60V, 4-Switch Synchronous Buck-Boost Controller Regulates Voltage from Wide Ranging Inputs and Charges Batteries at 98.5% Efficiency at 100W+ Keith Szolusha The LT®3791-1 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 well over a hundred 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 is needed. In addition to the high voltage, power and efficiency, it features short-circuit protection, a SYNC pin for synchronization to an external clock, a CLKOUT pin for driving an external SYNC pin or for parallel operation, OVLO (overvoltage lockout), SHORT output flag, C/10 detection and output flag for battery chargers, and a CCM pin for discontinuous or continuous conduction mode. The inclusion of DCM (discontinuous conduction mode) increases light load efficiency and prevents reverse current when it is undesirable. 120W, 24V 5A OUTPUT BUCK-BOOST VOLTAGE REGULATOR operates from an input voltage range of 12V to 58V. Adjustable undervoltage and overvoltage lockout protect the circuit. It has short-circuit protection and the SHORT output flag indicates The buck-boost converter shown in Figure 1 regulates 24V with 0A–5A load at up to 98.5% efficiency (Figure 2). It VIN 12V TO 58V 0.003Ω VIN 1µF 51Ω 499k INTVCC 0.1µF IVINP BST1 499k TG1 EN/UVLO 27.4k 100k 18.7k BG1 INTVCC LT3791-1 200k 0.1µF VOUT 24V 5A 715k M3 13.7k BG2 VC 1000pF RT 15k 10nF 22 | October 2012 : LT Journal of Analog Innovation SW2 TG2 ISP ISN FB SGND 147k 200kHz LTspice IV 38.3k SNSN PGND SS SYNC 33nF L1 10µH M4 COUT 220µF 35V ROUT ×2 7.5mΩ 4.7µF 50V ×2 Figure 1. 120W 24V 5A output buckboost voltage regulator accepts a 12V–58V input C1 47µF 80V 0.004Ω PWM 100k M2 0.1µF + SNSP SHORT C/10 CCM IVINMON ISMON CLKOUT CTRL VREF 33nF M1 SWI OVLO 56.2k + BST2 IVINN 4.7µF 100V 4.7µF D1 D2 470nF 10k when there is a short circuit on the output. It features DCM operation at light load for lowest power consumption and reverse current protection. ROUT limits the output current during both D1, D2: NXP BAT46WJ L1: COILCRAFT SER2915L-103KL 10µH M1, M2: RENESAS RJK0651DPB 60Vds M3, M4: RENESAS RJK0451DPB 40Vds C1: NIPPON CHEMICON EMZA800ADA470MJAOG COUT: SUNCON 35HVT220M 35V 220µF ×2 circuits.linear.com/589 design features The LT3791-1 can regulate both constant voltage and constant current. Large capacitive loads such as supercapacitors and batteries require constant current charging until they are charged up to a termination voltage, at which point they require constant voltage regulation. The LT3791-1 easily satisfies this requirement. 100 VIN = 14V IIN = 8.87A VOUT = 24V IOUT = 5A 95 EFFICIENCY (%) 90 85 80 75 70 VIN = 12V VIN = 24V VIN = 54V 65 Figure 2. Efficiency and worst case thermal results for the 24V converter in Figure 1 60 0 1 2 3 LOAD CURRENT (A) 4 5 a short circuit and an overload situation, making this a robust application. The 14V, 10A voltage regulator in Figure 3 takes a slightly different approach. It runs in CCM throughout its entire load current range 0A-10A to provide the lowest EMI at light load. It is still very efficient. The circuit retains short-circuit protection even though ROUT is replaced with a short. The main switch sense resistor RSW limits the short-circuit current at a higher level than ROUT, but hiccup mode during shortcircuit limits the power consumption of the IC, maintaining a low temperature rise on the components during a short. When DCM is not needed, ROUT may not be necessary and removing ROUT slightly increases circuit efficiency. OVLO is tied to the output to limit the output voltage transient during a 10A to 0A transition. This protects both the output capacitors and M3 and M4 switches from overvoltage. VIN 9V TO 36V 0.002Ω 470nF 499k IVINP EN/UVLO 51Ω CIN1 4.7µF 50V 1µF + IVINN VIN INTVCC CCM 76.8k CIN2 100µF 63V ×2 4.7µF 10V D1 D2 BST2 0.1µF BST1 C/10 IVINMON CLKOUT ISMON TG1 BG1 INTVCC LT3791-1 200k SHORT 100k 22nF VC 5.1k M3 COUT1 4.7µF 50V ×2 + COUT2 270µF 35V ×2 VOUT 14V 10A 100k 9.31k BG2 CTRL SS 10nF Figure 3. 140W (14V, 10A) CCM buck-boost voltage regulator with 9V–56V input has output OVLO for transient protection. SNSN PGND SYNC 100pF L1 3.3µH 0.0025Ω PWM M5 M2 M4 SNSP SHORT VREF 0.1µF M1 SWI 0.1µF RT SGND 147k 200kHz SW2 TG2 FB ISP ISN OVLO 499k D1, D2: NXP BAT46WJ L1: COILCRAFT SER2915H-332L 3.3µH 48A M1: RENESAS RJK0652DPB 60Vds M2: RENESAS RJK0651DPB 60Vds M3, M4: INFINEON BSC0904NSI 30Vds M5: NXP NX7002AK COUT2: SUNCON 35HVT270M CIN2: NIPPON CHEMICON EMZA630ADA101MJAOG 88.7k October 2012 : LT Journal of Analog Innovation | 23 0.003Ω VIN 12V TO 58V 499k 51Ω 470nF 499k IVINP EN/UVLO 1µF 27.4k CCM C/10 BST1 M1 TG1 SWI SHORT VREF M2 BG1 LT3791-1 PWM 0.1µF Figure 4. Parallel LT3791-1s in a 240W application L1 10µH 4.7µF 50V ×2 0.015Ω VOUT 24V 10A M4 51Ω M3 + COUT1 220µF 35V ×2 0.47µF SNSP CTRL 0.004Ω 100k SNSN PGND SS 33nF 4.7µF 10V D1 D2 0.1µF 200k 0.1µF C1 47µF 80V BST2 INTVCC1 SHORT + INTVCC1 IVINN VIN INTVCC OVLO 56.2k 4.7µF 100V BG2 IVINMON CLKOUT ISMON SYNC SW2 VC SGND RT 715k TG2 ISP ISN FB 13.7k 38.3k 147k 200kHz 3.3k 33nF INTVCC1 + LTC6240 – 45k 10k 10k 0.003Ω VIN 499k 470nF 499k 10k IVINP EN/UVLO 51Ω 1µF IVINN VIN INTVCC 27.4k 0.1µF C/10 BST1 TG1 200k SHORT 0.1µF PWM BG1 LT3791-1 M6 0.1µF L2 10µH 4.7µF 50V ×2 0.015Ω M8 M7 51Ω + 0.22µF SNSP 0.004Ω 100k 33nF M5 SWI SHORT VREF C2 47µF 80V 4.7µF 10V D3 D4 BST2 INTVCC2 45k + INTVCC2 CCM OVLO 56.2k 4.7µF 100V SNSN PGND SS BG2 CTRL IVINMON ISMON CLKOUT SYNC VC 10k 1000pF 2.2k RT SW2 TG2 ISP ISN FB SGND 147k 200kHz 22nF 24 | October 2012 : LT Journal of Analog Innovation 715k 13.7k 38.3k D1–D4: NXP BAT46WJ L1, L2: COILCRAFT SER2915L-103KL 10µH M1, M2, M5, M6: RENESAS RJK0651DPB 60Vds M3, M4, M7, M8: RENESAS RJK0451DPB 40Vds COUT1, COUT2: SUNCON 35HVT220M ×2 C1, C2: NIPPON CHEMICON EMZA800ADA470MJAOG COUT2 220µF 35V ×2 PARALLEL CONVERTERS FOR HIGH POWER USING CLKOUT AND SYNC The LT3791-1 has a CLKOUT output that can be used to synchronize other converters to its own clock with a 180° phase shift. By tying the CLKOUT of one converter to the SYNC input of another, the maximum output power is doubled while the output ripple is reduced. Figure 4 shows a 24V, 10A regulator formed by running two LT3791-1s in parallel. By using two parallel circuits, the maximum temperature rise seen on any one discrete component is only 28°C on the M3 and M7 MOSFETs at the lowest VIN . The top converter (master) in Figure 4 commands the current level provided by the bottom (slave) converter. The ISMON output of the master indicates how much current the master is providing, and by connecting ISMON to the CTRL input of the slave, the slave is forced to follow the master. A single op amp is needed to provide the simple 200mV level shift needed to match the CTRL input to the ISMON output levels. The master converter runs in constant voltage regulation while the slave converter is running in constant current regulation. 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 for it to be able to follow the master. design features The LT3791-1 features both continuous conduction mode (CCM) and discontinuous conduction mode (DCM). CCM provides continuous switching at light load and inductor current can be either positive or negative. When the LT3791-1 enters DCM operation at light load, it prevents backward running current (negative inductor current) and light load power dissipation is minimized. 100W+ 2.5A BUCK-BOOST 36V SLA BATTERY CHARGER In some battery charger applications, once termination voltage is reached and charge current tails off, a standby or float voltage regulation level is needed that is different from the charge voltage. The C/10 detection level of the LT3791-1 provides this capability. In the circuit in Figure 3 the C/10 function drops the battery voltage from charging (44V) to float (41V) when the battery is near full charge. When the battery voltage is then pulled down from an increased load, the voltage feedback loop returns the charger to its charge state of 44V. The LT3791-1 can regulate both constant voltage and constant current. Large capacitive loads such as supercapacitors and batteries require constant current charging until they are charged up to a termination voltage, at which point they require constant voltage regulation. The LT3791-1 easily satisfies this requirement. As an example, the buck-boost converter shown in Figure 5 charges a 36V 12Ah SLA battery at 44V with 2.5A DC from a 9V-to-58V input. DCM operation prevents reverse battery current when the output load is overcharged, protecting the circuit from large negative currents. Figure 5. SLA battery charger regardless of the voltage relationship between them. Furthermore, a microcontroller can be used to create a maximum power point tracking (MPPT) charger from a solar panel. The output diagnostics ISMON and IVINMON and current control pin CTRL make it easy to create a high power solar panel battery charger. DCM INCREASES EFFICIENCY AND PREVENTS REVERSE CURRENT The LT3791-1 features both continuous conduction mode (CCM) and discontinuous conduction mode (DCM). Figure 6 shows the difference between CCM and DCM. The mode is selected by simply connecting the CCM pin to either the INTVCC or C/10 pin. CCM provides continuous switching The LT3791-1 can be tailored to charge a range of battery chemistries and capacities from a variety of input sources PVIN 9V TO 57V RIN 0.003Ω 1µF 50Ω VIN INTVCC D1 IVINN TG1 BG1 OVLO LT3791-1 19.6k M1 0.1µF M4 M2 L1 10µH M3 SWI EN/UVLO RBAT 0.04Ω SNSN SHORT + PGND IVINMON ISMON CTRL BG2 SW2 TG2 ISP CLKOUT SGND 100k VREF PWM ISN FB CCM SS C/10 RT SYNC VC D1, D2: BAT46WJ 33nF L1: COILCRAFT SER2915L-103K M1-M4: RENESAS RJK0651DPB M5: NXP NX7002AK CIN2: ×2 NIPPON CHEMI-CON EMZA630ADA101MJA0G COUT2: ×3 NIPPON CHEMI-CON EMZA630ADA101MJA0G CIN2 100µF 63V ×2 COUT1 4.7µF 50V ×2 RSENSE 0.004Ω 200k 0.1µF + + COUT2 100µF 63V ×3 SNSP INTVCC CHARGE CURRENT CONTROL CIN1 4.7µF 100V ×2 0.1µF BST1 IVINP 24.3k D2 BST2 470nF 332k 4.7µF 3k 84.5k 300kHz 0.1µF 36V SLA BATTERY AGM TYPE 41V FLOAT 44V CHARGE AT 25°C 50Ω 1.00M INTVCC 10k 0.47µF 2.5A CHARGE 10k 402k 30.1k M5 22nF October 2012 : LT Journal of Analog Innovation | 25 5000 90 1000 CCM RISING THRESHOLD 600 EFFICIENCY (%) ILOAD (mA) FOR CCM OPERATION OVER ALL IOUT DCM FALLING THRESHOLD 400 INTVCC LT3791-1 LT3791-1 100k CCM CCM CCCM OPTIONAL 0 DCM (TG2 FOR M4 STAYS LOW) 18 12 24 30 36 VIN (V) 42 48 at light load and inductor current can be either positive or negative. Although zero-load inductor current in CCM is both positive and negative and more power is consumed than DCM, the switch node ringing associated with DCM is eliminated for those that do not want it. DCM operation at light load, the TG2 driver for M4 stays low and M4 no longer runs as a switch, but instead as a catch diode. This prevents backward running current (negative inductor current) and light load power dissipation is minimized. When DCM is selected, the converter remains in CCM until the load drops below about 10% of the programmed maximum output current. When the LT3791-1 enters The LT3791-1 synchronous buck-boost controller delivers over 100W at up to 98.5% efficiency to a variety of loads. Its wide, 4.7V to 60V input range and 0V to 1.2 1 40 0.8 30 INTVCC IVINN 470nF 0.01 0.1 IOUT (A) 1 TG1 EN/UVLO OVLO 0.1µF 18.7k 100k BG1 LT3791-1 200k 0.0125Ω M2 L1 22µH M4 M3 COUT1 4.7µF 100V ×2 + COUT2 330µF 63V ×3 Figure 7. 48V application VOUT 48V 3A 0.005Ω SNSN PGND BG2 SW2 TG2 ISP ISN FB SGND PWM 10k CIN2 100µF 80V ×2 SNSP SHORT C/10 CCM IVINMON ISMON CLKOUT VREF 0.1µF M1 SWI INTVCC 100k + BST1 0.1µF CTRL SS SYNC V C RT 40k 1µF 22nF 26 | October 2012 : LT Journal of Analog Innovation 105k 250kHz 1000pF 0.4 0.2 10 0 60V output range make it powerful and versatile, and its built-in short-circuit capabilities make for robust solutions in potentially hazardous environments. CCM and DCM operation make it useful for highest efficiency or lowest noise operation at light load. Its multiple control loops make it ideal for regulating constant voltage, constant current or both. This feature-rich IC easily fulfills buck-boost requirements where other topologies fail. n 0.1µF IVINP 499k 28.7k 4.7µF 10V D1 D2 BST2 CIN1 4.7µF 100V 0.6 DCM DCM CCM CCM c. DCM improves efficiency at light loads. CONCLUSION VIN 1.4 50 0.004Ω 1µF 51Ω 34.8k 60 1.6 POWER LOSS EFFICIENCY 0 0.001 54 b. DCM/CCM transition thresholds remain stable as the LT3791-1 moves through boost, buck-boost and buck modes of operation. 499k 70 10 a. DCM vs CCM setup VIN 18V TO 55V 1.8 20 200 INTVCC C/10 2 VIN = 24V VOUT = 24V 80 800 FOR DCM OPERATION AT IOUT < 10mV/ROUT 100 CCM POWER LOSS (W) Figure 6. Overview of continuous conduction mode (CCM) for low noise and discontinuous conduction mode (DCM) for light load efficiency 95.3k 2.15k D1, D2: NXP BAT46WJ L1: WÜRTH ELECTRONICS 74435572200 22µH 11A M1, M2, M4: RENESAS RJK0651DPB 60Vds M3: RENESAS RJK0652DPB 60Vds COUT2: YAGEO ST 330µF 63V ×3 CIN2: NIPPON CHEMICON EMZA630ADA101MJAOG 100µF 63V ×2 2.49k