LINEAR TECHNOLOGY NOVEMBER 2000 IN THIS ISSUE… COVER ARTICLE 3MHz Synchronous Boost Regulators Save Critical Board Space in Portable Applications ................ 1 Mark Jordan Issue Highlights ............................ 2 LTC® in the News ........................... 2 DESIGN FEATURES SOT-23 10MHz Rail-to-Rail Op Amp Saves Board Space and Power ...... 5 Glen Brisebois Low Distortion Rail-to-Rail Amplifiers Drive ADCs and Cables .................. 9 William Jett, Danh Tran and Glen Brisebois Phase-Shift Full-Bridge Controller Enables Efficient, Isolated Power Conversion for High Power Applications ............. 11 John Bazinet Zero-Drift Operational Amplifier Family in Small-Footprint Packages Features 3µ V Maximum DC Offset and 30nV/°C Maximum Drift ............... 15 David Hutchinson Low Dropout Linear Li-Ion Charge Controllers Prevent Overcharging, Save Board Space ....................... 18 James Herr DESIGN SOFTWARE SwitcherCAD™ III Provides Fast Spice Simulation of Switching Regulators and Built-In Schematic Capture ................................................... 21 Keith Szolusha and Robert Sheehan DESIGN INFORMATION A New, Fully Differential No Latency Delta-Sigma™ ADC Family .......... 25 Michael K. Mayes Dual 60µ A 10-Bit Serial DAC in MS-8 Saves Power and Space ............... 26 Vic Schrader DESIGN IDEAS .............................................. 28–36 (complete listing on page 28) New Device Cameos ..................... 37 Design Tools ................................ 39 Sales Offices ............................... 40 VOLUME X NUMBER 4 3MHz Synchronous Boost Regulators Save Critical Board Space in Portable Applications by Mark Jordan Introduction The proliferation of portable devices with ever increasing functionality has imposed a higher demand on power conversion circuitry, with a continued emphasis on maximizing battery life while reducing board real estate. Linear Technology’s new LTC3401 and LTC3402 synchronous boost converters operate at high frequency, facilitating the use of a small low cost inductor and tiny ceramic capacitors. Both the LTC3401 and LTC3402 come in a thermally enhanced MSOP-10 package, with the lead frame of the IC connected to ground (pin 5). With the converter housed in a small MSOP-10 package, the area of a complete 300mW converter is less than 0.08in2, with a low 1.2mm profile. For a 2W converter, the board area is less than 0.18in2. Efficiencies of up to 97% are achieved through internal features such as lossless current sensing, low gate charge, low RDS(ON) synchronous power switches and fast switching transitions to minimize power loss. An external Schottky diode is not required, but may be used to maximize efficiency. The LTC3401 is optimized for applications requiring less than 1 amp of input current, whereas the LTC3402 is optimized for applications requiring up to 2 amps of input current. The operating frequency is programmable from 100kHz to 3MHz, which allows these products to fit nicely in various applications where size and efficiency considerations can be traded off. The ICs start up with an input voltage below 1V and, once started, operate with an input below 0.5V. Proper operation below 0.5V protects against worst-case voltage droops in the battery during high current load transients. The output voltage is adjustable from 2.6V to 5.5V with a simple resistor voltage divider. The current mode control architecture, along with OPTI-LOOP TM compensation and adaptive slope compensation, allows the transient response to be optimized over a wide range of loads, input voltages, and output capacitors. At light loads, the user can choose to enter high efficiency Burst Mode™ operation. The IC consumes only 38µA of quiescent current in this mode. The part can also be commanded to shut down, drawing less than 1µA of quiescent continued on page 3 current. Figure 1. LTC3401, 3MHz single cell to 3V evaluation circuit , LTC and LT are registered trademarks of Linear Technology Corporation. Adaptive Power, Burst Mode, C-Load, DirectSense, FilterCAD, Hot Swap, LinearView, Micropower SwitcherCAD, No Latency ∆Σ, No RSENSE, Operational Filter, OPTI-LOOP, Over-The-Top, PolyPhase, PowerSOT, SwitcherCAD and UltraFast are trademarks of Linear Technology Corporation. Other product names may be trademarks of the companies that manufacture the products. DESIGN FEATURES LTC3401/02, continued from page 1 90 R4 5.1M 80 C1 3.3µF 1 CELL L1 1µH 3 + 10 2 0 = FIXED FREQ 1 = BURST MODE LTC3401 VIN SHDN SW VOUT MODE/SYNC 6 1 FB PGOOD RT VC GND 4 7 R2 866k 8 RT 10k VOUT 3V/100mA C2 4.7µF 9 5 Burst Mode OPERATION 70 R3 1M EFFICIENCY (%) VIN = 0.9V TO 1.5V D1 R5 39k 50 40 30 20 10 R1 619k C3 470pF 3MHz FIXED FREQUENCY 60 0 0.1 VIN = 1.2V 1 10 100 OUTPUT CURRENT (mA) C4 20pF 1000 Figure 3. Efficiency of the circuit in Figure 2 D1: CENTRAL SEMICONDUCTOR CMDSH-3 (631) 435-1110 L1: TAIYO YUDEN LB2016 (408) 573-4150 C1: TAIYO YUDEN JMK212BJ33MG C2: TAIYO YUDEN JMK212BJ47MG Figure 2. 1.2mm high, ultracompact single cell to 3V converter 1V to 3V, 300mW Converter in less than 0.08 in2 In applications where the physical size is the most critical design factor, the high switching frequency of the LTC3401 allows the use of small ceramic capacitors and a tiny chip inductor, as shown in the evaluation circuit photo in Figure 1. The circuit schematic is shown in Figure 2. This compact, 1.2mm high converter switches at a fixed frequency of 3MHz and can step up a single-cell alkaline battery to 3V with an output load up to 100mA. The efficiency peaks at 83% at 100mA output current, as shown in Figure 3, with the efficiency loss being primarily due to the series resistance of the chip inductor and the ICs switching losses. Using an inductor with lower series resistance, reducing the operating frequency and increasing the size of the filter capacitor result in efficiencies over 90% for this application, although the improved efficiency comes at the expense of added board area. The Burst Mode efficiency of the converter of Figure 2 is 70% at 500µA load, making it ideal for applications such as pagers, which power down for extended periods of time. The switching waveform of the SW pin at 3MHz is shown in Figure 4. The fast rise and fall times of less than 5ns along with short break-beforemake times between the synchronous switches of 20ns contribute to the high efficiency of the converter. High Efficiency 1.6W, 2 Cell to 3.3V Converter Many 2-cell applications require higher output power, but efficiency considerations are as important as 3 + 10 2 CELLS 2 0 = FIXED FREQ 1 = BURST MODE Linear Technology Magazine • November 2000 D1 6 1 LTC3401 VIN SHDN L1: C1: C2: D1: SW VOUT MODE/SYNC FB PGOOD VC RT RT 30k 50ns/DIV The LTC3402 is ideal for applications requiring higher power, such as a 4W Li-Ion to 5V converter shown in Figure 7. To minimize conduction losses at these higher currents, it is imperative to choose low ESR power components. Inductor saturation at high current is also a factor in the selection process. The efficiency of the circuit in Figure 7, with the Li-Ion battery at the nominal 3.6V, peaks at 94%, as shown in Figure 8. R3 1M C1 4.7µF Figure 4. 3MHz switching waveform on the SW pin The LTC3402 for Higher Power Applications L1 4.7µH VIN = 1.8V TO 3V VSW 1V/DIV board area. The circuit of Figure 5 operates at 1MHz and uses a 0.16in diameter Sumida power inductor along with all ceramic capacitors. The efficiency is 95% at 300mW output power, as shown in Figure 6. Removing the Schottky diode will reduce board area by approximately 5%, but at the cost of 4% less efficiency. SUMIDA CD43-4R7M TAIYO YUDEN JMK212BJ475MG TAIYO YUDEN JMK325BJ226MM ON SEMICONDUCTOR MBRM120T3 GND 4 R2 909k 7 8 VOUT 3.3V/500mA C2 22µF 9 5 R1 549k C3 470pF R5 82k C4 4.7pF (847) 956-0667 (408) 573-4150 (602) 244-6600 Figure 5. All-ceramic-capacitor 2-cell converter delivers 3.3V at 500mA 3 DESIGN FEATURES 100 EFFICIENCY (%) 80 70 60 R3 1M C1 10µF 1MHz FIXED FREQUENCY Li-Ion 50 20 1 7 VOUT MODE/SYNC FB PGOOD VC RT VIN = 2.4V WITH SCHOTTKY 8 5 GND Figure 6. Efficiency of the circuit in Figure 5 High Efficiency Li-Ion CCFL Backlight Application Small portable applications with a CCFL backlight, such as a PDA, require a highly efficient backlight converter solution to maximize operating time before recharging. A high efficiency Li-Ion CCFL supply is shown in Figure 9. The LTC3401 provides the tail current to the self-oscillating resonant Royer circuit, which generates the high voltage sinusoidal wave to the lamp. The lamp dimming is provided by means of a control voltage, but alternate dimming techniques can be used. (847) 639-6400 (408) 573-4150 (207) 282-5111 (602) 244-6600 enter Burst Mode operation. When the MODE/SYNC pin is driven high, full-time power saving Burst Mode operation is enabled. In Burst Mode operation, the converter delivers energy to the output until the regulation voltage is reached. At that point the IC ceases to switch and goes to “sleep” until the output voltage has drooped to typically 1% of the regulated value. The IC then wakes up and delivers energy again and the cycle repeats itself. The efficiency at light loads is improved in Burst Mode operation due to the dramatic reduction in switching and quiescent current losses. The MODE/SYNC pin serves an additional function of oscillator synchronization. The internal oscillator can be synchronized to an external clock at a higher frequency than the free-running frequency, with continued on page 8 C3 27pF, 1kV 6 1 VIN = 2.5V TO 4.2V EFFICIENCY (%) 10 2 R1 330Ω 4 3 CCFL Q2 Q1 C2 0.22µF L1 33µH D1 D4 Li-Ion R5 1M C1 10µF Burst Mode OPERATION 90 3 10 2 80 6 70 1 1MHz FIXED FREQUENCY 50 5 T1 Today’s portable electronics environment requires power conversion that is adaptable to varying conditions. The LTC3401 and LTC3402 allow the user to modify output voltage, operating frequency, Burst Mode operation and loop compensation with simple modifications to external components. The IC remains in fixed frequency mode until the user allows the IC to 60 C4 4.7pF Figure 7. Single Li-Ion cell to 5V application at 800mA Flexible Boost Converters 100 C2 150µF R1 549k C3 470pF R5 82k COILCRAFT DO3316-103 TAIYO YUDEN JMK212BJ106MM AVX TPSD157M63R ON SEMICONDUCTOR MBR0520L + 9 1000 L1: C1: C2: D1: VOUT 5V/0.8A R2 1.65M RT 30k 10 100 1 10 OUTPUT CURRENT (mA) 6 4 SW SHDN 2 0 = FIXED FREQ 1 = BURST MODE 0.1 VIN 10 30 0 LTC3402 3 + 40 D1 L1 10µH VIN = 2.5V TO 4.2V Burst Mode 90 OPERATION LTC3401 VIN SW VOUT SHDN MODE/SYNC FB PGOOD VC RT GND DIMMING INPUT 0V TO 2.5V 4 7 D2 D3 8 R2 10k 9 5 C5 1µF RT 150k 40 R4 20k R3 1k C4 0.1µF 30 20 10 0 VIN = 3.6V 0.1 1 100 10 LOAD CURRENT (mA) 1000 Figure 8. Efficiency of the circuit in Figure 7 4 T1: L1: Q1, Q2: C1: C2: D1: D2–D4: SUMIDA C1Q122 SUMIDA CD-54-330MC ZETEX FMMT-617 TAIYO YUDEN JMK212BJ106MM PANASONIC ECH-U ZETEX ZHCS-1000 1N4148 (847) 956-0667 (631) 543-7100 (408) 573-4150 (201) 348-7522 Figure 9. High efficiency, compact CCFL supply with remote dimming Linear Technology Magazine • November 2000 DESIGN FEATURES high, in which case the recovery time will rise and the output pulse width will increase. The higher capacitance BPV22NF shows this effect more than does the SFH213. This circuit is not suited to pulse width modulation schemes unless physical transmitter motion will be below the frequency of interest and the steady-state pulse width is noncritical. Convert Your Favorite Op Amp to a Rail-to-Rail Output Many of the world’s greatest op amps were not originally intended for operation on reduced supply voltages, the ultralow noise LT1028 being a good example. The LT1797 can help remedy this situation by converting the output stage of one of these amplifiers to a rail-to-rail output stage. Figure 8 shows the method. The LT1028 output drives the noninverting input of the LT1797, which is placed in a gain of three by R1 and R2. The feedback resistors R3 and R4 put the entire loop in a gain of 500, forcing the LT1028 to provide a gain of 167. This combination of the two amplifiers takes advantage of the ultralow noise, precision front end of the LT1028 and the rail-to-rail output of the LT1797. The circuit is stable from a gain-phase point of view without compensation components R5 and C1. However, when the input 5V 5V + IN + LT1028 – –5V R5 1k R2 4.99k LT1797 – –5V C1 2200pF R4 10Ω OUT R1 10k R3 4.99k Figure 8. Converting the LT1028 to a ±5V supply with rail-to-rail output; AV = 500 receives a transient or the output hits a rail, the two op amps begin a usually unrecoverable slew-rate contest. R5 and C1 fix this by slowing down the LT1028. Conclusion The LT1797 is a compelling choice where minimal footprint or rail-torail 10MHz gain bandwidth are essential. The efficient nature of the LT1797 design also makes it suitable for applications where power is at a premium and wide bandwidth and output drive are also required. Notes: 1 To cut to the chase, results will be given with sixteen feet of transmitter-receiver separation. 2 Some of the photodiodes tested had more capacitance than this, and some had less. Although it is tempting to place a trimpot at R1, the parasitic capacitance of a bulky trimpot would quickly complicate the matter. 3 Cascading the two 100kHz –3dB bandwidths results in a net bandwidth of 65kHz. However, the –3dB that is due to the photodiode capacitance and R1 will be more or less dependent on the photodiode used, and this will have an effect on the net bandwidth. 4 The bandwidth is chosen at about 80kHz because the low capacitance photodiode will not reduce the 100kHz bandwidth as much as would the design value of 16pF. For additional complexity, the bandwidth reduction due to input capacitance has effect on current noise and Johnson noise but not voltage noise. Also, the fact that measurements are made over a finite period of time introduces an inherent highpass characteristic. The skirt factor is next to impossible to determine because of the complexity of the various roll-off mechanisms. The value of 1.3 is a compromise. 5 Taking 100 measurements using a 50µs window, the average peak-to-peak noise was 7.7mVP-P with a standard deviation of 1.2mVP-P. Note that a 50µs window has a highpass effect above about 15kHz. For more information on parts featured in this issue, see http://www.linear-tech.com/go/ltmag Conclusion LTC3401/02, continued from page 4 a pulse width of less than 2µs. The state of the Mode condition remains unchanged because of internal filtering. For applications requiring a flag to indicate the condition of the output voltage, the PGOOD pin provides an open drain output, which pulls low when the output voltage is more than 9% below the regulation voltage. 8 With Linear Technology’s family of high performance synchronous boost converters, the designer of handheld electronics can easily extend operation time while saving critical board real estate. The high frequency operation of the LTC3401 and LTC3402 allows the use of all ceramic capacitors and a small inductor. The low voltage start-up makes these products ideal for single-cell alkaline portable applications, and the ability to program the operating frequency, output voltage, loop compensation and Burst Mode operation allows the designer to make the necessary decisions to optimize the power conversion for the given portable application. Low RON (0.16Ω NMOS, 0.18Ω PMOS) synchronous switches optimize efficiencies for all applications. All of this functionality is packed into a small MSOP-10 package. Linear Technology Magazine • November 2000