Hundreds of Watts, 60V In or Out: Synchronous 4-Switch Buck-Boost Converter is Easy to Parallel to Minimize Temperature Rise Keith Szolusha The LT3790 is a 4-switch synchronous buck-boost DC/DC converter that regulates both constant voltage and constant current at up to 98.5% efficiency using only a single inductor. It can deliver hundreds of watts and features a 60V input and output rating, making it an ideal DC/DC voltage regulator and battery charger when both step-up and step-down conversion are needed. A single LT3790 converter can deliver high power due to its synchronous switching topology, but eventually the switching and/or conduction losses at higher power can overwhelm a single converter with excessive board heating. Although heat can VIN 8V TO 56V RIN 1.5mΩ 4.7µF 100V ×2 INTVCC VIN 1µF TG1 499k M1 0.1µF M4 M2 L1 10µH M3 SWI EN/UVLO BG1 OVLO INTVCC LT3790 200k 4.7µF 50V ×2 BG2 SW2 TG2 ISP ISN FB CTRL CSS 33nF 1000pF RT RC 15k CC 10nF SGND 147k 200kHz VOUT 24V 5A (12A*) 71.5k 1.37k SNSN SS SYNC VC 28 | January 2015 : LT Journal of Analog Innovation ROUT 8mΩ PGND PWM 100k COUT 220µF 35V ×2 RSENSE 2mΩ ISMON CLKOUT PWMOUT VREF 0.1µF + SNSP SHORT C/10 CCM IVINMON 33nF Figure 1. 120W, 24V, 5A output buck-boost voltage regulator with 8V–56V input has up to 98.5% efficiency and is easy to parallel. CVCC 4.7µF 0.1µF BST1 IVINP 100k C1 47µF 80V BST2 470nF 27.4k + D1 D2 IVINN 88.7k The buck-boost converter shown in Figure 1 regulates 24V with 0A–5A load at up to 98.5% efficiency. It operates from an input voltage range of 8V to 56V. Adjustable undervoltage and overvoltage lockout protect the circuit. It has short-circuit protection and the SHORT output flag indicates when there is a short circuit on the output. It features DCM operation at light load for lowest power consumption and reverse current protection. The sense resistor ROUT sets the output current limit during be mitigated with bulked up heat sinks, additional external gate drivers, and/or forced airflow, it may be better to simply tie together two or more converters in parallel to spread the load. This is easy to do with the LT3790 buck-boost regulator. 51Ω 499k 120W, 24V, 5A OUTPUT BUCK-BOOST VOLTAGE REGULATOR D1, D2: NXP BAT46WJ L1: COILCRAFT SER2915L-103KL 10µH M1, M2: INFINEON BSC100N06LS3 60Vds M3, M4: INFINEON BSC032N04LS 40Vds C1: NIPPON CHEMICON EMZA800ADA470MJAOG COUT: SUNCON 35HVT220M ×2 *WITH VIN > 20V, CAN DELIVER 300W USING ROUT = 4mΩ, RC = 5k, CC = 22nF 3.83k design features Figure 3. Two LT3790 24V voltage regulators are easy to parallel for double the output with limited discrete component temperature rise. VIN 8V TO 56V 1.5mΩ 499k Figure 2. Single 24V, 5A converter shown in Figure 1 has a maximum of 20°C temp rise on any component at 12V input (a) and 50°C at 9V input (b). Even at 8V input (c), the hottest component reaches only 96.5°C without forced airflow or heat sinking. 470nF 499k IVINP EN/UVLO EN 51Ω 1µF IVINN VIN INTVCC 27.4k C/10 TG1 M1 SWI SHORT VREF BG1 LT3790 PWM M2 0.1µF VOUT 24V 10A (25A*) L1 10µH ROUT1 8mΩ M4 51Ω M3 + COUT1 220µF 35V ×2 + COUT2 220µF 35V ×2 0.47µF SNSP 2mΩ 100k SNSN PGND SS 33nF 4.7µF 50V ×2 BST1 CTRL VIN = 12V VOUT = 24V IOUT = 5A SINGLE PCB NO FORCED AIR 4.7µF 10V D1 D2 0.1µF 200k 0.1µF C1 47µF 80V BST2 INTVCC1 SHORT + INTVCC1 CCM OVLO 88.7k 4.7µF 100V ×2 BG2 IVINMON CLKOUT ISMON SYNC SW2 VC (a) 1.37k 3.83k 147k 200kHz 5k 1000pF SGND RT 71.5k TG2 ISP ISN FB 47nF VIN 1.5mΩ 499k VIN = 9V VOUT = 24V IOUT = 5A SINGLE PCB NO FORCED AIR 470nF 499k IVINP EN/UVLO EN 51Ω 1µF IVINN VIN INTVCC 27.4k 0.1µF C/10 0.1µF TG1 SHORT VREF BG1 LT3790 M6 0.1µF L2 10µH M8 M7 ROUT2 8mΩ 51Ω 0.47µF SNSP 2mΩ 100k 33nF M5 SWI PWM 4.7µF 50V ×2 BST1 200k SHORT C2 47µF 80V 4.7µF 10V D3 D4 BST2 INTVCC2 (b) + INTVCC2 CCM OVLO 88.7k 4.7µF 100V ×2 SNSN PGND SS BG2 1nF VIN = 8V VOUT = 24V IOUT = 5A SINGLE PCB NO FORCED AIR CTRL IVINMON ISMON CLKOUT SYNC VC 470Ω (c) 22nF RT SW2 TG2 ISP ISN FB SGND 147k 200kHz 71.5k 14.0k D1–D4: NXP BAT46WJ 3.83k L1, L2: COILCRAFT SER2915L-103KL 10µH M1, M2, M5, M6: INFINEON BSC100N06LS3 60Vds M3, M4, M7, M8: INFINEON BSC032N04LS 40Vds COUT1, COUT2: SUNCON 35HVT220M ×2 C1, C2: NIPPON CHEMICON EMZA800ADA470MJAOG *25A FOR VIN > 20V AND ROUT1,2 =4mΩ January 2015 : LT Journal of Analog Innovation | 29 The CLKOUT pin of the master can be directly tied to the SYNC input pin of the slave for 180° phase-interleaving of the two parallel converters. The 180° phase difference between the converters reduces overall converter output ripple, instead of doubling it. If more than two converters are connected in parallel, they can be synchronized to either operate phase-shifted or in-phase with an external clock source, or daisy-chaining CLKOUT pins. 0.5 IL1(MASTER) 5A/DIV IL2(SLAVE) 5A/DIV IL1(MASTER) 2A/DIV IL2(SLAVE) 2A/DIV 0.4 0.3 0.2 ISMON1 500mV/DIV ISMON2 500mV/DIV ∆I (A) 0.1 ISMON1 500mV/DIV ISMON2 500mV/DIV 0.0 –0.1 –0.2 –0.3 –0.4 VIN = 12V VOUT = 24V ILOAD = 10A 2µs/DIV VIN = 24V VOUT = 24V ILOAD = 10A 2µs/DIV –0.5 0 2 6 4 LOAD CURRENT (A) 8 10 Figure 4. Parallel converter inductor and output current matching both a short-circuit and overload situations, making this a robust application. The temperature rise of this 120W board at 12V input is only 20°C on the hottest component (a switching MOSFET) as shown in Figure 2a. There is still margin for either higher output power at 12V input, or the same 120W from a lower VIN without excessive component temperature rise— note that higher output power requires a correspondingly increased output current limit. When operated down to 8V input with 120W output, the components on this standard 4-layer LT3790 PCB remain below 97°C (at room temp) without forced airflow or heat sinking. To deliver significantly higher power with the same, limited temperature rise and input voltage range, two or more LT3790 converters can easily be connected in parallel. 30 | January 2015 : LT Journal of Analog Innovation PARALLEL CONVERTERS, CONSTANT VOLTAGE MASTER, CONSTANT CURRENT SLAVE Ideally, paralleled switching converters share the load equally throughout the entire output range. The LT3790’s ability to run in either constant voltage or constant current operation allows one master converter to control the output voltage, while its current monitor output (ISMON) tells one or more slave converters how much output current to regulate (CTRL input) in order to match its own output level. Current matching between multiple converters is nearly ideal using this technique. The CLKOUT pin of the master can be directly tied to the SYNC input pin of the slave for 180° phase-interleaving of the two parallel converters. The 180° phase difference between the converters reduces overall converter output ripple, instead of doubling it. If more than two converters are connected in parallel, they can be synchronized to either operate phase-shifted or in-phase with an external clock source, or daisy-chaining CLKOUT pins. Figure 3 shows a 24V, 10A (or 25A under certain conditions, see figure) voltage regulator formed by running two LT3790s in parallel. By using two parallel circuits, the maximum temperature rise on any one discrete component is only 20°C for the M3 and M7 MOSFETs at 12V input and 50°C at 9V input. The top converter (master) in Figure 3 regulates the 24V output voltage and commands the current level that is regulated by the bottom (slave) converter. The ISMON output of the master indicates how much current the master is providing, and by connecting ISMON directly to the CTRL input of the slave, the slave is forced to follow the master. The LT3790 ISMON output level and CTRL input level are identically mapped so that a direct connection from one to the other is possible, and doing so forces the total output current to be shared equally between the design features Ideally, paralleled switching converters share the load equally throughout the entire output range. The LT3790’s ability to run in either constant voltage or constant current operation allows one master converter to control the output voltage, while its current monitor output (ISMON) tells one or more slave converters how much output current to regulate (CTRL input) in order to match its own output level. VOUT 1V/DIV (AC COUPLED) ISMON1 (MASTER) 500mV/DIV ISMON1 (MASTER) 500mV/DIV ISMON2 (SLAVE) 500mV/DIV ISMON2 (SLAVE) 500mV/DIV VIN = 12V 500µs/DIV VOUT = 24V ILOAD = 5A TO 10A The constant current slave must have its loop broken and signal injected in the current loop feedback path instead of the traditional voltage feedback path since that is the feedback loop in use during parallel operation. The master bode plot in Figure 7 demonstrates the stability of the system. 500µs/DIV VIN = 24V VOUT = 24V ILOAD = 5A TO 10A CONCLUSION The LT3790 synchronous buck-boost controller delivers over 100W at up to 98.5% efficiency to a variety of loads, and it is easy to parallel multiple converters for even higher power outputs. The ability to control either output voltage or current, combined with the levelmatching of the ISMON output amplifier and the CTRL input amplifier, simplifies the connection of a master voltage regulator and one or more slave current regulators. The result is high power 60V buck-boost regulation that can deliver hundreds of watts at high efficiency. n Figure 5. Parallel converter transient response evenly shares current parallel converters, as shown in Figure 4. Note that the output voltage of the slave is set slightly higher (28V) so that the voltage feedback loop of the slave is not in regulation, allowing it to follow the master. demonstrates a properly compensated converter and equally shared load current. Further analysis with the network analyzer gives us the details of the separate converters. The noise injection point and measurement to generate control loop bode plots is different for the constant voltage regulator master and the constant current regulator slave. Separately, each loop can be measured by injecting the LOOP ANALYSIS FOR STABILITY Transient response and network analyzer loop analysis can be used to measure stability. A transient response of 50% to 100% current, shown in Figure 5, IOUT ROUT1 + + 100Ω CH1 – + CH2 – CURRENT LOOP BODE PLOT MEASUREMENT SETUP FOR SLAVE 72.8k IOUT ROUT2 VOUT + COUT1 – CH1 + NOISE INJECT ISP – CH2 + 39Ω VOUT COUT2 NOISE INJECT 60 180 50 150 40 120 30 90 PHASE 20 GAIN (dB) VOLTAGE LOOP BODE PLOT MEASUREMENT SETUP FOR MASTER 30 0 3.83k ISN Figure 6. Loop response measurement of parallel converters 0 GAIN –10 –30 –20 –60 –30 VIN = 18V VOUT = 24V ILOAD = 10A –40 –50 FB 60 10 –60 PHASE (°) VOUT 1V/DIV (AC COUPLED) perturbation signal and measuring the loop response, as shown in Figure 6. 0.2 –90 –120 –150 PARALLEL 1 10 FREQUENCY (kHz) 50 –180 Figure 7. Bode plot shows measured results for parallel system. January 2015 : LT Journal of Analog Innovation | 31