LINEAR TECHNOLOGY FEBRUARY 1998 IN THIS ISSUE… COVER ARTICLE Universal Continuous-Time Filter Challenges Discrete Designs .......... 1 Max Hauser Issue Highlights ............................ 2 LTC® in the News ........................... 2 DESIGN FEATURES An SMBus-Controlled 10-Bit, Current Output, 50µ A, Full-Scale DAC ........ 6 Ricky Chow Micropower 600kHz Fixed-Frequency DC/DC Converters Step Up from a 1-Cell or 2-Cell Battery .................. 8 Steve Pietkiewicz New 333ksps, 16-Bit ADC Offers 90dB SINAD and –100dB THD .............. 11 Marco Pan Ultralow Power 14-Bit ADC Samples at 200ksps .................................. 14 Dave Thomas A 10MB/s Multiple-Protocol Chip Set Supports Net1 and Net2 Standards ................................................... 17 David Soo DESIGN IDEAS High Clock-to-Center Frequency Ratio LTC1068-200 Extends Capabilities of Switched Capacitor Highpass Filter ................................................... 23 Frank Cox LT1533 Ultralow Noise Switching Regulator for High Voltage or High Current Applications .......... 24 Jim Williams A Complete Battery Backup Solution Using a Rechargeable NiCd Cell .. 26 L.Y. Lin and S.H. Lim Zero-Bias Detector Yields High Sensitivity with Nanopower Consumption ............................... 28 Mitchell Lee DESIGN INFORMATION Micropower Octal 10-Bit DAC Conserves Board Space with SO-8 Footprint ..................................... 29 Kevin R. Hoskins Tiny MSOP Dual Switch Driver is SMBus Controlled ........................ 31 Peter Guan New Device Cameos ..................... 34 Design Tools ................................ 35 Sales Offices ............................... 36 VOLUME VIII NUMBER 1 Universal Continuous-Time Filter Challenges Discrete Designs by Max Hauser The LTC1562 is the first in a new family of tunable, DC-accurate, continuous-time filter products featuring very low noise and distortion. It contains four independent 2nd order, 3-terminal filter blocks that are resistor programmable for lowpass or bandpass filtering functions up to 150kHz, and has a complete PC board footprint smaller than a dime. Moreover, the part can deliver arbitrary continuous-time pole-zero responses, including highpass, notch and elliptic, if one or more programming resistors are replaced with capacitors. The center frequency (f0) of the LTC1562 is internally trimmed, with an absolute accuracy of 0.5%, and can be adjusted independently in each 2nd order section from 10kHz to 150kHz by an external resistor. Other features include: ❏ Rail-to-rail inputs and outputs ❏ Wideband signal-to-noise ratio (SNR) of 103dB ❏ Total harmonic distortion (THD) of –96dB at 20kHz, –80dB at 100kHz ❏ Built-in multiple-input summing and gain features; capable of 118dB dynamic range ❏ Single- or dual-supply operation, 4.75V to 10.5V total ❏ “Zero-power” shutdown mode under logic control ❏ No clocks, PLLs, DSP or tuning cycles required The LTC1562, in the SSOP package, provides eight poles of programmable continuous-time filtering in a total surface mount board area (including the programming resistors) of 0.24 square inches (155 mm2 )—smaller than a U.S. 10-cent coin. This filter can also replace op amp–R-C active filter circuits and LC filters in applications requiring compactness, flexibility, high dynamic range or fewer precision components. What’s Inside? As shown in Figure 1, the LTC1562 includes four identical 3-terminal blocks. Each contains active circuitry, precision capacitors and precision resistors, forming a flexible 2nd order filter core. These blocks are designed to make filters as easy to configure as op amps. The 3-terminal arrangement minimizes the number of external parts necessary for a complete 2nd order filter with arbitrarily programmable f0, Q and gain. Figure 2 shows the contents of one block, along with three external resistors, forming a complete lowpass/bandpass filter (the most basic application of the LTC1562). In Figure 2, a lowpass response appears between the VIN source and the LP output pin, and simultaneously a bandpass response is available at the BP output pin. Both outputs have rail-to-rail capability for the maximum possible signal swing, and hence, maximum signalto-noise ratio (SNR). continued on page 3 , LTC and LT are registered trademarks of Linear Technology Corporation. Adaptive Power, Burst Mode, C-Load, FilterCAD, Linear View, Micropower SwitcherCAD, Operational Filter and SwitcherCAD are trademarks of Linear Technology Corporation. Other product names may be trademarks of the companies that manufacture the products. DESIGN FEATURES INV V+ BP LP INV BP LP *R1 AND C ARE PRECISION INTERNAL COMPONENTS 1 sR1C* V+ SHUTDOWN SWITCH V– A C B – 2ND ORDER SECTIONS SHUTDOWN SWITCH SHDN AGND D + C LP V– INV INV BP LP INV BP BP RQ R2 LP 1562 F01 RIN 1562 F02 + – Figure 1. LTC1562 block diagram VIN LTC1562, continued from page 1 The LTC1562 is versatile; it is not limited to the lowpass/bandpass filter of Figure 2. Cascading multiple sections, of course, yields higherorder filters (Figure 3a). A highpass response results if the external input resistor (RIN of Figure 2) is replaced by a capacitor, CIN, which sets only gain, not critical frequencies (Figure 3b). Responses with arbitrary zeroes (for example, elliptic or notch responses) are implemented with feedforward connections with multiple 2nd order blocks, as shown in the application circuit in Figure 8. Moreover, the virtual-ground INV input gives each 2nd-order section the built-in capability for analog operations such as gain (preamplification), summing and weighting of multiple inputs, or accepting current or charge signals directly. These flexible 3-terminal elements are Operational Filter™ blocks. Although the LTC1562 is offered in a 20-pin SSOP package, the LTC1562 is a 16-pin circuit; the extra pins are connected to the die substrate and should be returned to the negative power supply. In single-supply appli- Figure 2. Single 2nd order section, illustrating connection with external resistors R2, RIN and RQ cations, these extra V– pins should be connected directly to a PC board’s ground plane for the best grounding and shielding of the filter. 16-pin plastic DIP packaging is also available (consult the factory). DC Performance and Power Shutdown The LTC1562 operates from single or dual supply voltages, nominally 5V to 10V total. It generates an internal half-supply reference point (the AGND pin), establishing a reference voltage for the inputs and outputs in single-supply applications. In these applications, the AGND pin should be bypassed with a capacitor to the ground plane (at V–); the pin can be connected directly to ground when a split supply is used. The DC offset voltage from the filter input to the LP output for a typical 2nd order section (unity DC gain) is typically 5mV. Both outputs swing to within approximately 100mV of each supply rail with loads of 5kΩ and 30pF. VIN To save power in a “sleep” situation, a logic high input on the SHDN pin will put the LTC1562 into its shutdown mode, in which the chip’s power supply current is reduced to only junction leakage (typically 2µA from a single 5V supply). The shutdown pin is designed to accept CMOS levels with 5V swing, for example, 0V and 5V logic levels when the LTC1562 is powered from either a single 5V or a split ±5V supply. Note that in the LTC1562, unlike some other products, a small bias current source (approximately 2µA) at the SHDN pin causes the chip to default to the shutdown state if this pin is left open. Therefore, the user must remember to connect the SHDN pin to a logic low for normal operation if the shutdown feature is not used. (This default-toshutdown convention saves system power in the shutdown state, since it eliminates even the microampere current that would otherwise flow from the driving logic to the bias-current source.) VIN CIN VOUT INV BP LP 2ND ORDER INV BP LP 2ND ORDER Figure 3a. Two 2nd order sections cascaded for higher order response Linear Technology Magazine • February 1998 VOUT INV BP LP 2ND ORDER Figure 3b. 2nd order section configured for highpass output 3 DESIGN FEATURES 1 RQ1, 5.62k R21, 10k 2 3 4 5V 0.1µF 5 R23, 10k RIN3 10k VIN1 6 7 RQ3, 5.62k 8 INV B INV C BP B BP C LP B LP C V– V + LTC1562 SHDN AGND LP A LP D BP A BP D INV D INV A 0 16 –10 15 RQ2, 13k –20 14 R22, 10k GAIN (dB) VIN2 10 RIN2, 10k RIN1 10k VOUT2 –5V 13 0.1µF 12 –40 –50 VOUT1 11 –30 10 R24, 10k –60 RQ4, 13k –70 9 –80 10k RIN4, 10k 1562 TA01 100k FREQUENCY (Hz) 1M 1562 TA02 Figure 4. Dual, matched 4th order 100kHz Butterworth lowpass filter Frequency Responses Lowpass filters with standard all-pole responses (Butterworth, Chebyshev, Bessel, Gaussian and so on) of up to 8th order (eight poles) can be realized with LTC1562 sections connected as in Figures 2 and 3a; practical examples appear later in this article. Resistor ratios program the standard filter parameters f0, Q and gain; required values of these filter parameters can be found from tables or from software such as FilterCAD™ for Windows®, available free from LTC. The “LP” and “BP” outputs of each 2nd order section, although named after their functions in Figure 2, can display other responses than lowpass and bandpass, respectively, if the external components are not all resistors. The highpass configuration of Figure 3b has a passband gain set by the ratio CIN/C, where C is an internal 160pF capacitor in the LTC1562. The two resistors in Figure 3b control f0 and Q, as in the other modes. The LTC1562 is the first truly compact universal active filter, yet it offers instrumentation-grade performance rivaling much larger discrete-component designs. Bandpass applications can use the LTC1562 in either of two ways. In the basic configuration, with the only external components being resistors (Figure 2), the BP output has a bandpass response from VIN. With an input capacitor, as in Figure 3b, the BP output has a highpass response as noted above and the LP pin shows a bandpass response. The f 0 range is approximately 10kHz–150kHz, limited mainly by the magnitudes of the external resistors required. At high f0 these resistors fall below 5k, heavily loading the outputs of the LTC1562 and leading to increased THD and other effects. A lower Figure 5. Frequency response of Figure 4’s circuit f0 limit of 10kHz reflects an arbitrary resistor magnitude limit of 1 Megohm. The LTC1562’s MOS input circuitry can accommodate higher resistor values than this, but junction leakage current from the input-protection circuitry may cause DC errors. Design formulas and further details on frequency-response programming appear in the LTC1562 data sheet. Low Noise and Distortion The active (that is, amplifier) circuitry in the LTC1562 was designed expressly for filtering. Because of this, filter noise is due primarily to the circuit resistors rather than to the amplifiers. The amplifiers also exhibit exceptional linearity, even at high frequencies (patents pending). The noise and distortion performance for filters built with the LTC1562 compares favorably with filters using expensive, high performance, off-theshelf op amps that demand many more external parts and far more board area (we know, because we’ve 10 TO CIN3 CIN1 150pF RQ1, 10.2k 2 R21, 35.7k 3 4 5V 0.1µF 5 6 CIN3 150pF FROM HP C R23, 107k RQ3, 54.9k 7 8 INV B INV C BP B BP C LP C LP B V + LTC1562 –10 15 RQ2, 22.1k 14 R22, 66.5k 13 V– SHDN AGND LP A LP D BP A BP D INV A INV D 12 CIN2 150pF –5V 0.1µF –20 –30 –40 –50 –60 11 10 R24, 127k 9 RQ4, 98.9k CIN4 150pF –70 –80 –90 VOUT 1562 TA08 Figure 6. 8th order Chebyshev highpass filter with 0.05dB ripple (fCUTOFF = 30kHz) 4 0 16 GAIN (dB) 1 CIN 1k 10k 100k FREQUENCY (Hz) 1M 1562 TA09 Figure 7. Frequency response of Figure 6’s circuit Linear Technology Magazine • February 1998 DESIGN FEATURES 10 RFF1, 10k CIN1, 87pF RIN1, 19.6k RQ1, 13k 2 R21, 8.87k 3 4 5V 0.1µF 5 6 R23, 8.87k RIN3, 69.8k 7 RQ3, 28k 8 –10 INV C INV B BP C BP B LP C LP B SHDN AGND LP A LP D BP D INV D INV A CIN3, 47pF –20 15 RQ2, 8.87k 14 R22, 12.1k 13 V– V + LTC1562 BP A 16 12 GAIN (dB) 1 VIN 0 RIN2, 8.06k –5V –30 –40 –50 –60 0.1µF –70 11 10 R24, 17.8k 9 RQ4, 6.98k –80 –90 RIN4, 7.32k 1k 10k 100k FREQUENCY (Hz) 1M 1562 TA04 Figure 9. Frequency response of Figure 8’s circuit. VOUT RFF2, 17.8k 1562 TA03 ALL RESISTORS = 1% METAL FILM Figure 8. 8th order 100kHz elliptic lowpass filter built them). The details of this performance depend on Q and other parameters and are reported for specific application examples below. As with other low distortion circuits, accurately measuring distortion performance requires both an input signal and distortion-analyzing equipment with adequately low distortion floors. Low level signals can exploit a low noise preamplification feature in the LTC1562. A 2nd order section operated with unity gain, Q = 1 and f0 = 100kHz shows a typical output noise of 24µVRMS, which gives a 103dB SNR with full-scale output from a 10V total supply. However, reducing the value of RIN in Figure 2 increases the gain without a proportional increase in the output noise (unlike many active filters). A gain of 100 (40dB) with the same Q and f0 gives a measured output RIN1A 6.19k CIN1 680pF RQ1, 10k 2 R21, 10k 3 4 0.1µF 5 R23, 10k RIN3B 3.83k VIN3 The practical circuit in Figure 4 is a dual lowpass filter with a Butterworth (maximally-flat-passband) frequency response. Each half gives a DC-accurate, unity-passband-gain lowpass response with rail-to-rail input and output. With a 10V total power supply, the measured output noise for one filter is 36µVRMS in a 200kHz bandwidth, and the largeVOUT2 1 5V RIN3A 6.19k Dual 4th Order 100kHz Butterworth Lowpass Filter VOUT1 RIN1B 3.83k VIN1 noise of 449µVRMS or an input-referred noise of 4.5µVRMS—a 78dB output SNR with an input that is 40dB down. Thus, the same circuit can handle a wide range of input levels with high SNR by changing (or switching) the input resistor. In the example just cited, the ratio of maximum input signal to minimum input noise, by changing RIN, is 118dB. 6 7 RQ3, 10k CIN3 680PF 8 INV B INV C BP B BP C LP B LP C V + LTC1562 AGND LP A LP D BP A BP D INV A VOUT3 15 14 13 V– SHDN INV D RIN2B 3.83k 16 12 RQ2, 10k RIN2A 6.19k VIN2 –5V 0.1µF 11 10 R24, 10k 9 RQ4, 10k RIN4B 3.83k VOUT4 RIN4A 6.19k VIN4 CIN4 680pF 1562 TA07 ALL RESISTORS = 1% METAL FILM Figure 10. Quad 3-pole 100kHz Butterworth lowpass filter Linear Technology Magazine • February 1998 8th Order 30kHz Chebyshev Highpass Filter Figure 6 shows a straightforward use of the highpass configuration in Figure 3b with some practical values. Each of the four cascaded 2nd order sections has an external capacitor in the input path, as in Figure 3b. The resistors in Figure 6 set the f0 and Q values of the four sections to realize a Chebyshev (equiripple-passband) response with 0.05dB ripple and a 30kHz highpass corner. Figure 7 shows the frequency response. Total output noise for this circuit is 40µVRMS. 8th Order 100kHz Elliptic Lowpass Filter CIN2 680pF R22, 10k signal output SNR is 100dB. Measured THD at 1VRMS input is –83.5dB at 50kHz and –80dB at 100kHz. Figure 5 shows the frequency response of one filter. Figure 8 illustrates how sharp-cutoff filtering can exploit the Operational Filter capabilities of the LTC1562. In this design, two external capacitors are added and the virtual-ground inputs of the LTC1562 sum parallel paths to obtain two notches in the stopband of a lowpass filter, as plotted in Figure 9. This response falls 80dB in one octave; the total output noise is 46µVRMS and the Signal/ continued on page 32 5 CONTINUATIONS Tracking the growing popularity of portable communication systems, the LTC1623 makes a very handy singleslot 3.3V/5V PC Card switch matrix. As shown in Figure 3, this circuit enables a system controller to switch either a 3.3V or a 5V supply to any of its SMBus-addressed peripherals. Besides N-channel switches, the LTC1623 can also be used to control a P-channel switch, as shown in Figure 4. As a result, the load connected to the P-channel switch will be turned on upon power-up of the LTC1623, whereas the other load must wait for a valid address and command to be powered. VCC 2.7V TO 5.5V 10µF VCC DATA Adding Optional Test Signal In some cases, the optional test signals local loopback (LL), remote loopback (RL) and test mode (TM) are required but there are not enough drivers and receivers available in the Q1 Si3442DV 0.1µF 1k LTC1623 GB (PROGRAMMABLE) 0.1µF AD0 Q2 Si6433DQ AD1 GND DISPLAY FAN 1623 F02 Figure 4. LTC1623 controlling a P-channel switch (Q2) Conclusion With a standby current of only 17µA and a tiny 8-lead MSOP (or SO) footprint, the LTC1623 offers a simple and efficient solution for managing system peripherals using the SMBus architecture. Conclusion LTC1543, continued from page 17 The mode pins are routed to the connector and are left unconnected (1) or wired to ground (0) in the cable. The internal pull-up current sources ensure a binary 1 when a pin is left unconnected and also ensure that the LTC1543/LTC1544/LTC1344A enter the no-cable mode when the cable is removed. In the no-cable mode, the LTC1543/LTC1544 power supply current drops to less than 200µ A and all of the LTC1543/ LTC1544 driver outputs will be forced into the high impedance state. 1k GA CLK (FROM SMBus) LTC1543/LTC1544 to handle these extra signals. The solution is to combine the LTC1544 with the LTC1343. By using the LTC1343 to handle the clock and data signals, the chip set gains one extra single-ended driver/ receiver pair. This configuration is shown in Figure 5. In the world of network equipment, the product differentiation is mostly in the software and not in the serial interface. The LTC1543, LTC1544 and LTC1344A provide a simple yet comprehensive solution to standards compliance for multiple-protocol serial interface. Compliance Testing A European standard EN 45001 test report is available for the LTC1543/ LTC1544/LTC1344A chip set. The report provides documentation on the compliance of the chip set to Layer 1 of the NET1 and NET2 standard. A copy of this test report is available from LTC or from Detecon, Inc. at 1175 Old Highway 8, St. Paul, MN 55112. Authors can be contacted at (408) 432-1900 LTC1562 continued from page 5 Quadruple 3rd Order 100kHz Butterworth Lowpass Filter Another example of the flexibility of the virtual-ground inputs is the ability to add an extra, independent real pole by replacing the input resistor in Figure 2 with an R-C-R “T” network. In Figure 10, a 10k input resistor has been split into two parts and the parallel combination of the two forms a 100kHz real pole with the 680pF 32 Conclusion external capacitor. Four such 3rd order Butterworth lowpass filters can be built from one LTC1562. The same technique can add additional real poles to other filter configurations as well, for example, augmenting Figure 4’s circuit to obtain a dual 5th order filter from a single LTC1562. The LTC1562 is the first truly compact universal active filter, yet it offers instrumentation-grade performance rivaling much larger discrete-component designs. It serves applications in the 10kHz–150kHz range with an SNR as high as 100dB or more (16+ equivalent bits). The LTC1562 is ideal for modems and other communications systems and for DSP antialiasing or reconstruction filtering. Linear Technology Magazine • February 1998