19-1948; Rev 1; 3/01 Miniature Electronically Trimmable Capacitor Customization Features ♦ Tiny SC70 Package (1.1mm x 2.2mm x 2.4mm) ♦ High-Performance Electronically Trimmable Capacitance ♦ Very Simple Digital Interface ♦ Eliminates the Need for Mechanical Tuning ♦ Enabling Technology for Low-Cost Production Line Automation ♦ Fully Static Operation After Programming (No Switching) Maxim can customize the MAX1474 for specific highvolume applications. Contact Maxim for further information. Applications Ordering Information Post-Trim of Low-Cost Regenerative Receivers PART Tunable RF Stages Low-Cost, Low-Temperature Drift Oscillators MAX1474AXT-T PINPACKAGE -40°C to +125°C 6 SC70-6 TEMP. RANGE TOP MARK AAT Garage Door Openers Keyless Entry Functional Diagram Industrial Wireless Control Capacitive Sensor Trimming 0.2pF RFID Tags CP CM 0.4pF Pin Configuration 6pF 6pF 0.8pF 1.6pF TOP VIEW MAX1474 VDD 2 EN 3 AAT CP 1 SC70 VSS VDD 6 CM 5 VSS 4 DAT 3.2pF EN LOGIC INTERFACE ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. DAT 1 MAX1474 General Description The MAX1474 is a fine-line (geometry) electronically trimmable capacitor (FLECAP) programmable through a simple digital interface. There are 32 programmable capacitance values ranging from 6.4pF to 13.3pF in 0.22pF increments (Table 1). The quartz dielectric capacitance is highly stable and exhibits a very low voltage coefficient. It has virtually no dielectric absorption and has a very low temperature drift coefficient (<33ppm/°C). The MAX1474 is programmed through two digital interface pins, which have Schmidt triggers and pulldown resistors to secure capacitance programming. MAX1474 Miniature Electronically Trimmable Capacitor ABSOLUTE MAXIMUM RATINGS Supply Voltage, VDD to VSS..................................... -0.3V to +6V All Other Pins ...................................(VSS - 0.3V) to (VDD + 0.3V) RMS Current into Any Pin....................................................50mA Continuous Power Dissipation (TA = +70°C) 6-Pin SC70 (derate 3.1mW/°C above +70°C) .............245mW Operating Temperature Range MAX1474AXT................................................-40°C to +125°C Storage Temperature Range .............................-65°C to +150°C Junction Temperature ......................................................+150°C Lead Temperature (soldering, 10s) .................................+300°C Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (VDD = +5V, VSS = 0, TA = +25°C, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS 5.5 V GENERAL CHARACTERISTICS Supply Voltage 2.7 Supply Current Normal DC operation only, EN, DAT pins Supply Current During programming at 1MHz 200 µA 15 DAT pulses 960 MHz f0 = 315MHz 12 10 µA CAPACITOR CHARACTERISTICS Self-Resonant Frequency Quality Factor SRF Q Absolute Accuracy ±15 Capacitance Increments (Note 1) Capacitance at 0 DAT Pulses (Note 1) Capacitance Range (Note 1) % 0.22 pF CMIN 6.4 pF CMAX - CMIN 6.9 pF 33 ppm/°C Temperature Drift DIGITAL CHARACTERISTICS Low-Level Input Voltage VIL High-Level Input Voltage VIH 0.25 x VDD 0.75 x VDD V V Input Pulldown Resistor 50 kΩ Input Leakage Current 200 µA TIMING CHARACTERISTICS Maximum Clock Rate on DAT fDAT 25 MHz tDAT_HIGH 20 ns Minimum Setup Time for DAT tSETUP 20 ns Minimum Data Load Time tLOAD 20 ns Minimum Reset Time tRESET 20 ns Minimum DAT High Pulse Width Note 1: Measurements made at CP with CM = GND. 2 _______________________________________________________________________________________ Miniature Electronically Trimmable Capacitor DAT = 31 PULSES, SEE FIGURE 3a. 15.6 11.05 10.95 15.2 VCC = +5V 14.8 10.94 CS (pF) 10.96 CS (pF) CS (pF) DAT = 31 PULSES, VCC = +5V, SEE FIGURE 3b. VCC = +3.3V 10.97 10.93 11.00 10.95 14.4 10.92 MAX1474 toc03 10.98 11.10 MAX1474 toc02 DAT = 31 PULSES 10.99 16.0 MAX1474 toc01 11.00 CAPACITANCE vs. VOLTAGE (DIFFERENTIAL) CAPACITANCE vs. VOLTAGE (SINGLE ENDED) CAPACITANCE vs. TEMPERATURE 10.91 14.0 10.90 -15 10 35 60 85 10.90 0 110 125 1 EFFECTIVE CAPACITANCE vs. FREQUENCY 4 5 -5 -4 -3 -2 33 0 1 2 3 4 5 QUALITY FACTOR vs. CAPACITANCE 20 MAX1474 toc05 100 MAX1474 toc04 36 -1 VOLTAGE (V) QUALITY FACTOR vs. FREQUENCY 39 18 16 31 DAT PULSES 14 27 12 15 DAT PULSES 21 10 Q Q 24 18 10 8 0 DAT PULSES 15 6 12 4 2 9 6 1 350 500 650 800 950 1100 0 10 100 FREQUENCY (MHz) 1000 6 FREQUENCY (MHz) 1150 8 9 10 11 12 13 14 CAPACITANCE (pF) SMITH CHART: 0 DAT PULSES SELF-RESONANT FREQUENCY vs. DAT PULSES 1200 7 MAX1474 toc08 200 MAX1474 toc07 50 1100 SRF (MHz) CAPACITANCE (pF) 3 VOLTAGE (V) TEMPERATURE (°C) 30 2 MAX1474 toc06 -40 1050 f1 1000 950 900 850 800 0 4 8 12 16 20 24 28 32 f1 = 1200MHz DAT PULSES _______________________________________________________________________________________ 3 MAX1474 Typical Operating Characteristics (VDD = +5V, VSS = 0, TA = +25°C, unless otherwise noted.) Typical Operating Characteristics (continued) (VDD = +5V, VSS = 0, TA = +25°C, unless otherwise noted.) SMITH CHART: 15 DAT PULSES SMITH CHART: 31 DAT PULSES f1 MAX1474 toc10 MAX1474 toc09 MAX1474 Miniature Electronically Trimmable Capacitor f1 f1 = 962MHz f1 = 848MHz Pin Description PIN NAME FUNCTION 1 CP Capacitor Pin Connected to High-Impedance Node. See Detailed Description. 2 VDD Positive Supply Pin. Connect a 0.01µF capacitor to VSS. 3 EN Programming Enable Pin. Has an internal 50kΩ pulldown resistor to VSS. 4 DAT Programming Data Pin. Has an internal 50kΩ pulldown resistor to VSS. 5 VSS Negative Supply Pin 6 CM Capacitor Pin Connected to Low-Impedance Node. See Detailed Description. Detailed Description The MAX1474 consists of a binary-weighted array of capacitors that can be switched in and out of parallel to provide 32 monotonic steps. The switches implemented allow the DC bias of either terminal of the capacitor to be anywhere from ground to the supply voltage. There is no inherent polarity. The signal swing range is limited to 300mV above VDD and 300mV below VSS. Therefore, if a large-signal swing range is desired, care must be taken to bias the terminal(s) requiring high swing capability near midsupply. If the capacitance is measured differentially from one terminal to the other, the greatest min/max ratio of the binary array can be realized, allowing tuning from 4 0.42pF to 10.9pF in 0.34pF steps. If one terminal is grounded, and the capacitance is measured to ground, the parasitics inherent in the package will become lumped with the switched array, allowing tuning from 6.4pF to 13.3pF in 0.22pF steps. When using the MAX1474, it is recommended for optimal performance to connect the CM terminal to the lowest impedance node (i.e., ground or supply if possible) and CP to the higher impedance node (i.e., the resonant node in a ground-referenced LC tank circuit). Digital Interface The digital interface sets the desired capacitance value. There are only two pins, EN and DAT, required for this operation. EN is an active-high control signal. _______________________________________________________________________________________ Miniature Electronically Trimmable Capacitor The FLECAP zero step differential capacitance is low, typically 0.42pF. The parasitic capacitances are the reason for the 6pF of input capacitance and the decrease in range when configured as a single-ended capacitor. The MAX1474 is an excellent choice in applications where only differential capacitance matters. Table 1. Capacitance Values NUMBER OF PULSES ON DAT TYPICAL VALUES (CM = GND) (pF) TYPICAL VALUES (DIFFERENTIAL) (pF) 0 6.41 0.424 1 6.63 0.769 2 6.86 1.11 3 7.09 1.45 4 7.31 1.79 5 7.54 2.13 6 7.78 2.47 7 8.00 2.81 8 8.22 3.13 9 8.45 3.48 10 8.67 3.82 11 8.90 4.16 12 9.12 4.49 13 9.34 4.84 • Send pulses on DAT. The total number of pulses determines the capacitance value (Table 1). 14 9.57 5.18 15 9.77 5.52 • Deassert EN. 16 10.02 5.84 17 10.25 6.18 18 10.47 6.53 19 10.67 6.87 20 10.87 7.20 21 11.10 7.54 22 11.33 7.88 23 11.57 8.22 24 11.80 8.55 25 12.02 8.89 26 12.24 9.24 27 12.46 9.58 28 12.68 9.91 29 12.91 10.25 30 13.14 10.59 31 13.33 10.93 Q Enhancement Circuit Since the capacitors have the multiplexer series resistance associated with them, the quality factor of the trimmable capacitors is not high. For example, a 10pF capacitor only has a theoretical Q of about 10 at 315MHz with 5Ω series resistance. As in Figure 2, an impedance transformer-like circuit is needed to overcome this low Q. With this circuit, a much higher Q is achievable using high-Q external capacitors. This configuration allows a tuning range of approximately 314MHz to 319MHz with 160kHz steps when resonated with a 27nH inductor. Figure 2 shows a Q enhancement circuit. Startup Sequence The MAX1474 must be programmed before use. When powered up, the following sequence must occur: • Deassert both EN and DAT. • Assert EN. Applications Information The MAX1474 offers users a wide range of applications, including trimming of regenerative receivers, oscillators, RFID tags, and capacitive sensors. A capacitive sensor is usually made of two capacitors. One capacitor (CM) changes with the sensed function, such as pressure, acceleration, or humidity. The second capacitor (CR) is used as a reference capacitance that does not vary with the sensed function. In most applications, it is needed to detect the difference between these capacitance values. Due to manufacturing tolerances, there are normally offsets between CM and CR, which can easily be nulled out by the FLECAP as shown in Figure 4. To increase the adjustment capacitance range, several FLECAPs can be placed in parallel. Several capacitors can be tied together to increase the number of steps or the resolution, as shown in Figures 5a and 5b. _______________________________________________________________________________________ 5 MAX1474 While EN is asserted, the internal counter counts positive edges of DAT. The number of sequential pulses on DAT determines the capacitance setting. As EN is deasserted, the counter value is latched into the capacitance control registers first, and then the counter is set to zero. The programmed capacitance value then appears between CP and CM. During the period when EN is asserted, the capacitance value between CP and CM stays unchanged. Figure 1 is a digital timing diagram. Miniature Electronically Trimmable Capacitor MAX1474 Test Circuits/Timing Diagrams EN DAT tLOAD tSETUP tRESET COUNT tSETUP tDAT_HIGH Figure 1. Digital Timing Diagram CAPACITANCE METER f = 1MHz 0.01µF 2MΩ 12pF CIN: 9.3 TO 9.6pF CM VGEN 35pF Figure 2. Q Enhancement Circuit 6 CP C: 6.41 TO 13.33pF MAX1474 VDD VSS EN DAT Figure 3a. Capacitance vs. Voltage (Single Ended) _______________________________________________________________________________________ Miniature Electronically Trimmable Capacitor CAPACITANCE METER 0.01µF CFIXED 0.01µF 2MΩ CP CM MAX1474 VGEN1 VGEN1 VDD MAX1474 VSS MAX1474 VGEN1 + VGEN2 = VDD EN DAT Figure 5b. Increasing Capacitance Range and Resolution Figure 3b. Capacitance vs. Voltage (Differential) Chip Information TRANSISTOR COUNT: 634 CR CM CS MAX1474 Figure 4. Electronic Offset Trimming CP CM VDD VSS MAX1474 EN DAT CP CM VDD VSS MAX1474 EN DAT Figure 5a. Increasing Capacitance Range _______________________________________________________________________________________ 7 MAX1474 Test Circuits/Timing Diagrams (continued) MAX1474 Miniature Electronically Trimmable Capacitor SC70, 6L.EPS Package Information Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. 8 _____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.