advertisement A Positive-to-Negative Voltage Converter Can Be Used for Stable Outputs Even with a Widely Varying Input – Design Note 433 Victor Khasiev + – , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 01/08/433 Q PWM D VO– + C L RLOAD DN433 F01 Figure 1. Simplifed Block Diagram of Positive-to-Negative Converter VIN+ + – V1 D VO– + – V1 Q D VO– + L C RLOAD L + C RLOAD DN433 F02 (2a) Transistor Q is On (2b) Transistor Q is Off Figure 2. Equivalent Circuits Show the Operation of the Positive-to-Negative Converter Q GATE ON OFF ON OFF IL A new generation of Linear Technology high voltage synchronous step-down converters, such as the LT®3845, make it possible to implement positive-to-negative conversions for a variety of applications. Basic Operation Figure 1 shows a simplified block diagram of a positive-tonegative converter. Figure 2 shows an equivalent circuit, which helps in understanding the basic operation of the circuit in Figure 1. When transistor Q is on (Figure 2a), diode D is reverse biased and the current in inductor L increases. When Q is off (Figure 2b), inductor L changes polarity, diode D becomes forward biased, and current flows from inductor L to the load and capacitor C. The voltage across capacitor C and the load is negative, relative to system ground. Figure 3 shows a timing diagram. V1 + This topology is particularly useful when the input varies above or below the output. In such cases, a traditional step-down regulator would not be able to regulate once the battery voltage drops below the output, thus shortening the useful battery run time. Buck-boost solutions and other topologies such as a SEPIC solve this problem, but they tend to be more complicated and expensive. The positive-to-negative converter topology presented here combines the simplicity of a step-down converter and the regulation range of a buck-boost topology. VIN+ + An obvious application of a positive-to-negative converter is generating a negative voltage output from a positive input. However, a not-so-obvious use is to produce a stable output voltage in an application that has a widely varying input. For example, a converter in a battery-powered device, which has an inherently variable input voltage, can produce a stable output voltage even if input voltage falls below the absolute value of the output voltage. However, an obvious drawback is reverse polarity, which can be easily overcome in this application. The supplied circuitry can use the negative output as the system ground and the negative battery terminal as the “positive” voltage source. VIN VL VOUT DN433 F03 Figure 3. Converter Timing Diagram The duty cycle range can be found from following expression: D= VO VIN + VO DMAX = DMIN = VO VIN(MIN) + VO VO VIN(MAX ) + VO Component Stress in a Positive-to-Negative Topology VMAX is the maximum voltage across transistor Q and diode D (Figure 2), where: VMAX = VIN(MAX) + |VO| The maximum current, IMAX, through transistor Q, inductor L and diode D can be derived based on the following equations, assuming continuous conduction mode: IL = VIN(MIN) • t •DMAX IO dI , dI= , IMAX =IL + 2 1–DMAX L where t is a switching period. Circuit Description Figure 4 shows a 9V to 15V input to –12V at 3A output converter. The high voltage LT3845 is used for several VIN 9V TO 15V R2 51.1k C5 0.47μF 1 2 3 4 C1 0.1μF 5 –12V (IC GND) 6 R3 143k 7 R4 16.2k R6 49.9k 8 Conclusion Very often electrical engineers have to design a negative voltage source supplied from a positive voltage rail. The positive-to-negative converter discussed in the article can be a good alternative to a flyback or a SEPIC approach. Q1 PH3075L GND VIN BOOST SHDN SS The entire converter power path contains the LT3845 high voltage PWM controller, MOSFETs Q1 and Q2, inductor L1, diode D1 and output filter capacitors COUT1–COUT3. Diode D2 is a bootstrap diode and diode D3 provides bias voltage for internal MOSFET drivers. VIN CIN1 22μF 25V R1 249k reasons, including the ability of its SW pin to withstand 65V, its integrated high side driver and differential current sense. The LT3845 can also provide synchronous rectification, which allows the use of efficient MOSFETs over less efficient switching diodes. TG LT3845 SW VCC BURST FB BG VC PGND IS+ SYNC IS– fSET SGND 17 16 15 14 13 C3 0.1μF D2 BAS521 R8 10Ω L1 13μH PB2020.153 C6 OPT RS1 6mΩ R7 10Ω COUT1 16ME470WF D1 B160 12 11 Q2 PH1875L D3 BAS521 10 9 C2 1μF + COUT3 10μF 25V COUT2 10μF 25V GND VOUT –12V 3A –12V (IC GND) DN433 F04 C4 2.2μF R5 61.9k Figure 4. Conversion of 9V-15V into –12V at 3A Based on the LT3845 High Voltage PWM Controller 91.5 14V 91.0 15V EFFICIENCY (%) 90.5 13V 12V 90.0 9V 10V 89.5 89.0 88.5 88.0 1.0 1.5 2.0 2.5 3.0 LOAD CURRENT (A) DN433 F05 Figure 6. Transient Response to an Output Load Step of 1A to 2A Figure 5. Efficiency for the Figure 4 Circuit with Varying Input Voltage to a Fixed –12V Output Figure 7. Start-Up Waveform for the Circuit in Figure 4 with VIN = 14V, VOUT = –12V, IOUT = 2A Data Sheet Download For applications help, call (408) 432-1900, Ext. 3161 www.linear.com Linear Technology Corporation dn433f LT/TP 0108 387K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2008 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●