Miniature Electronically Trimmable Capacitor

www.DataSheet4U.com 19-1948; Rev 1; 3/01 Miniature Electronically Trimmable Capacitor 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. Features o Tiny SC70 Package (1.1mm x 2.2mm x 2.4mm) o High-Performance Electronically Trimmable Capacitance o Very Simple Digital Interface o Eliminates the Need for Mechanical Tuning o Enabling Technology for Low-Cost Production Line Automation o Fully Static Operation After Programming (No Switching) MAX1474 Customization Maxim can customize the MAX1474 for specific highvolume applications. Contact Maxim for further information. Applications Post-Trim of Low-Cost Regenerative Receivers Tunable RF Stages Low-Cost, Low-Temperature Drift Oscillators Garage Door Openers Keyless Entry Industrial Wireless Control Capacitive Sensor Trimming RFID Tags CP Ordering Information PART MAX1474AXT-T PINPACKAGE -40°C to +125°C 6 SC70-6 TEMP. RANGE TOP MARK AAT Functional Diagram 0.2pF CM 0.4pF Pin Configuration 6pF 0.8pF 6pF TOP VIEW CP 1 1.6pF MAX1474 VDD VSS 3.2pF AAT SC70 6 CM VDD 2 5 VSS EN 3 4 DAT EN LOGIC INTERFACE DAT ________________________________________________________________ Maxim Integrated Products 1 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. www.DataSheet4U.com Miniature Electronically Trimmable Capacitor MAX1474 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 GENERAL CHARACTERISTICS Supply Voltage Supply Current Supply Current CAPACITOR CHARACTERISTICS Self-Resonant Frequency Quality Factor Absolute Accuracy Capacitance Increments (Note 1) Capacitance at 0 DAT Pulses (Note 1) Capacitance Range (Note 1) Temperature Drift DIGITAL CHARACTERISTICS Low-Level Input Voltage High-Level Input Voltage Input Pulldown Resistor Input Leakage Current TIMING CHARACTERISTICS Maximum Clock Rate on DAT Minimum DAT High Pulse Width Minimum Setup Time for DAT Minimum Data Load Time Minimum Reset Time fDAT tDAT_HIGH tSETUP tLOAD tRESET 25 20 20 20 20 MHz ns ns ns ns VIL VIH 0.75 x VDD 50 200 0.25 x VDD V V kΩ µA CMIN CMAX - CMIN SRF Q 15 DAT pulses f0 = 315MHz 960 12 ±15 0.22 6.4 6.9 33 % pF pF pF ppm/°C MHz Normal DC operation only, EN, DAT pins During programming at 1MHz 200 2.7 5.5 10 V µA µA SYMBOL CONDITIONS MIN TYP MAX UNITS Note 1: Measurements made at CP with CM = GND. 2 _______________________________________________________________________________________ www.DataSheet4U.com Miniature Electronically Trimmable Capacitor Typical Operating Characteristics (VDD = +5V, VSS = 0, TA = +25°C, unless otherwise noted.) CAPACITANCE vs. VOLTAGE (SINGLE ENDED) MAX1474 toc01 MAX1474 toc02 MAX1474 CAPACITANCE vs. TEMPERATURE 11.00 10.99 10.98 10.97 CS (pF) CS (pF) 10.96 10.95 10.94 10.93 10.92 10.91 10.90 -40 -15 10 35 60 85 110 125 TEMPERATURE (°C) 14.0 0 14.4 15.2 16.0 DAT = 31 PULSES CAPACITANCE vs. VOLTAGE (DIFFERENTIAL) DAT = 31 PULSES, VCC = +5V, SEE FIGURE 3b. 11.05 CS (pF) MAX1474 toc03 11.10 DAT = 31 PULSES, SEE FIGURE 3a. 15.6 VCC = +3.3V 14.8 VCC = +5V 11.00 10.95 10.90 1 2 3 4 5 -5 -4 -3 -2 -1 0 1 2 3 4 5 VOLTAGE (V) VOLTAGE (V) EFFECTIVE CAPACITANCE vs. FREQUENCY MAX1474 toc04 QUALITY FACTOR vs. FREQUENCY MAX1474 toc05 QUALITY FACTOR vs. CAPACITANCE 18 16 14 12 MAX1474 toc06 39 36 33 CAPACITANCE (pF) 30 27 24 21 18 15 12 9 6 50 200 350 500 650 800 950 1100 FREQUENCY (MHz) 15 DAT PULSES 0 DAT PULSES 31 DAT PULSES 100 20 10 10 8 6 4 2 Q 1 10 100 FREQUENCY (MHz) 1000 Q 0 6 7 8 9 10 11 12 13 14 CAPACITANCE (pF) SELF-RESONANT FREQUENCY vs. DAT PULSES MAX1474 toc07 SMITH CHART: 0 DAT PULSES MAX1474 toc08 1200 1150 1100 SRF (MHz) 1050 1000 950 900 850 800 0 4 8 12 16 20 24 28 f1 32 f1 = 1200MHz DAT PULSES _______________________________________________________________________________________ 3 www.DataSheet4U.com Miniature Electronically Trimmable Capacitor MAX1474 Typical Operating Characteristics (continued) (VDD = +5V, VSS = 0, TA = +25°C, unless otherwise noted.) SMITH CHART: 15 DAT PULSES MAX1474 toc09 SMITH CHART: 31 DAT PULSES MAX1474 toc10 f1 f1 f1 = 962MHz f1 = 848MHz Pin Description PIN 1 2 3 4 5 6 NAME CP VDD EN DAT VSS CM FUNCTION Capacitor Pin Connected to High-Impedance Node. See Detailed Description. Positive Supply Pin. Connect a 0.01µF capacitor to VSS. Programming Enable Pin. Has an internal 50kΩ pulldown resistor to VSS. Programming Data Pin. Has an internal 50kΩ pulldown resistor to VSS. Negative Supply Pin 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. _______________________________________________________________________________________ www.DataSheet4U.com Miniature Electronically Trimmable Capacitor 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. 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. MAX1474 Table 1. Capacitance Values NUMBER OF PULSES ON DAT 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 TYPICAL VALUES (CM = GND) (pF) 6.41 6.63 6.86 7.09 7.31 7.54 7.78 8.00 8.22 8.45 8.67 8.90 9.12 9.34 9.57 9.77 10.02 10.25 10.47 10.67 10.87 11.10 11.33 11.57 11.80 12.02 12.24 12.46 12.68 12.91 13.14 13.33 TYPICAL VALUES (DIFFERENTIAL) (pF) 0.424 0.769 1.11 1.45 1.79 2.13 2.47 2.81 3.13 3.48 3.82 4.16 4.49 4.84 5.18 5.52 5.84 6.18 6.53 6.87 7.20 7.54 7.88 8.22 8.55 8.89 9.24 9.58 9.91 10.25 10.59 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. • Send pulses on DAT. The total n