LINEAR TECHNOLOGY JUNE 2009 IN THIS ISSUE… COVER ARTICLE Two New Controllers for Boost, Flyback, SEPIC and Inverting DC/DC Converters Accept Inputs up to 100V ............................................................1 Wei Gu Linear in the News…............................2 DESIGN FEATURES Charge Li-Ion Batteries Directly from High Voltage Automotive and Industrial Supplies Using Standalone Charger in a 3mm × 3mm DFN..................................5 Jay Celani Power Management IC Combines USB On-The-Go and USB Charging in Compact Easy-to-Use Solution..........8 George H. Barbehenn and Sauparna Das Power Management IC with Pushbutton Control Generates Six Voltage Rails from USB or 2 AA Cells Via Low Loss PowerPath™ Topology........................12 John Canfield Improve Hot Swap Performance and Save Design Time with Hot Swap™ Controller that Integrates 2A MOSFET and Sense Resistor..........16 David Soo Compact No RSENSE™ Controllers Feature Fast Transient Response and Regulate to Low VOUT from Wide Ranging VIN ..........................................................18 VOLUME XIX NUMBER 2 Two New Controllers for Boost, Flyback, SEPIC and Inverting DC/DC Converters Accept Inputs up to 100V by Wei Gu Introduction Two new versatile DC/DC controller ICs, the LT®3757 and LT3758, are optimized for boost, flyback, SEPIC and inverting converter applications. The LT3757 operates over an input range of 2.9V to 40V, suitable for applications from single-cell lithium-ion battery portable electronics up to high voltage automotive and industrial power supplies. The LT3758 extends the input voltage to 100V, providing flexible, high performance operation in high voltage, high power telecommunications equipment. Both ICs exhibit low shutdown quiescent cur- 4.7µF 50V X5R Space-Saving, Dual Output DC/DC Converter Yields Plus/Minus Voltage Outputs with (Optional) I2C Programming........22 200k VIN 10µH SHDN/UVLO 43.2k SYNC Mathew Wich (complete list on page 26) continued on page VIN 10V TO 30V Terry J. Groom DESIGN IDEAS .....................................................26–36 rent of 1µA, making them an ideal fit for battery-operated systems. Both integrate a high voltage, low dropout linear (LDO) regulator. Thanks to a novel FBX pin architecture, the LT3757 and LT3758 can be connected directly to a divider from either the positive output or the negative output to ground. They also pack many popular features such as soft-start, input undervoltage lockout, adjustable frequency and synchronization in a small 10-lead MSOP package or a 3mm × 3mm QFN package. LT3757 FBX GND INTVCC VC 8.45k 0.1µF M1 Si7850 SENSE RT SS 42.2k GATE IHLP-5050EZ-01 MBRM360 10nF 4.7µF 10V X5R 48V 1A 590k 1% 20.0k 1% 0.01Ω New Device Cameos............................37 4.7µF 50V X5R s2 Design Tools.......................................39 Sales Offices......................................40 Figure 1. A 10V–30V input, 48V at 1A output boost converter L, Linear Express, Linear Technology, LT, LTC, LTM, BodeCAD, Burst Mode, FilterCAD, LTspice, OPTI-LOOP, Over-The-Top, PolyPhase, SwitcherCAD, µModule and the Linear logo are registered trademarks of Linear Technology Corporation. Adaptive Power, Bat-Track, C-Load, DirectSense, Easy Drive, FilterView, Hot Swap, LTBiCMOS, LTCMOS, LinearView, Micropower SwitcherCAD, Multimode Dimming, No Latency ∆Σ, No Latency Delta-Sigma, No RSENSE, Operational Filter, PanelProtect, PowerPath, PowerSOT, SafeSlot, SmartStart, SNEAK-A-BIT, SoftSpan, Stage Shedding, Super Burst, ThinSOT, TimerBlox, Triple Mode, True Color PWM, UltraFast and VLDO are trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. DESIGN FEATURES L Sensing Output Voltage Made Easier Unlike traditional controllers, which can only sense positive outputs, the LT3757 and LT3758 have a novel FBX pin architecture that simplifies the design of inverting and non-inverting VIN 4.5V TO 36V 4.7µF 50V X5R 215k Precision UVLO Voltage and Soft-Start Input supply UVLO for sequencing or start-up over-current protection is easily achieved by driving the UVLO with a resistor divider from the VIN supply. The divider output produces 1.25V at the UVLO pin when VIN is at the desired UVLO rising threshold voltage. The UVLO pin has an adjustable input hysteresis, which allows the IC to resist a settable input supply droop before disabling the converter. During a UVLO event, the IC is disabled and VIN quiescent current drops to 1µA or lower. L1 6.8µH VIN SHDN/UVLO 100k SYNC LT3757 GATE M1 Si7850 SENSE 8.45k 10nF 10µF 50V X5R 4.7µF 10V X5R C1: SANYO 50CE22BS L1, L2: VISHAY IHLP4040DZ-11 Figure 3. A 4.5V–36V to –5V at 3A inverting converter Linear Technology Magazine • June 2009 95 94 VIN = 12V VIN = 24V 93 200 300 400 500 600 700 800 900 1000 ILOAD (mA) Figure 2. Efficiency of the converter in Figure 1 The SS pin provides access to the soft-start feature, which reduces the peak input current and prevents output voltage overshoot during start-up or recovery from a fault condition. The SS pin reduces the inrush current by not only lowering the current limit but also reducing the switching frequency. In this way soft-start allows the output capacitor to charge gradually towards its final value. Adjustable/Synchronizable Switching Frequency The operating frequency of the LT3757 and LT3758 can be programmed from 100kHz to 1MHz range with a single resistor from the R T pin to ground, or synchronized to an external clock via the SYNC pin. The adjustable operating frequency allows it to be set outside certain frequency bands to fit applications that are sensitive to spectral noise. L2 6.8µH VOUT –5V 3A 88 86 84 105k 1% D1 PDS1045 42.2k 95 94 FBX GND INTVCC VC 0.1µF 96 C1 22µF 50V 0.01Ω RT SS 98 EFFICIENCY (%) In high voltage applications, the LT3757 and LT3758 eliminate the need for an external regulator or a slow-charge hysteretic start scheme through the integration of an onboard linear regulator, allowing simple start-up and biasing. This regulator generates INTVCC, the local supply that runs the IC from the converter input VIN. The internal LDO can operate the IC continuously, provided the input voltage and/or MOSFET gate charge currents are low enough to avoid excessive power dissipation in the part. When the INTVCC pin is driven externally above its regulated voltage during operation—from the input, the output or a third winding—the internal LDO is automatically turned off, reducing the power dissipation in the IC. The LDO also provides internal current limit function to protect IC from excessive on-chip power dissipation. The current limit decreases as VIN increases. If the current limit is exceeded, the INTVCC voltage falls and triggers the soft-start. converters. The LT3757 and LT3758 each contain two internal error amplifiers; one senses positive outputs and the other negative. When the converter starts switching and the output voltage starts ramping up or down, depending on the topologies, one of the error amplifiers seamlessly takes over the feedback control, while the other becomes inactive. The FBX pin can be connected directly to a divider from either a positive output or a negative output. This direct connection saves space and expense by eliminating the traditional glue circuitry normally required to level-shift the feedback signal above ground in negative converters. The power supply designer simply decides the output polarity he needs, the topology he wants to use and the LT3757 or LT3758 does the rest. EFFICIENCY (%) Internal High Voltage LDO + LT3757/58, continued from page 82 80 78 76 20k 1% 100µF 6.3V, X5R s2 VIN = 5V VIN = 12V VIN = 36V 74 72 0 500 1000 1500 2000 2500 3000 ILOAD (mA) Figure 4. Efficiency of the converter in Figure 3 L DESIGN FEATURES 2.2µF 100V 232k • VIN L1A WURTH 744 870 470 SHDN/UVLO 20k SYNC LT3758 GATE 90 85 L1B 0.02Ω 280k 1% + • FBX GND INTVCC VC 95 VOUT 24V 1A 4.7µF M1 FDMS2572 100V SENSE RT SS D1 PDS3100 EFFICIENCY (%) VIN 18V TO 72V COUT1 22µF 35V x2 100pF 30.9k 4.7nF VIN = 18V VIN = 24V VIN = 36V VIN = 48V VIN = 72V 60 COUT2 3.3µF 25V, X5R 20k 1% 4.7nF 75 70 42.2k 0.1µF 80 55 100 200 300 400 500 600 700 800 900 1000 LOAD CURRENT (mA) Figure 6. Efficiency of the converter in Figure 5 Figure 5. A 18V–72V input, 24V/1A output SEPIC converter In space constrained applications, higher switching frequencies can be used to reduce the overall solution size and the output ripple. If power loss is a concern, switching at a lower frequency reduces switching losses, improving efficiency. Current Mode Control The LT3757 and LT3758 use a current mode control architecture to enable a higher supply bandwidth, thus improving response to line and load transients. Current mode control also requires fewer compensation components than voltage mode control architectures, making it much easier to compensate over all operating conditions. A 10V–30V Input, 48V/1A Output Boost Converter Figure 1 shows a 48V, 1A output converter that takes an input of 10V to 30V. The LT3757 is configured as a boost converter for this applications where the converter output voltage is higher than the input voltage. Figure 2 shows the efficiency for this converter. A 4.5V–36V Input, –5V/3A Output Inverting Converter Figure 3 shows the LT3757 in an inverting converter that operates from a 4.5V to 36V input and delivers 3A to a –5V load. The negative output can be either higher or lower in amplitude than the input. It has output short circuit protection, which is further enhanced by the frequency foldback feature in the LT3757. The 300kHz operating frequency allows the use of small inductors. The ceramic capacitor used for the DC coupling capacitor provides low ESR and high RMS current capability. The output power can easily scaled by the choice of the components around the chip without modifying the basic design. Figure 4 shows the efficiency for this converter at different input voltages. An 18V–72V Input, 24V/1A Output SEPIC Converter A SEPIC converter is similar to the inverting converter in that it can step up or step down the input, but with a positive output. It also offers output disconnect and short-circuit protection. Figure 5 illustrates an 18V–72V input, 24/1A output SEPIC power supply using LT3758 as the controller. Figure 6 shows the efficiency for this converter at different input voltages. An 18V–72V Input, –3.3V/2A Output Flyback Converter Figure 7 shows the LT3758 in a nonisolated flyback converter with an 18V to 72V input voltage range and a –3.3V / 2A output. It provides robust output short-circuit protection thanks to the frequency foldback feature in the LT3758. The circuit can also be used for different negative voltages simply by changing the value of the resistor divider on the FBX pin. continued on page 21 T1 PA1277NL VIN 18V TO 72V CIN 1µF ×2 4.7nF 105k SHDN/UVLO 8.66k GATE RT SS VC GND UPS840 0.1µF 31.6k COUT 100µF ×2 BAS516 t M1 Si4848 SENSE FBX 36.5k 10k VOUT –3.3V 2A 51.1Ω 4.7µF LT3758 t BAV21W VIN INTVCC SYNC 10k D1 t 0.04Ω 10k 2.2nF Figure 7. 18V–72V input, –3.3V/2A output flyback converter Linear Technology Magazine • June 2009 DESIGN FEATURES L margin to switch from tracking the TRACK/SS input voltage to regulating to the internal reference. Figure 7 shows typical tracking waveforms of the application in Figure 6. VOUT and the reference supply voltage, VMASTER, are equal and track together during start-up until they reach 1.2V, at which point VOUT regulates to 1.2V while VMASTER continues ramping to 1.8V. Conclusion The LTC3878 and the LTC3879 support a VIN range from 4V to 38V (40V abs max). The regulated output voltage is programmable from 90% VIN down to 0.8V (for the LTC 3878) and 0.6V (for the LTC3879). The output regulation accuracy is ±1% over the full –40°C to 85°C temperature range. The operating frequency is resistor programmable and is compensated for variations in VIN. Current limit is continuously programmable and is measured without a sense resistor by using the voltage drop across the external synchronous bottom MOSFET. The valley current mode architecture is ideal for low duty factor operation and allows very low output voltages at reasonable current loop bandwidths. Compensation is easy to design and offers robust and stable operation even with low ESR ceramic output capacitors. The LTC3878 offers current limited start-up, while the LTC3879 has separate run and output voltage tracking pins. The LTC3878 is available in the GN16 package, and the LTC3879 is available in thermally enhanced MSE16 and QFN (3mm × 3mm) packages. Excellent performance and compact size make the LTC3878 and LTC3879 well suited to small, tightly constrained applications such as distributed power supplies, embedded computing and point of load applications. L to provide feedback from the isolated secondary to the LT3758. Figure 8 shows an 18V–72V input, 5V/2A output isolated flyback converter. pologies. Both offer a particularly wide input voltage range. These ICs produce space saving, cost efficient and high performance solutions in any of these topologies. The range of applications extends from single-cell, lithiumion powered systems to automotive, industrial and telecommunications power supplies. L LT3757/58, continued from page An 18V–72V Input, 5V/2A Output Isolated Flyback Converter The basic design shown in Figure 7 can be modified to provide DC isolation between the input and output with the addition of a reference, such as the LT4430, on the secondary side of the transformer and an optocoupler VIN 18V TO 72V GND Conclusion The LT3757 and LT3758 are versatile control ICs optimized for a wide variety of single-ended DC/DC converter toPA1277NL T1 +VOUT t CIN 1µF ×2 4.7nF 4.7µF 10k 51.1Ω BAV21W COUT 100µF ×2 t BAT54C 100pF 6.81k M1 Si4848DY 162k 1k 0.1µF VIN SS VIN INTVCC 0.04Ω 1µF RT SENSE 8.66k GND OPTO 10pF 6.81k 0.47µF SHDN/UVLO FBX VIN GND COMP GATE VC 330pF LT4430ES6 BAS516 LT3758 105k –VOUT UPS840 t BAS516 5V, 2A OC PS2801-1 SYNC 36.5k 2200pF FB 22.1k Figure 8. 18V–72V input, 5V/2A isolated flyback converter Linear Technology Magazine • June 2009 21