July 2012 I N T H I S I S S U E pushbutton controller 10 15V, 2.5A monolithic buckboost with 95% efficiency and low noise 17 easy 2-supply current sharing 20 Volume 22 Number 2 Combined Voltage and Current Control Loops Simplify LED Drivers, High Capacity Battery/Supercap Chargers & MPPT* Solar Applications Xin (Shin) Qi * Maximum Power Point Tracking sub-1mm height 24V, 15A monolithic regulator 26 deliver 25A at 12V from inputs to 60V 28 The rapid expansion of constant-current/constant-voltage (CC-CV) applications, especially in LED lighting and high capacity battery and supercapacitor chargers challenges power supply designers to keep pace with the increasingly complicated interplay of current and voltage control loops. A switch-mode converter designed specifically for CC-CV offers a clear advantage, especially when the supply has limited power, or its power is allocated among several competing loads. The LTC4155 is a monolithic switching battery charger that efficiently delivers 3.5A charge current in a compact PCB footprint. See page 13. Caption w w w. li n ea r.com Consider, for instance, the challenge of charging a supercapacitor in a minimum amount of time from a power-limited supply. To maintain constant input power, the controlled charging current must decrease as the output (supercapacitor) voltage increases. The LT®3796 solves the problem of power limited or constant current/constant voltage regulation by seamlessly combining a current regulation loop and two voltage regulation loops to control an external N-channel power switch. The inherent wired-OR behavior of its three transconductance error amplifiers summed into the compensation pin, VC , ensures that the correct loop (that is, the one closest to regulation) dominates. (continued on page 4) The LT3796’s wide VIN range (6V to 100V) and rail-to-rail (0V to 100V) output current monitoring and regulation allow it to be used in a wide variety of applications from solar battery chargers to high power LED lighting systems. (LT3796, continued from page 1) HIGH POWER LED DRIVER WITH ROBUST OUTPUT SHORT CIRCUIT PROTECTION The additional, standalone current sense amplifier can be configured for any number of functions, including input current limit and input voltage regulation. Figure 1 shows the LT3796 configured as a boost converter to drive a 34W LED string from a wide input range. The LED current is derated at low input voltages to prevent external power components from overheating. The front-end current sense amplifier monitors the input current by converting the input current to a voltage signal at the CSOUT pin with The LT3796’s wide VIN range (6V to 100V) and rail-to-rail (0V to 100V) output current monitoring and regulation allow it to be used in a wide variety of applications from solar battery chargers to high power LED lighting systems. The fixed switching frequency, current-mode architecture results in stable operation over a wide range of supply and output voltages. The LT3796 incorporates a high side current sense, enabling its use in boost, buck, buckboost or SEPIC and flyback topologies. Figure 1. A 34W LED driver with robust output short-circuit protection. VCSOUT = IIN • RSNS1 • The LT3796 includes short-circuit protection independent of the LED current sense. The short-circuit protection feature prevents the development of excessive switching currents and protects the power components. The protection threshold (375mV, typ) is designed to be 50% higher than the default LED current sense threshold. R6 R5 The resistor network at the FB1 pin provides OPENLED protection, which limits the output voltage and prevents the ISP pin, ISN pin and several external VIN 9V TO 60V 100V (TRANSIENT) R1 1M R2 118k R3 499k OPTIONAL INPUT CURRENT REPORTING R4 97.6k VS CSN CSP EN/UVLO GATE R6 40.2k PWM SYNC LED CURRENT REPORTING INTVCC R10 100k R9 100k FAULT VMODE RSNS 15mΩ FB1 LT3796 SYNC ISP ISMON Q1 M2 TG FAULT INTVCC VMODE R11 402k (OPT) RLED 620mΩ ISN C4 0.1µF R11 OPTIONAL FOR FAULT LATCHOFF UP TO 400mA GND PWM VREF M1: INFINEON BCS160N10NS3-G M2: VISHAY SILICONIX Si7113DN L1: COILTRONICS DR127-220 D1: DIODES INC PDS5100 D2: VISHAY ES1C Q1: ZETEX FMMT589 LED: CREE XLAMP XR-E R8 13.7k M1 SENSE CSOUT C3 10nF C2 2.2µF ×4 100V R7 1M CTRL CSOUT D1 R5 2k VIN circuits.linear.com/558 L1 22µH RSNS1 50mΩ IIN C1 2.2µF ×3 LTspice IV 4 | July 2012 : LT Journal of Analog Innovation components from exceeding their maximum rating. If an LED fails open or if the LED string is removed from the high power driver, the FB constant voltage loop takes over and regulates the output to 92.5V. The VMODE flag is also asserted to indicate an OPENLED event. SS FB2 RC 10k C6 0.1µF CC 10nF D2 C5 4.7µF RT VC INTVCC 85V LED RT 31.6k 250kHz design features The LT3796 solves the problem of power limited, or constant-current/constant-voltage regulation by seamlessly combining a current regulation loop and two voltage regulation loops to control an external N-channel power switch. SS 2V/DIV SS 2V/DIV LED+ 50V/DIV LED+ 50V/DIV FAULT 10V/DIV FAULT 10V/DIV IM2 1A/DIV IM2 1A/DIV If there is no resistor between the SS pin and VREF pin, the converter enters hiccup mode and periodically retries as shown in the Figure 2. If a resistor is placed between VREF and SS pin to hold SS pin higher than 0.2V during LED short, then the LT3796 enters latchoff mode with GATE pin low and TG pin high, as shown in Figure 3. To exit latchoff mode, the EN/UVLO pin must be toggled low to high. 5ms/DIV 5ms/DIV Figure 2. Short LED protection: hiccup mode (without R11 in Figure 1) Figure 3. Short LED protection: latchoff mode (with R11 in Figure 1) LED DRIVER WITH HIGH PWM DIMMING RATIO Once the LED overcurrent is detected, the GATE pin drives to GND to stop switching, the TG pin is pulled high to disconnect the LED array from the power path and the FAULT pin is asserted. The Schottky diode D2 is added to protect the drain of PMOS M2 from swinging well below ground when shorting to ground through a long cable. The PNP helper Q1 is included to further limit the transient short-circuit current. Using an input referred LED string allows the LT3796 to act as a buck mode controller as shown in Figure 4. The 1MHz operating frequency enables a high PWM dimming ratio. The OPENLED regulation voltage is set to 1.25V • VIN 16V TO 36V RLED 250mΩ 1A Figure 4. A buck mode LED driver with 3000:1 PWM dimming ratio R3 100k R1 1M VIN R2 100k VS VREF CSN LT3796 R4 100k 8V LED TG CSP PWM PWM ISN ISP EN/UVLO CTRL CSOUT FB1 LED CURRENT REPORTING C5 0.1µF R6 124k L1 10µH SENSE INTVCC RSNS 33mΩ R9 100k FAULT VMODE GND FAULT VMODE VC M1: VISHAY SILICONIX Si73430DV M2: VISHAY SILICONIX Si7113DN D1: ZETEX ZLLS2000TA L1: WÜRTH 744066100 LED: CREE XLAMP XM-L M1 GATE ISMON INTVCC SYNC RT RC 10k CC 4.7nF RT 6.65k 1MHz SS C4 0.1µF C2 10µF ×4 25V R5 1M FB2 R8 100k through the independent current sense amplifier at CSP, CSN and CSOUT pins. During the PWM off phase, the LT3796 disables all internal loads to the VC pin LED+ M2 C3 4.7µF R3 R5 • + 1 R6 R4 Figure 5. 3000:1 PWM dimming ratio of the circuit in Figure 4 at VIN = 24V and PWM = 100Hz D1 C1 2.2µF ×2 50V PWM 5V/DIV IL 1A/DIV INTVCC IL 1A/DIV 1µs/DIV July 2012 : LT Journal of Analog Innovation | 5 Voltage drops in wiring and cables can cause load regulation errors. These errors can be corrected by adding remote sensing wires, but adding wires is not an option in some applications. As an alternative, the LT3796 can adjust for wiring drops, regardless of load current, provided that the parasitic wiring or cable impedance is known. OUT • 1:1 C1 10µF M1 C3 10µF L1B + C4 100µF 25V RWIRE VLOAD 12V, 1A CURRENT LIMIT RSNS 33mΩ VIN C8 0.1µF VREF GATE SENSE GND EN/UVLO ISP ISMON ISN PWM LT3796 SYNC C5 0.1µF R7 100k SS VS CTRL FAULT FAULT R4 287k FB1 VMODE TG VC RT 19.6k 400kHz and preserves the charge state. It also turns off the PMOS switch M2 to disconnect the LED string from the power path and prevent the output capacitor from discharging. These features combine to greatly improve the LED current recovery time when PWM signal goes high. Even with a 100Hz PWM input signal, this buck mode LED driver can achieve a 3000:1 dimming ratio as illustrated in Figure 5. OUT CSOUT RT L1: WÜRTH 744871220 D1: ZETEX ZLLS2000TA M1: VISHAY SILICONIX Si4840DY C7 1µF CSP R6 100k VMODE R2 38.3k R3 154k VREF INTVCC R1 38.3k CSN FB2 INTVCC RC 24.9k CC 10nF R5 12.4k INTVCC C6 4.7µF SEPIC CONVERTER WITH R WIRE COMPENSATION Voltage drops in wiring and cables can cause load regulation errors. These errors can be corrected by adding remote sensing wires, but adding wires is not an option in some applications. As an alternative, the LT3796 can adjust for wiring drops, regardless of load current, provided that the parasitic wiring or cable impedance is known. Figure 6 shows a 12V SEPIC converter that uses the RWIRE compensation feature. RSNS1 is selected to have 1A load current 6 | July 2012 : LT Journal of Analog Innovation RSNS1 250mΩ D1 • Figure 6. This SEPIC converter compensates for voltage drops in the wire between the controller and the load (RWIRE) C2 10µF L1A 22µH VIN 12V limit controlled by the ISP, ISN pins. The resistor network R1–R5, along with the LT3796’s integrated current sense amplifier (CSAMP in Figure 7), adjusts the OUT node voltage (VOUT) to account for voltage drops with respect to the load current. This ensures that VLOAD remains constant at 12V throughout the load range. Figure 7 shows how the LT3796’s internal CSAMP circuit plays into the operation. The LT3796’s voltage loop regulates the FB1 pin at 1.25V so that I3 stays fixed at 100µ A for R5 = 12.4k. In Figure 7, VOUT changes design features The LT3796 in a 28-lead TSSOP package performs tasks that would otherwise require a number of control ICs and systems. It offers a reliable power system with simplicity, reduced cost and small solution size. ILOAD R3 2 • (R WIRE ) = R1 R SNS1 RWIRE CSN CSP – I1 CSAMP R4 RWIRE = 0.5Ω VOUT 800mA 12.5 VLOAD LT3796 + R3 12V R2 R1 13.0 I2 VOUT/VLOAD (V) VOUT RSNS1 IOUT 500mA/DIV 12.0 11.5 11.0 VOUT 500mV/DIV (AC-COUPLED) 10.5 10.0 CSOUT FB1 = 1.25V R5 200mA 0 200 400 600 800 1000 1200 500µs/DIV ILOAD (mA) I3 Figure 7. RWIRE voltage drops are compensated for via the LT3796’s CSAMP circuit Figure 8. Measured VLOAD and VOUT with respect to ILOAD Figure 9. Load step response of the circuit in Figure 6 with current I2 as VOUT = 1.25V + I2 • R4. If the change of I2 • R4 can offset the change of ILOAD • (RSNS1 + RWIRE), then VLOAD will stay constant. output current is what gives VOUT the positive load regulation characteristic. The positive load regulation is just what is needed to compensate for the cable drop. SOLAR PANEL BATTERY CHARGER Referring to Figure 7, the divider R1/R3 from VOUT sets the voltage regulated at CSP by the current I1 flowing in R2. I1 is conveyed to the FB1 node where it sums with I2 . The measured VLOAD and VOUT with respect to ILOAD are shown in Figure 8. Clearly, VLOAD is independent of ILOAD when ILOAD is less than the 1A current limit. When ILOAD approaches 1A, the current loop at ISP and ISN pins begins to interfere with the voltage loop and drags the output voltage down correspondingly. The load transient response is shown in Figure 9. As the output current increases, I1 decreases due to the increasing voltage drop across RSNS ; its decrease must be compensated by a matching increase in the current I2 to maintain the constant 100µ A into FB2. This increase in I2 with Solar powered devices rely on a highly variable energy source, so for a device to be useful at all times, energy from solar cells must be stored in a rechargeable battery. Solar panels have a maximum power point, a relatively fixed voltage at which the panel can produce the most power. Maximum power point tracking (MPPT) is usually achieved by limiting a converter’s output current to keep the panel voltage from straying from this value. The LT3796’s unique combination of current and voltage loops make it an ideal MPPT battery charger solution. July 2012 : LT Journal of Analog Innovation | 7 WÜRTH SOLAR PANEL VOC = 37.5V VMPP = 28V C6 2.2µF 100V OUT BAT RSNS1 250mΩ • D1 D2 15V 1:1 C1 4.7µF 50V • VIN L1A 33µH M2 R4 301k INTVCC R1 10k C2 10µF L1B R10 30.1k R9 10k NTC VCHARGE = 14.6V VFLOAT = 13.5V AT 25°C + BAT R5 137k R2 475k VIN VS CSN CSP M1 GATE EN/UVLO R3 20k SENSE RSNS 15mΩ CTRL CSOUT C3 0.1µF GND CSOUT R6 100k R11 93.1k FB1 FB2 PWM LT3796 VREF ISP OUT ISN BAT R8 113k M3 ISMON C6 0.1µF SS C4 0.1µF VMODE SYNC TG M1: VISHAY SILICONIX Si7456DP M2: VISHAY SUD19P06-60-E3 M3: ZETEX ZXM61N03F L1: COILCRAFT MSD1260-333 D1: ON SEMI MBRS260T3G D2: CENTRAL SEMI CMDZ15L R9: MURATA NCP18XH103F03RB R12 10.2k INTVCC VC RT RC 499Ω CC 22nF FAULT RT 19.6k 400kHz VMODE R7 49.9k C5 4.7µF INTVCC R13 49.9k FAULT Figure 10. A solar panel battery charger maximum power point tracking (MPPT) Figure 10 shows a solar panel to sealed lead acid (SLA) battery charger driven by the LT3796. The charger uses a three-stage charging scheme. The first stage is a constant current charge. Once the battery is charged up to 14.35V, the charging current The charging current is programmed by the resistor network at the CSP and CSOUT (CTRL) pins as follows, 1.2 1.0 V −V V VCTRL = R6 • IN INTVCC − INTVCC , R4 R5 R4 FOR VIN ≥ VINTVCC 1+ R5 VCTRL = 0 V, ICHARGE (A) 0.8 0.6 0.4 R4 FOR VIN < VINTVCC 1+ R5 0.2 0 begins to decrease. Finally, when the required battery charge current falls below 100m A, the built-in C/10 termination disables the charge circuit by pulling down VMODE, and the charger enters float charge stage with VFLOAT = 13.5V to compensate for the loss caused by self-discharge. 20 25 30 35 VIN (V) Figure 11. ICHARGE vs VIN for the solar charger in Figure 10 8 | July 2012 : LT Journal of Analog Innovation 40 Maximum power point tracking is implemented by controlling the maximum output charge current. Charge current is reduced as the voltage on the solar panel output falls toward 28V, which corresponds to 1.1V on the CTRL pin and full charging current, as shown in Figure 11. This servo loop thus acts to dynamically reduce the power requirements of the charger system to the maximum power that the panel can provide, maintaining solar panel power utilization close to 100%. SUPERCAPACITOR CHARGER WITH INPUT CURRENT LIMIT Supercapacitors are rapidly replacing batteries in a number of applications from rapid-charge power cells for cordless tools to short term backup systems for microprocessors. Supercapacitors are longer lasting, greener, higher performance and less expensive over the long run, but charging supercapacitors requires precise control of charging current and voltage design features RSNS1 150mΩ 1.33A MAX VS L1B CSP CSN EN/UVLO OUTPUT CURRENT REPORTING GATE ISMON PWM VREF INPUT CURRENT REPORTING AND LIMIT CSOUT SYNC R2 124k C4 0.1µF VOUT = 0V TO 28V D1 R1 20k VIN C3 0.1µF C6 10µF • C1 10µF C7 0.1µF L1A 33µH • VIN 28V C2 4.7µF ×2 50V R8 536k R9 24.9k M1 SENSE RSNS 33mΩ LT3796 1.67A MAX GND CSOUT FB1 FB2 ISP RSNS2 150mΩ SS ISN SUPERCAP TG INTVCC R7 100k R6 100k INTVCC FAULT FAULT CHGDONE C6 4.7µF VMODE CTRL VREF RT VC RC 499Ω VOUT L1: COILCRAFT MSD1260-333 D1: ON SEMI MBRS260T3G M1: VISHAY SILICONIX Si7850 Q1: ZETEX FMMT591A R3 499k C5 0.1µF R10 499k R5 1M R4 30.1k CC 22nF Q1 RT 19.6k 400kHz Figure 12. A 28V/1.67A supercapacitor charger with input current limit regulation until the input current moves close to the 1.33A input current limit. Some applications require that the input current is limited to prevent the input supply from crashing. Figure 12 shows a 1.67A supercapacitor charger with 28V regulated output voltage and 1.33A input current limit. The input current is sensed by RSNS1, converted to a voltage signal and fed to the FB2 pin to provide input current limit. CONCLUSION In each charging cycle, the supercapacitor is charged from 0V. The feedback loop from VOUT to the RT pin through R3, C5, R5, R10, R4, and Q1 to RT works as frequency foldback to keep regulation under control. In Figure 13, the input current and output charging current are plotted against output voltage for this charger, showing the LT3796 maintaining the output current The LT3796 is a versatile step-up DC/DC controller that combines accurate current and voltage regulation loops. Its unique combination of a single current loop and two voltage loops makes it easy to solve the problems posed by applications that require multiple control loops, such as LED drivers, battery or supercapacitor chargers, MPPT solar battery chargers, and step-up or SEPIC converters with input and output current limit. It also includes a number of fault protection and reporting functions, a top gate driver and current loop reporting. The LT3796 in a 28-lead TSSOP package performs tasks that would other wise require a number of control ICs and systems. It offers a reliable power system with simplicity, reduced cost and small solution size. n 1800 IOUT 1600 INPUT/OUTPUT CURRENT (mA) limiting to prevent any system-wide damage or damage to the supercapacitor. 1400 1200 IIN 1000 800 600 400 200 0 0 5 10 15 20 25 30 VOUT (V) Figure 13. Input/output current vs output voltage for 28V/1.67A supercapacitor charger in Figure 12 July 2012 : LT Journal of Analog Innovation | 9