advertisement Monolithic Converter Delivers High Output Voltages Design Note 545 Jesus Rosales Introduction Industrial, telecom, medical and automotive applications use a wide variety of regulated voltages to operate effectively, including high voltage and negative rails. When the designer is faced with producing industrial power supplies, the job can be simplified by minimizing the parts count, as well as the number of required controller ICs. The LT®8331 achieves both of these goals with an integrated 140V, 500mA switch, programmable frequency, ultra-quiescent current and light load Burst Mode ® operation. A high voltage negative output rail can easily be attained by configuring the converter as shown in Figure 2. This converter allows full switch voltage utilization to produce a negative output, by adding a capacitor and a diode. A coupling capacitor, C5, adds input to output disconnect during shutdown, similar to a CUK converter. Figure 3 shows a CUK converter using the LT8331, and Figure 4 shows the efficiency curve for this inverter with a 12V input. Efficiency for this converter peaks at 84%. Burst Mode operation kicks in when the load drops to about 40mA, which allows the converter to maintain a respectable efficiency level of 73% even with a 1mA load. High voltage applications are easily accomplished using a simple boost converter, as shown in Figure 1. This application can output 120V at 25mA to 80mA in a straight boost configuration or 240V at 12mA to 35mA as a two stage boost by adding a few components, as shown inside the dotted line. The load can be taken entirely from VOUT2, or a combination of VOUT1 and VOUT2. D1, D2, D3 : BAV21W L1: CDRH8D43RT125NP-331 C1 10µF D2 0.1µF L1 VIN 10V to 48V L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. VIN BIAS EN/UVLO R3 215k SS GND C4 0.22µF *** R2 2M SW LT8331 R5 154k FBX SYNC/MODE RT 1µF D1 330µH R1 1M D3 100Ω 1/4W R4 13.3k VOUT2 240V 12mA to 35mA VOUT2 VOUT1 120V C7 4.7pF C2 2µF INTVCC C3 1µF *** R2: USE 1M VALUE AND TIE TO VOUT1 FOR SINGLE STAGE BOOST R2: USE 2M VALUE, AND TIE TO VOUT2 FOR DUAL STAGE BOOST R2: MAY REQUIRE MILTIPLE SERIES RESISTORS TO COMPLY WITH VOLTAGE RATING Figure 1. 120V or 240V Output Boost Converter 12/15/545 The LT8331 has very low quiescent current. It features an operation mode where the switching frequency is allowed to decrease progressively when a light load is detected. This mode enables the converter to maintain both high efficiency and low output ripple at light loads. The input current is just 29µA when the output is unloaded and about 11µA comes from the FBX resistor divider. If the converter is turned off by pulling the EN pin to ground, the input current drops to about 1µA for a 5V input, or 2µA with a 12V input. A good portion of the shutdown current is drawn by the EN/UVLO resistor divider. input to output DC path, a desirable characteristic in applications where the output must be disconnected from the input. This has the added benefit of removing the current drawn by the FBX resistor divider. The switch voltage in these converters equals the sum of the input and output voltages. Conclusion The LT8331 simplifies the design of high output voltage and wide input voltage applications by reducing the number of external components. Its 140V, 500mA internal switch, 100V input, programmable frequency, ultra-low quiescent current and light load Burst Mode operation make it ideal for a broad range of applications. The LT8331’s 4.5V to 100V input range and its 140V rated switch make it an ideal candidate for SEPIC and CUK converters. Coupling capacitor, C5, breaks the C5 1µF VIN 10V to 48V 100Ω D1 1/4W VOUT –120V 25mA to 65mA 330µH VIN D2 SW 1M 4.7pF BIAS 10µF 1M LT8331 FBX EN/UVLO SYNC/MODE 215k SS GND 0.22µF 1µF x2 6.7k RT INTVCC 1µF 154k D1, D2 : BAV21W L1: CDRH8D43RT125NP-331 Figure 2. –120V Inverting Converter 95 1M 4.7µF VIN SW 4.7µF 120µH D2 1M 10pF VOUT –12V 90mA to 180mA BIAS LT8331 FBX 71.5k 22µF 90 85 EFFICIENCY (%) C5 120µH VIN 5V to 80V 80 75 70 EN/UVLO SYNC/MODE SS GND RT INTVCC 60 1µF 55 665k 0.22µF 63.4k 65 50 1 20 40 60 80 100 120 140 160 LOAD (mA) Figure 3. –12V Output CUK Converter Data Sheet Download www.linear.com/LT8331 Linear Technology Corporation Figure 4. Efficiency Curve for Figure 3 (with a 12V Input) For applications help, call (408) 432-1900 DN545f LT/AP 1215 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