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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 -40C to +125C 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
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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 = +70C) 6-Pin SC70 (derate 3.1mW/C above +70C) .............245mW Operating Temperature Range MAX1474AXT................................................-40C to +125C Storage Temperature Range .............................-65C to +150C Junction Temperature ......................................................+150C Lead Temperature (soldering, 10s) .................................+300C
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 = +25C, 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 k A 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 A A SYMBOL CONDITIONS MIN TYP MAX UNITS
Note 1: Measurements made at CP with CM = GND.
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Miniature Electronically Trimmable Capacitor
Typical Operating Characteristics
(VDD = +5V, VSS = 0, TA = +25C, 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
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Miniature Electronically Trimmable Capacitor MAX1474
Typical Operating Characteristics (continued)
(VDD = +5V, VSS = 0, TA = +25C, 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 0.01F capacitor to VSS. Programming Enable Pin. Has an internal 50k pulldown resistor to VSS. Programming Data Pin. Has an internal 50k 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
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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.
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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 5 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 number of pulses determines the capacitance value (Table 1). * Deassert 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.
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Miniature Electronically Trimmable Capacitor MAX1474
Test Circuits/Timing Diagrams
EN
DAT
tSETUP tRESET COUNT tDAT_HIGH tSETUP
tLOAD
Figure 1. Digital Timing Diagram
CAPACITANCE METER f = 1MHz 0.01F 2M
12pF CIN: 9.3 TO 9.6pF 35pF C: 6.41 TO 13.33pF
VGEN
CP
CM
MAX1474 VDD VSS
EN
DAT
Figure 2. Q Enhancement Circuit
Figure 3a. Capacitance vs. Voltage (Single Ended)
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Miniature Electronically Trimmable Capacitor MAX1474
Test Circuits/Timing Diagrams (continued)
CAPACITANCE METER
0.01F 2M CP VGEN1 VDD VGEN1 + VGEN2 = VDD EN DAT CM
0.01F
CFIXED
VGEN1
MAX1474 MAX1474
MAX1474
VSS
Figure 5b. Increasing Capacitance Range and Resolution
Figure 3b. Capacitance vs. Voltage (Differential)
CR
Chip Information
TRANSISTOR COUNT: 634
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
Miniature Electronically Trimmable Capacitor MAX1474
Package Information
SC70, 6L.EPS
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 (c) 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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