEN UMK325BJ106MMHT VOUT* 400mA SW LTC7138 GND DN543 F03 10 ILOAD (mA) D1 ANODE VFB ISET OVLO 536k 100k COUT 10µF 50V X5R GND DN543 F06 Figure 3. High Efficiency 24V, 800mA Buck Using Two LTC7138s in Parallel L1: SUMIDA CDRH104RNP-101NC COUT : TAIYO YUDEN UMK325BJ106MMHT D1: DIODES INC SBR1U200P1-7 * WHEN VIN > 32V, LTC7138 SWITCHES AND VOUT IS REGULATED TO 32V; WHEN VIN ≤ 32V, LTC7138 OPERATES IN DROPOUT AND VOUT FOLLOWS VIN. Figure 6. 32V, 400mA Surge Stopper 127V IL1 500mA/DIV VIN 50V/DIV IL2 500mA/DIV 24V 32V CLAMP VOUT 20V/DIV VOUT 500mV/DIV AC-COUPLED LOAD = 400mA 20µs/DIV DN543 F04 Figure 4. Parallel Operation of the 24V, 800mA Buck at VIN = 140V, IOUT = 600mA Data Sheet Download www.linear.com/LTC7138 Linear Technology Corporation 20ms/DIV DN543 F07 Figure 7. Output of the Surge Stopper (Figure 6) Is Clamped to 32V During 127V Input Transient For applications help, call (408) 432-1900, Ext. 3720 dn543f LT/AP 1015 111K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2015