LINEAR TECHNOLOGY MARCH 2006 IN THIS ISSUE… COVER ARTICLE Take the Easy Road to Digitally Managed Power ...................1 Andy Gardner Issue Highlights ..................................2 Linear Technology in the News….........2 DESIGN FEATURES Cascadable, 7A Point-of-Load Monolithic Buck Converter ..................6 Peter Guan I2C Bus Buffer Resolves Stuck Buses, Eliminates Heavy Load Limitations and Provides Level Translation .........10 George Humphrey Negative Voltage Hot Swap™ Controller with 10-Bit ADC and I2C Monitoring ............................13 Zhizhong Hou High Efficiency Nickel Charger Operating to 34V is Easily Configured to Deliver a Safe Fast Charge...............................19 James A. McKenzie Tiny Controller Makes It Easy to Charge Large Capacitors ...............22 David Ng MOSFETs Make Sense for Tracking and Sequencing Power Supplies ........24 Thomas DiGiacomo High Voltage Boost/LED Controller Provides 3000:1 PWM Dimming Ratio ..........................31 Eugene Cheung 3A Monolithic Buck Regulator in 4mm × 4mm QFN ...........................34 Theo Phillips DESIGN IDEAS ....................................................37–45 (complete list on page 37) New Device Cameos ...........................46 Design Tools ......................................47 Sales Offices .....................................48 VOLUME XVI NUMBER 1 Take the Easy Road to Digitally Managed Power Introduction by Andy Gardner Digital management of high availability q A 7-channel ADC multiplexer power supplies holds great promise, with four external differential but it often comes at the cost of compliinputs, a 12V input, a 5V VDD incated multichip circuit solutions. For put, and an input for the on-chip example, an application with voltagetemperature sensor. current monitoring and supply voltage q Two continuous time, 8-bit, curmargining can require a number of rent output DACs with voltage ICs, including a low-drift reference, buffered outputs. The outputs of a multichannel, differential input the voltage buffers can be placed ADC with at least 12 bits of resoluin a low leakage, high impedance tion, an 8-bit DAC, and a dedicated state. microcontroller. Add to this the con- q A built-in, closed-loop servo siderable software algorithm that development efadjusts the The LTC2970 fort required for point-of-load simplifies the design of margining algovoltage of a digitally managed power rithms, voltage DC/DC converter and current monito the desired supplies by incorporating tor functions, and value. The range important features into one the cost, comand resolution of easy-to-use device. plexity, spacious the voltage servo board real-estate is user adjustrequirements and blossoming designable with two external resistors. debug time can deter even the most q Extensive, user configurable dedicated power supply designer from overvoltage and undervoltage trying digitally managed power. fault monitoring. The LTC2970 simplifies the design q An I2C and SMBus compliant of digitally managed power supplies 2-wire serial bus interface, two by incorporating important features GPIO pins, and an ALERT pin. into one easy-to-use device: a dual q An on-chip, 5V, linear regulator power supply monitor and controller. that allows the LTC2970 to operFigure 1 shows a block diagram of the ate from an external 8V to 15V LTC2970, highlighting the following voltage supply. features: q Another part in the family, the q A 14-bit, differential input, ΔΣ LTC2970-1, adds a tracking ADC with a maximum total unadalgorithm that allows two or more justed error (TUE) of ±0.5% over power supplies to be ramped up the industrial temperature range and down in a controlled manner. when using the on-chip reference. continued on page 3 L, LT, LTC, LTM, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered trademarks of Linear Technology Corporation. Adaptive Power, BodeCAD, C-Load, DirectSense, Easy Drive, FilterCAD, Hot Swap, LinearView, µModule, Micropower SwitcherCAD, Multimode Dimming, No Latency ΔΣ, No Latency Delta-Sigma, No RSENSE, Operational Filter, PanelProtect, PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT, True Color PWM, UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names may be trademarks of the companies that manufacture the products. DESIGN FEATURES L LTC2970, continued from page 1 linear search algorithm that compares the digitized point-of-load voltage against the target. The current being sourced by IDAC0 is then adjusted as needed one LSB per servo iteration. This current develops a point-of-load ground referenced correction voltage across resistor R40 which is buffered to the VOUT0 pin. The resulting voltage differential between the VOUT0 pin and the converter’s feedback node is multiplied by a factor of –R20/R30 and added to the nominal output voltage of the DC/DC converter, thus closing Margining and Monitoring Application Figure 2 shows a typical application circuit for monitoring and margining a DC/DC converter with external feedback resistors. The LTC2970’s VIN0_A differential inputs sense the voltage directly at the point-of-load while inputs VIN0_B monitor the voltage across sense resistor R50. The DC/DC converter’s output voltage can be margined to precise, user-programmable set points by a 5V REGULATOR VIN VOUT 12VIN 10 2R the servo loop. When voltage margining is disabled, the converter’s feedback node can be isolated from the LTC2970 by placing the VOUT0 pin in a high impedance state. Figure 3 shows the LTC2970 applied to a DC/DC converter with a TRIM pin. As in Figure 2, two external resistors are required: VOUT0 connects to the TRIM pin through resistor R30 and IOUT0 is terminated at the DC/DC converter’s point-of-load ground by R40. Following power-up, the VOUT0 pin defaults to a high impedance 0µA TO 255µA IDAC0 8 BITS R 14 IOUT0 VDD VDD 9 + 12VP CMP0 12VM VDDP GND 25 + VDDM TEMP SENSOR – VBUF0 VDD TSNSP 11 VOUT0 – UVLO POR TSNSM VIN0_AP 1 CH0_AP VIN0_AM 2 CH0_AM VIN0_BP 3 CH0_BP VIN0_BM 4 CH0_BM VIN1_AP 5 CH1_AP VIN1_AM 6 CH1_AM VIN1_BP 7 CH1_BP VIN1_BM 8 CH1_BM REF 23 0µA TO 255µA + – IDAC1 8 BITS + CMP1 ADC CLOCKS VDD 3.5k 7:1 MUX RAM ADC_Results MONITOR LIMITS SERVO TARGETS SCL 18 ASEL0 22 POR CLOCK GENERATION OSCILLATOR ASEL1 21 GPIO_0 16 SERVO CONTROLLER GPIO_1 15 DAC SOFT CONNECT FUNCTION SERVO FUNCTION MONITOR FUNCTION MANAGE FAULT REPORTING WATCH DOG TRACKING CONTROL (LT2970-1) ALERT 17 GPIO_CFG 20 7 12 VOUT1 VBUF1 – REFERENCE 1.229V (TYP) 20Ω I2C BUS INTERFACE (400kHz, SMBUS COMPATIBLE) – + 6.65X (TYP) RGND 24 SDA 19 13 IOUT1 14-BIT DELTA-SIGMA A/D 18 REGISTERS I/O CONFIGURATION IDAC0 IDAC1 ADC MONITOR FAULT ENABLE INSTANTANEOUS FAULTS LATCHED FAULTS 2 POR Figure 1. Block diagram of the LTC2970 Linear Technology Magazine • March 2006 3 L DESIGN FEATURES state allowing the DC/DC converter to power-up to its nominal output voltage. After power-up, the LTC2970’s soft-connect feature can be used to automatically find the IDAC code that most closely approximates the TRIM pin’s open-circuit voltage before enabling VOUT0. Applications that need to be sequenced can be configured to hold off the DC/DC converter when the LTC2970 powers-up by tying the GPIO_CFG pin high. This causes the GPIO_0 pin to automatically pull the DC/DC converter’s RUN pin low until the SMBus compatible I2C interface releases it. The absolute accuracy of the LTC2970 is demonstrated in Figure 4. The LTC2970 is configured to servo one of the outputs of a LTC3728 DC/ DC converter to 1V if the converter’s voltage deviates by more than ±0.1%. The LTC2970 is easily able to hold the output voltage to within ±1mV of 1V while both it and the DC/DC converter are heated from –50°C to 100°C. When the LTC2970 is isolated from the LTC3728, the output voltage drifts between 1.002V and 1.0055V over the same temperature range. 8V TO 15V VIN IN OUT VIN0_BM I– VIN0_BP R30 R20 RUN/SS 0.1µF GPIO_CFG I+ DC/DC CONVERTER FB R10 ALERT VOUT0 SCL VIN0_AP SDA I2C BUS + LOAD VDC0 – GPIO_0 IOUT0 R40 VIN0_AM GND SGND REF GND ASEL0 ASEL1 0.1µF Figure 2. Application circuit for DC/DC converter with external feedback resistors 8V TO 15V 0.1µF 12VIN VIN VDD VO+ 1/2 LTC2970 0.1µF GPIO_CFG R30 TRIM DC/DC CONVERTER VOUT0 ALERT VIN0_AP I2C BUS SCL SDA VSENSE+ + ON/OFF LOAD VDC0 – VSENSE– GPIO_0 IOUT0 R40 VIN0_AM VO– The LC2970’s features offer several benefits that differentiate it from competitive solutions: 4 VDD 1/2 LTC2970 Features REF GND ASEL0 ASEL1 0.1µF Figure 3. Application circuit for a DC/DC converter with a trim attenuates ripple components that have the potential to alias to DC. The ADC’s differential inputs can monitor supply voltages at the point of load and sense resistor voltages. The differential and common mode input ranges span –0.3V to 6V. With its 500µV/LSB resolution, the ADC can resolve voltages for a wide range of load current across sense resistor values of only a few milliohms. For 1.006 LOAD CURRENT 1.004 R1 L RL RL = INDUCTOR DCR R1 = R2 =10kΩ C1 = C2 = 0.1µF C1 UNCORRECTED VOUT (V) Delta-Sigma ADC The LTC2970’s ADC is a second-order delta-sigma modulator followed by a sinc2 digital filter that converts the modulator’s serial data into a 14-bit word at a conversion rate of 30Hz. The ADC’s TUE is less than ±0.5% when using the on-chip reference. One advantage delta-sigma ADCs offer over conventional ADCs is on-chip digital filtering. Combined with a large over-sampling ratio (OSR = 512), this feature makes the LTC2970 insensitive to the effects of noise when sampling power-supply voltages. The LTC2970’s sinc2 digital filter provides high rejection except at integer multiples of the modulator sampling frequency, fs = 30.72kHz. Adding a simple RC lowpass filter at the input of the ADC 0.1µF VIN R50 R2 1.002 1 0.998 −50 DRIFT AND OFFSET CORRECTED C2 – + −25 50 0 25 TEMPERATURE (°C) 75 14-BIT DELTA-SIGMA A/D 100 Figure 4. Corrected and uncorrected DC/DC converter output voltage vs temperature Figure 5. Network for sensing load current with inductor DCR Linear Technology Magazine • March 2006 DESIGN FEATURES L disturbance to the converter’s output voltage. There is no body diode from the VOUT0 pin to the LTC2970’s VDD supply, and the VOUT0 pin goes into a high impedance state when VDD drops below the LTC2970’s undervoltage lockout threshold. So no special precautions need to be taken in the event the DC/DC converter is still active when the LTC2970 powers down. DC/DC CONVERTER OUT R20 – + 11 R30 ADJUST VOUT0 – + R10 Voltage Servo The voltage servo feature can be configured to trigger on under voltage and/or over voltage events, run continuously, or run just once. The LTC2970 relies on a simple linear search algorithm to find the IDAC code that results in an ADC input voltage that most closely corresponds to the servo target. The polarity of the servo algorithm can be programmed as inverting (default) or noninverting. SOFT CONNECT COMPARATOR IDAC0 8 BITS 14 R40 GND IOUT0 POINT OF LOAD GROUND Figure 6. DAC connections to DC/DC converter with an external feedback resistors switching power supply applications without sense resistors, measure the load current via the DC resistance of the inductor using the application circuit shown in Figure 5. The ADC inputs are also isolated from the LTC2970’s internal supply. So the user can measure differential and common mode input voltages that are greater than VDD without turning on body diodes, and no special precautions need to be taken if the LTC2970 loses power while monitoring DC/DC converter voltages powered from a different supply. Voltage Buffered IDACs Figure 6 illustrates how each of the LTC2970’s continuous-time IDACs connects to a DC/DC converter with an external feedback network. The servo’d correction voltage is set by resistor R40. Since R40 is terminated at the point-of-load ground, the correction voltage is insensitive to load induced ground bounce. The correction voltage is buffered to the VOUT0 pin by a unity-gain amplifier whose output can be placed in a low-leakage (<100nA), high impedance state. Resistor R30 connects the VOUT0 pin to the feedback node of the DC/DC converter. The range and resolution over which the correction voltage can move the converter’s output is adjustable via resistor R30. A “soft-connect” feature allows the LTC2970 to automatically find the VOUT0 voltage that most closely approximates the DC/DC converter’s feedback node voltage before enabling the voltage buffer thus minimizing any Voltage Monitor The LTC2970 is able to perform ADC conversions on any combination of seven different input channels. Overvoltage and undervoltage threshold continued on page 46 8V TO 15V 0.1µF 12VIN VDD 10k Q10, Q11: 2N7002 D10, D11: MMBD4448V *SOME DETAILS OMITTED FOR CLARITY 0.1µF GPIO_CFG 100k GPIO_0 RUN/SS D10 LTC2970-1 Q10 VOUT0 R30A R30B ALERT I2C BUS FB R10 SCL IOUT0 SDA 0.1µF R31B R20 FB R11 IOUT1 VDC0 IN VIN DC/DC CONVERTER Q11 GND OUT RUN/SS D11 R31A VIN R40 GPIO_1 VOUT1 IN DC/DC CONVERTER OUT VDC1 R21 R41 Figure 7. The LTC2970-1 enables supply tracking Linear Technology Magazine • March 2006 5 L NEW DEVICE CAMEOS New Device Cameos Wide Dynamic Range RF/IF Log Detector The LT5537 is a wide dynamic range RF/IF detector, which operates from below 10MHz to 1000MHz. The lower limit of the operating frequency range can be extended to near DC by the use of an external capacitor. The input dynamic range at 200MHz with ±3dB nonlinearity is 90dB (from –76dBm to 14dBm, single-ended 50Ω input). The detector output voltage slope is nominally 20mV/dB, and the typical temperature coefficient is 0.01dB/°C at 200MHz. The LT5537 is available in a tiny 8-Lead (3mm × 2mm) DFN package. Dual Output Synchronous DC/DC Controller Draws Only 80µA Quiescent Current in an Automotive System The LTC3827 is a low quiescent current, 2-phase dual output synchronous step-down DC/DC controller. The LTC3827 draws only 80µA when one output is active and only 115µA LTC2970, continued from page 5 registers allow the user to define instantaneous and/or latched faults in the event one of the input voltages deviates outside an acceptable window. The GPIO_0 and FAULT pins can be configured to assert if a fault occurs. Tracking Two or More Supplies with the LTC2970-1 The LTC2970-1 enables power supply tracking with the addition of a few external components. A special global address and synchronization command allow multiple LTC2970-1s to track and sequence multiple pairs of power supplies. A typical LTC2970-1 tracking application circuit is shown in Figure 7. The GPIO_0 and GPIO_1 pins are tied directly to their respective DC/ DC converter RUN/SS pins. When GPIO_CFG is pulled-up to VDD, the LTC2970-1 automatically holds off 46 when both outputs are active, making it ideally suited for automotive applications, such as navigation systems, where one or more supplies remains active while the engine is off. The LTC3827’s input supply range of 4V to 36V is wide enough both to protect against high input voltage transients and to continue to operate during automotive cold crank. The LTC3827 features a ±1% internal reference and can provide output voltages from 0.8V up to 10V, making it perfect for the higher voltage supplies typically required for audio systems, analog tuners, and CD/DVD players in many automobiles. Each output can deliver up to 20A of current at efficiencies as high as 95%. The LTC3827 is rated for operation from –40°C to 85°C, and has a maximum operating junction temperature of 125°C. The LTC3827’s constant frequency, current mode architecture provides excellent line and load regulation, and its 2-phase operation reduces input capacitance requirements. The LTC3827 smoothly ramps each output voltage during startup using separate adjustable soft-start and tracking input pins. It operates at a selectable frequency between 250kHz and 550kHz, and can be synchronized to an external clock from 140kHz to 650kHz using its phase-locked loop (PLL). Output overvoltage and overcurrent (short circuit) protection are provided internally. With both outputs shut down, the LTC3827 draws a mere 8µA. The LTC3827 is offered in two packages: a 28-lead SSOP (LTC3827-1) and a 32-pin 5mm × 5mm QFN (LTC3827). the DC/DC converters after power-up. N-channel FETs Q10 and Q11 and diodes D10 and D11 form unidirectional range switches around R30A and R31A while GPIO_CFG is high, which allow the VOUT0 and VOUT1 pins to drive the converter outputs all the way to/from ground through resistors R30B and R31B. When GPIO_CFG pulls low, FETs Q10 and Q11 turn off. R30A and R31A then combine in series with R30B and R31B for normal margin operation. The 100kΩ/0.1µF lowpass filter in series with the gates of Q10 and Q11 minimizes charge injection into the feedback nodes of the DC/DC converters when GPIO_CFG pulls low. power applications into one easy-touse device. A multiplexed, differential input 14-bit delta-sigma and a low drift on-chip reference deliver less than ±0.5% total unadjusted error. Two continuous-time, 8-bit, voltage-buffered IDACs can also be programmed through the I2C and SMBus compatible interface to servo power-supplies to the desired voltages. Extensive, user configurable fault monitoring and a built-in servo algorithm reduce the burden on system computing resources and shorten software development time. The LTC2970 and LTC2970-1 are available in a 24-lead QFN package. Conclusion The LTC2970 dual power supply monitor and controller combines the necessary features essential for digitally managed, high availability For further information on any of the devices mentioned in this issue of Linear Technology, use the reader service card or call the LTC literature service number: 1-800-4-LINEAR Ask for the pertinent data sheets and Application Notes. L for the latest information on LTC products, visit www.linear.com Linear Technology Magazine • March 2006