January 2011 I N T H I S TimerBlox: Function-Specific ICs Quickly and Reliably Solve Timing Problems I S S U E solar battery charger tracks panel maximum power 10 I2C system monitor combines temperature, voltage and current Andy Crofts measurements 22 Your design is nearly complete, but a nagging timing requirement has suddenly cropped up. It might call for a variable frequency oscillator, a low frequency timer, a pulse-width modulator, a controlled one-shot pulse generator, or an accurate delay. Regardless of the requirement, you need a quick, reliable, stable solution—there is no time to develop code for a microcontroller. You could build something out of discrete components and a comparator or two, or maybe the good old 555 timer could do the job, but will the accuracy be there? Will it take up too much room on the board? What about time to test and specify the bench-built timer? isolated data transmission and power conversion combo in surface mount package 30 isolated power supplies made easy 38 nanopower buck converter for energy harvesting apps 41 POWER SUPPLY Volume 20 Number 4 CONNECTOR DROPS WIRING DROPS CONNECTOR DROPS WIRING DROPS CONNECTOR DROPS LOAD CONNECTOR DROPS Figure 1. The simplest model for load regulation over resistive interconnections. There is a better way. Linear Technology’s TimerBlox® family of silicon timing devices solves specific timing problems with minimal effort. TimerBlox devices easily drop into designs with a fraction of the design effort or space requirements that a microcontroller or discrete-component solution would demand. It only takes a few resistors to nail down the frequency or time duration you require. That’s it, no coding or testing required. Complete solutions are tiny, composed of a 2mm × 3mm DFN, or a popular 6-lead SOT-23, plus a couple of resistors and decoupling cap. A TOOLBOX OF TIMERBLOX DEVICES All TimerBlox devices use Linear’s silicon oscillator technology, featuring low component count, vibration-immunity, fast startup, and ease-of-use. Each TimerBlox device is purpose-built to solve a specific timing problem (see Table 1), so the performance TimerBlox devices solve timing problems w w w. li n ea r.com (continued on page 2) …continued from the cover In this issue... COVER STORY TimerBlox: Function-Specific ICs Quickly and Reliably Solve Timing Problems Andy Crofts 1 (LTC699x, continued from page 1) DESIGN FEATURES of each device is specified for its intended application, eliminating the guesswork involved with configuring and applying do-it-all timers. Battery Charger’s Unique Input Regulation Loop Simplifies Solar Panel Maximum Power Point Tracking Jay Celani 10 Two High Power Monolithic Switching Regulators Include Integrated 6A, 42V or 3.3A, 42V Power Switches, Built-in Fault Protection and Operation up to 2.5MHz Matthew Topp and Joshua Moore 16 I2C System Monitor Combines Temperature, Voltage and Current Measurements for Single-IC System Monitoring David Schneider 22 Isolated Data Transmission and Power Conversion Integrated Into a Surface Mount Package Keith Bennett 30 Isolated Power Supplies Made Easy John D. Morris It only takes a few resistors to nail down the frequency or time duration you require. That’s it, no coding or testing required. Complete solutions are tiny, composed of a 2mm × 3mm DFN, or a popular 6-lead SOT-23, plus a couple of resistors and decoupling cap. 38 DESIGN IDEAS Nanopower Buck Converter Runs on 720nA, Easily Fits into Energy Harvesting and Other Low Power Applications Michael Whitaker 41 product briefs 42 back page circuits 44 Because each TimerBlox device is designed to perform a specific timing function, the most significant design decision is choosing the proper part number. To further simplify design, five of the six package pins in all TimerBlox devices share the same name and function—with the remaining pin unique to the device function. Figure 1 details the function of each pin (SOT-23 shown). Each Timerblox device offers eight different timing ranges and two modes of operation (which vary for each device). The operational state is represented by a 4-bit DIVCODE value, which is set by the voltage on the DIV pin. For the ultimate in simplicity, a resistor divider can be used to set the DIVCODE. For example, Figure 2 shows how changing the voltage at the DIV pin sets the functionality of the LTC6992 by selecting a DIVCODE from 0–15. The MSB of DIVCODE is a “mode” bit, in this case selecting the output polarity. The remaining bits choose the frequency range. Once the proper DIVCODE has been determined, the frequency or timing duration is fine-tuned by a simple calculation for RSET. The set resistor establishes the frequency of an internal silicon oscillator master clock. The resulting circuit has guaranteed accuracy over the full 2.25V–5.5V supply range and –40°C to 125°C temperature range. (continued on page 4) Figure 1. All TimerBlox devices share common pin functions DEVICE-SPECIFIC FUNCTION PIN LTC6990: Output Enable LTC6991: Reset LTC6992: Modulation Control LTC6993: Trigger LTC6994: Logic Input Scan This SCAN THIS CODE WITH YOUR SMART PHONE Most smart phones support QR Code scanning using their built-in cameras. Some phones may require that you download a QR Code scanner/reader application. This code links to www.linear.com/ltjournal. 2 | January 2011 : LT Journal of Analog Innovation OUTPUT Sources and Sinks 20mA SUPPLY VOLTAGE 2.25V TO 5.5V OUT GND RVCO (OPTIONAL) CONTROL VOLTAGE Modulates Output Frequency V+ LTC699x CONTROL RESISTOR Allows for VCO Operation SET V+ C1 0.1µF R1 DIV R2 RSET SET RESISTOR Only a Single Resistor is Required to Set the Master Frequency RESISTOR DIVIDER SETS VDIV Voltage at DIV (Divider) Pin Selects One of 16 States, which Sets the Output Frequency Range and Mode Bit The LTC6990 can easily be used as a voltage-controlled frequency modulator. Although this technique can be used with other silicon oscillators, they typically are limited in accuracy and suffer from poor supply rejection. The LTC6990 does not have these limitations. Figure 2. LTC6992 Frequency Range and “POL” Bit vs DIVCODE (LTC699x, continued from page 2) While it can be used as a fixed-frequency oscillator, the LTC6990 can easily be applied as a frequency modulator. A second SET-pin resistor, RVCO, allows a control voltage to vary ISET and change the VCTRL 0V to 3.3V 9 6 8 7 0V •VSET is GND-referenced, allowing for a GND-referenced control voltage that is easy to work with. ¼V+ ½V+ ¾V+ V+ C1 0.1µF R1 976k DIV R2 102k Figure 3 shows the LTC6990 configured as a VCO that translates a 0V to 3.3V control kHz V DIVCODE = 1 (NDIV = 2, Hi-Z = 0) OE 2V/DIV VCTRL 2V/DIV OUT 2V/DIV 20µs/DIV 4 | January 2011 : LT Journal of Analog Innovation V+ •All TimerBlox devices allow for a wide 16:1 timing range within each NDIV setting, but only the LTC6990 uses a small 2× step through divider settings. That allows for maximum overlap between ranges to accommodate any 8:1 range of VCO frequencies (or 16:1 with a reduced-accuracy extended range). And since each TimerBlox device has eight different timing ranges, the LTC6990 still maintains a large 4096:1 total frequency range. Figure 4. Performance of the voltage-controlled oscillator shown in Figure 3 f OUT = 400kHz − VCTRL • 109 LTC6990 V+ 10 5 •VSET (the SET pin voltage) is regulated to 1V and is accurate to ±30mV over all conditions. This allows RVCO to establish an accurate VCO gain. OUT SET 11 4 INCREASING VDIV 40kHz TO 400kHz RSET 86.6k 12 1 Figure 3. LTC6990 voltage-controlled oscillator RVCO 232k 13 3 output frequency. Although this technique can be used with other silicon oscillators, they typically are limited in accuracy and suffer from poor supply rejection. The LTC6990 does not have these limitations because of three important enhancements: where NDIV = 1, 2, 4, …, 128 GND 2 0.01 1MHz • 50k ISET • NDIV VSET OE 14 0.1 The LTC6990 is a resistor-programmable oscillator featuring 1.5% accuracy and an output enable function to force the output low or into a high-impedance state. The output frequency is determined by the NDIV frequency divider and RSET (which replaces VSET/ISET): OUTPUT ENABLE 15 1 10 0.001 DIVCODE MSB (“POL” BIT) = 1 0 100 VOLTAGE-CONTROLLED OSCILLATOR CAN BE USED FOR FIXED FREQUENCY OR FREQUENCY MODULATION fOUT = DIVCODE MSB (“POL” BIT) = 0 1000 fOUT (kHz) For an even easier design process, download “The TimerBlox Designer” from www.linear.com/timerblox— a free Excel-based tool that generates component values, schematics, and timing diagrams automatically. cover story PWM CONTROL TRIGGER INPUT LTC6991 LTC6992 OUTPUT RESET LTC6993 VCTRL LTC6994 OUTPUT ENABLE LTC6990 DEVICE FUNCTION OPTIONS RANGE Voltage-Controlled Silicon Oscillator Configurable frequency gain and voltage range 488Hz to 2MHz Low Frequency Oscillator Period range from 1ms to 9.5 hours 29µHz to 977Hz LTC6992-1 0%–100% Duty Cycle LTC6992-2 5%–95% Duty Cycle LTC6992-3 0%–95% Duty Cycle LTC6992-4 5%–100% Duty Cycle LTC6993-1 Rising-Edge Triggered LTC6993-2 Rising-Edge Re-Triggerable LTC6993-3 Falling-Edge Triggered LTC6993-4 Falling-Edge Re-Triggerable LTC6994-1 1-Edge Delay Voltage-Controlled PWM 3.8Hz to 1MHz One-Shot 1µs to 34 sec Delay 1µs to 34 sec LTC6994-2 2-Edge Delay Table 1. TimerBlox family members voltage into a 40kHz to 400kHz frequency. Due to the LTC6990’s high modulation bandwidth, the output responds quickly to control voltage changes, as can be seen in Figure 4. LOW FREQUENCY SOLUTIONS The LTC6991 picks up in frequency where the LTC6990 leaves off, with an enormous 29µ Hz to 977Hz range (a period range of 1ms to 9.5 hours). It incorporates a fixed 10-stage frequency divider and a programmable 21-stage divider. Since the applications for frequency modulation are rare at such low frequencies, the emphasis for this part is on covering as wide a range as possible. Therefore, the LTC6991 uses large 8× steps between NDIV settings. The trade-off is a smaller 2× overlap between ranges. The output interval relationship is: t OUT = NDIV • RSET • 1.024ms 50k where NDIV = 1, 8, 64, …, 221 The LTC6991 is designed to handle long duration timing events. In place of an output enable, it includes a similar reset function. The RST pin can truncate the output pulse or prevent the output from oscillating at all, but it has no effect on the timing of the next rising edge. This function allows the LTC6991 to initiate an event with a variable duration, perhaps controlled by another circuit. Otherwise, if RST is inactive, the LTC6991 produces a square wave. Figure 5 shows how a simple camera intervalometer can be constructed from the LTC6991 and a handful of discrete January 2011 : LT Journal of Analog Innovation | 5 Figure 5. An LTC6991-based camera intervalometer 3 SEC PULSE WIDTH RPW 100k RST CPW 33µF LTC6991 GND RS3 95.3k 8 SEC TO 64 SEC RS1 1M OUT V+ + V R1B 1M R1A 332k SET VARIABLE INTERVAL (8 SEC TO 8.5 MIN) SHUTTER OUT DIV 1µF RST “SLOW RANGE” 1.1 MIN TO 8.5 MIN RS2 2M R2 130k Figure 6. An upgraded camera intervalometer— the LTC6994-1 is added to allow shutter speed adjustment IN RST OUT GND 47.5k RSHUTTER 1M V + 1M SET DIV 3M RSHUTTER: SHUTTER SPEED ADJUSTMENT 0Ω FOR 0.25 SEC 1M FOR 4 SEC components. An intervalometer is used in time-lapse photography to capture images at specific intervals. The shutter rate might range from a few seconds to a few hours. In this example, the photographer can choose any interval between 8 seconds and 8.5 minutes. An RC delay from OUT to RST allows for a 3-second shutter pulse before resetting the output. Potentiometer RS1 varies the total resistance at the SET pin from 95.3k to 762k to adjust the period 100 DIVCODE = 0 90 3 PARTS LTC6992-1/ LTC6992-4 DUTY CYCLE (%) 80 LTC6992-2/ LTC6992-3 70 60 50 40 LTC6992-2/ LTC6992-4 30 20 10 0 LTC6992-1/LTC6992-3 0 0.2 0.4 0.6 VMOD/VSET (V/V) 0.8 6 | January 2011 : LT Journal of Analog Innovation 681k RINTERVAL 1M 1 95.3k 2.25V TO 5V GND V+ SET DIV 332k from 8 seconds to 64 seconds, with DIVCODE set to 4 by R1A and R2. Closing the SLOW RANGE switch changes the DIVCODE to 5, increasing NDIV by 8× to extend the interval up to 8.5 minutes. Figure 6 shows how easy it is to add timing functions on top of each other using TimerBlox devices. Here the LTC6994-1 is added to the intervalometer in Figure 5 to create an intervalometer with shutter-speed adjustment. PULSE-WIDTH MODULATOR The MOD pin accepts a control voltage with a range of 0.1V to 0.9V that linearly regulates the output duty cycle. The 0.1V “pedestal” ensures that an op-amp or other input driver is 1M 1µF “SLOW INTERVAL RANGE” 1 MIN TO 8 MIN 2M 130k RINTERVAL: INTERVAL ADJUSTMENT 0Ω FOR 8 SEC 1M FOR 64 SEC The LTC6992 TimerBlox oscillator features pulse-width modulation—the ability to control output duty cycle with a simple input voltage. The LTC6992 makes quick work of a technique that is useful for many applications: light dimming, isolated proportional control, and efficient load control, to name a few. Figure 7. Measured transfer function of the LTC6992 family SHUTTER OUT LTC6991 LTC6994-1 able to reach the bottom of the control range. The duty cycle is given by: DutyCycle = VMOD 1 V − 100mV − ≈ MOD 0.8 • VSET 8 800mV The output frequency is governed by the simple relationship shown below. The total frequency range of the LTC6992 covers 3.8Hz to 1MHz, using 4× divider steps in the eight NDIV settings. fOUT = 1MHz • 50k NDIV • RSET where NDIV = 1, 4, 16, …, 16384 The LTC6992-1 allows for the full duty cycle range, covering 0% (for VMOD ≤ 0.1V) to 100% (for VMOD ≥ 0.9V). At the extremes, the output stops oscillating, resting at GND (0% duty) or V+ (100% duty). Some applications (such as coupling a control signal across an isolation transformer) require continuous oscillation. For such applications, choose the LTC6992-2, which limits the output duty cycle to 5% min and 95% max. The LTC6992-3 and LTC6992-4 complete cover story The LTC6992 makes quick work of producing a voltage-controlled PWM signal—useful for many applications: light dimming, isolated proportional control and efficient load control, to name a few. the family by limiting the duty cycle at only one extreme. Figure 7mV shows the measured response for the LTC6992 family. Figure 8 shows a typical circuit. With the frequency divider (NDIV) set to 1 and RSET = 200k, this PWM circuit is configured for a 250kHz output frequency. Figure 9 demonstrates the circuit in action for both the LTC6992-1 and the LTC6992-2. The high modulation bandwidth allows the output duty cycle to quickly track changes in the modulation voltage. VMOD 0.5V/DIV 250kHz VMOD 0.1V TO 0.9V MOD OUT LTC6992 LTC6992-1 OUT 1V/DIV 2.25V TO 5.5V GND V+ SET DIV RSET 200k C1 0.1µF TIE DIV TO GND FOR DIVCODE = 0 LTC6992-2 OUT 1V/DIV 10µs/DIV Figure 8. An LTC6992 pulse-width modulator Figure 9. Performance of the PWM shown in Figure 7 ONE-SHOT EVENTS Of course, not all timing applications require a stable frequency oscillator output. Some circuits require an eventtriggered fixed-duration pulse, like that produced by the LTC6993 monostable (one-shot) pulse generator, which offers eight different logic functions and a huge 1µs to 34-second timing range. The oneshot duration tOUT is established by RSET: NDIV • RSET • 1µs 50k where NDIV = 1, 8, 64, …, 221 t OUT = The LTC6993 is triggered by a rising or falling transition on its TRIG pin, which initiates an output pulse with pulse width tOUT. Some variations include the ability to “retrigger” the pulse, extending the output pulse duration with additional trigger signals. And each version can be configured to produce logic high or low output pulses using the MSB of the DIVCODE. Table 2 summarizes the different options. Figure 10 shows a basic circuit, with the DIVCODE set to 3 (NDIV = 512, POL = 0) by a resistor divider and a 97.6k RSET defining a TRIG 2V/DIV 1ms 1ms TRIG OUT LTC6993 2.25V TO 5.5V LTC6993-1 OUT 2V/DIV 1ms V+ GND 0.1µF SET R1 1M DIVCODE = 3 DIV RSET 97.6k R2 280k LTC6993-2 OUT 2V/DIV 200µs/DIV Figure 10. An LTC6993 monostable pulse generator (one-shot) Figure 11. The LTC6993 non-retriggerable and retriggerable functionality IN 2V/DIV IN OUT LTC6994 GND 100µs 2.25V TO 5.5V V+ 0.1µF SET RSET 619k DIV LTC6994-1 OUT 2V/DIV R1 976k DIVCODE = 1 R2 102k LTC6994-2 OUT 2V/DIV tDELAY = 100µs Figure 12. LTC6994 delay interval generator 100µs/DIV Figure 13. LTC6994 single and double-edge delay functionality January 2011 : LT Journal of Analog Innovation | 7 The LTC6993 is triggered by a rising or falling transition on its TRIG pin, which initiates an output pulse with pulse width tOUT. Some variations include the ability to “retrigger” the pulse, extending the output pulse duration with additional trigger signals. 1ms output pulse width. To demonstrate the difference between retriggerable and non-retriggerable functionality, Figure 11 shows the results of using either the LTC6993-1 or LTC6993-2 in this circuit. THE LTC6994 FOR PROGRAMMABLE DELAY AND PULSE QUALIFICATION The LTC6994 is a programmable delay or pulse qualifier. It can perform noise filtering, which distinguishes its function from a delay line. The LTC6994 is available in two versions, as detailed in Table 3. The LTC6994-1 delays the rising or falling edge of the input signal. The LTC6994-2 delays any input transition, rising or falling, and can invert the output signal. The LTC6994’s programmable delay (denoted as tDELAY below) can vary from 1µs to 34 seconds, accurate to ±3% in most conditions. tDELAY = Table 2. LTC6993 options Table 3. LTC6994 options DEVICE INPUT POLARITY RE-TRIGGER LTC6993-1 Rising-Edge No LTC6993-2 Rising-Edge Yes LTC6993-3 Falling-Edge No LTC6993-4 Falling-Edge Yes delays either the rising or falling transition, and the LTC6994-2, which delays transitions in both directions. Both versions will reject narrow pulses, but the LTC6994-2 preserves the original signal’s pulse width. In addition to this type of noise filtering, the LTC6994 is useful for delay matching, generating multiple clock phases, or doubling the clock frequency of the input signal, as shown in Figure 14. NDIV • RSET • 1µs 50kΩ where NDIV = 1, 8, 64, …, 221 The output will only respond to input changes that persist longer than the delay period. This operation is well suited for pulse qualification, switch debouncing, or guaranteeing minimum pulse widths. The basic circuit in Figure 12 is configured for a 100µs delay. Figure 13 demonstrates the difference between the LTC6994-1, which 74AC86 FREQ2X 500kHz fIN 250kHz IN OUT LTC6994 2.25V TO 5.5V GND V+ SET DIV RSET 49.9k OUT 250kHz C1 0.1µF TIE DIV TO GND FOR DIVCODE = 0 4µs IN More Online Learn more about TimerBlox devices at www.linear.com/timerblox. There you can find data sheets, TimerBlox Designer software, even an introductory video about the products. 8 | January 2011 : LT Journal of Analog Innovation 1µs OUT FREQ2X Figure 14. 90° phase-shifted (quadrature) signal generator and frequency doubler DEVICE DELAY FUNCTION LTC6994-1 or LTC6994-2 or MOTOR SPEED ALARM There is no limit to how TimerBlox devices can be combined to easily produce esoteric timing functions. For instance, the design in Figure 15 combines one shots and delay blocks with a VCO to produce a high/low motor speed alarm. The circuit sounds a high frequency tone if a motor is spinning too fast and a low frequency tone if too slow. The input is taken from a motor shaft encoder or other rotational sensor and used to trigger a one shot to produce a 1ms pulse per revolution. The fast alarm threshold can be set between 10,000 rpm and 1500 rpm which, in time, is one pulse every 6ms to 40ms. Re-triggerable one shot, U3, is adjusted for a time interval equal to the warning threshold value. If it is continually re-triggered and not allowed to time out, then the motor is turning too fast. For time-filtering, a delay timer, U4, is programmed by the same threshold adjust voltage to delay an output signal until the motor has exceeded the threshold speed for 100 revolutions (600ms to 4000ms). The delayed cover story The output of the LTC6994 will only respond to input changes that persist longer than the delay period. This operation is well suited for pulse qualification, switch debouncing, or guaranteeing minimum pulse widths. output signal enables an LTC6990 oscillator to produce a 5kHz warning tone. Another time filter is created with delay block U7 which sounds a lower frequency alarm if the motor remains too slow for 10 revolutions (500ms to 5000ms). Two OR gates are used to detect when the motor has stopped completely. The slow alarm threshold can be set between 1200 rpm and 120 rpm or one pulse every 50ms to 500ms. The delay timer, U5, pulses its output if allowed to time out because the motor speed is too slow. This output re-triggers one shot U6 and keeps its output high as long as the speed remains too slow. design effort to produce accurate and reliable circuits. Several of the five core products are available in multiple versions to cover more applications and reduce the need for external components. Each part is designed to be as flexible as possible with a 2.25V to 5.5V supply range, up to –40°C to 125°C temp range and wide timing ranges. In addition, the parts are offered in a small 2mm × 3mm DFN or a low-profile SOT-23 (ThinSOT™) package when a leaded package is required. n CONCLUSION The Linear Technology TimerBlox family of silicon oscillators fills a designer’s toolbox with simple and dependable timing solutions that require minimal Figure 15. Motor speed alarm OUT U3 LTC6993-2 IN TRIG GND V+ SET DIV 5V OUT U4 LTC6994-1 GND V+ SET DIV 5V 1M 1M 5V TRIG 392k 681k 137k 681k OE 10k 1N4148 10k RPM U2 LTC6990 GND 5V ALARM OUTPUT TOO SLOW = 250Hz TOO FAST= 5kHz V+ 1M A2 SET 5V 97.6k DIV 887k V+ GND 976k IN 182k GND V+ SET DIV DIV 787k OUT U5 LTC6994-1 OUT U6 LTC6993-4 IN TRIG 5V 5V OUT U7 LTC6994-1 GND V+ GND V+ SET DIV SET DIV 523k 5V TOO SLOW WARNING 10k THRESHOLD ADJUST OUT 383k A1 OUT U1 LTC6993-1 SET 88.7k 1.5k RPM TOO FAST WARNING 10k THRESHOLD ADJUST MOTOR SPEED INPUT 1 PULSE/REV 432k 1N4148 383k 84.5k 1M 5V 1M 383k 86.6k 383k 84.5k 681k 120 RPM 1.2k RPM U1: MOTOR SPEED INPUT. Output is 1ms, one-shot pulse per revolution. U2: ALARM OUTPUT. Output of this VCO is alarm oscillator tone. FAST SPEED SENSOR U3: RETRIGGERABLE ONE SHOT. Output stops pulsing if rpm > fast threshold. U4: DELAY TIME FILTER. If rpm too fast for 100 cycles, ouput alarm is sounded. SLOW SPEED SENSOR U5: DELAY TIMER. Output never pulses if rpm > slow threshold. U6: RETRIGGERABLE ONE SHOT. If triggered output goes high and stays high if rpm below threshold. U7: DELAY TIME FILTER. If rpm too slow for 10 cycles, output alarm is sounded. A1, A2: Logic to sound alarm if motor too slow or stopped. January 2011 : LT Journal of Analog Innovation | 9