General Description
The MAX1474 is a fine-line (geometry) electronically trim-
mable 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 volt-
age 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.
Customization
Maxim can customize the MAX1474 for specific high-
volume applications. Contact Maxim for further informa-
tion.
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
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)
MAX1474
Miniature Electronically Trimmable Capacitor
________________________________________________________________ Maxim Integrated Products 1
Ordering Information
19-1948; Rev 1; 3/01
PART TEMP. RANGE PIN-
PACKAGE
TOP
MARK
MAX1474AXT-T -40°C to +125°C 6 SC70-6 AAT
CP
0.2pF
0.4pF
0.8pF
1.6pF
6pF 6pF
3.2pF
CM
DATEN LOGIC INTERFACE
VDD VSS
VDD
DATEN
16CM
5VSS
CP
MAX1474
SC70
TOP VIEW
2
34
AAT
Functional Diagram
Pin Configuration
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.
MAX1474
Miniature Electronically Trimmable Capacitor
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VDD = +5V, VSS = 0, TA= +25°C, unless otherwise noted.)
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.
Note 1: Measurements made at CP with CM = GND.
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
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
GENERAL CHARACTERISTICS
Supply Voltage 2.7 5.5 V
Supply Current Normal DC operation only, EN, DAT pins 10 µA
Supply Current During programming at 1MHz 200 µA
CAPACITOR CHARACTERISTICS
Self-Resonant Frequency SRF 15 DAT pulses 960 MHz
Quality Factor Q f0 = 315MHz 12
Absolute Accuracy ±15 %
Capacitance Increments (Note 1) 0.22 pF
C ap aci tance at 0 D AT P ul ses (Note 1) CMIN 6.4 pF
C ap aci tance Rang e (Note 1) CMAX - CMIN 6.9 pF
Tem p er atur e D r i ft 33 ppm/°C
DIGITAL CHARACTERISTICS
Low-Level Input Voltage VIL 0.25 x
VDD V
High-Level Input Voltage VIH 0.75 x
VDD V
Input Pulldown Resistor 50 kΩ
Input Leakage Current 200 µA
TIMING CHARACTERISTICS
Maximum Clock Rate on DAT fDAT 25 MHz
Minimum DAT High Pulse Width 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
MAX1474
Miniature Electronically Trimmable Capacitor
_______________________________________________________________________________________ 3
10.90
10.93
10.92
10.91
10.95
10.94
10.99
10.98
10.97
10.96
11.00
-40 -15 10 35 60 85 110 125
CAPACITANCE vs. TEMPERATURE
MAX1474 toc01
TEMPERATURE (°C)
CS (pF)
DAT = 31 PULSES
14.0
14.4
15.2
14.8
15.6
16.0
021345
CAPACITANCE vs. VOLTAGE
(SINGLE ENDED)
MAX1474 toc02
VOLTAGE (V)
CS (pF)
VCC = +3.3V
VCC = +5V
DAT = 31 PULSES,
SEE FIGURE 3a.
10.90
10.95
11.05
11.00
11.10
-5 -3 -2 -1 0-4 12345
MAX1474 toc03
VOLTAGE (V)
CS (pF)
CAPACITANCE vs. VOLTAGE
(DIFFERENTIAL)
DAT = 31 PULSES,
VCC = +5V,
SEE FIGURE 3b.
6
9
36
33
15
12
21
18
27
24
30
39
50 350 500 650200 800 950 1100
EFFECTIVE CAPACITANCE
vs. FREQUENCY
MAX1474 toc04
FREQUENCY (MHz)
CAPACITANCE (pF)
0 DAT PULSES
31 DAT PULSES
15 DAT PULSES
10 100 1000
QUALITY FACTOR vs. FREQUENCY
MAX1474 toc05
FREQUENCY (MHz)
Q
1
10
100
0
4
2
8
6
12
10
14
18
16
20
6897 1011121314
QUALITY FACTOR vs. CAPACITANCE
MAX1474 toc06
CAPACITANCE (pF)
Q
Typical Operating Characteristics
(VDD = +5V, VSS = 0, TA= +25°C, unless otherwise noted.)
800
900
850
1000
950
1050
1100
1150
1200
08124 1620242832
SELF-RESONANT
FREQUENCY vs. DAT PULSES
MAX1474 toc07
DAT PULSES
SRF (MHz)
SMITH CHART:
0 DAT PULSES
MAX1474 toc08
f1 = 1200MHz
f1
MAX1474
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 implement-
ed 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
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
4 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(VDD = +5V, VSS = 0, TA= +25°C, unless otherwise noted.)
f1
SMITH CHART:
15 DAT PULSES
MAX1474 toc09
f1 = 962MHz
Pin Description
PIN NAME FUNCTION
1 CP Capacitor Pin Connected to High-Impedance Node. See Detailed Description.
2V
DD 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.
5V
SS Negative Supply Pin
6 CM Capacitor Pin Connected to Low-Impedance Node. See Detailed Description.
f1
SMITH CHART:
31 DAT PULSES
MAX1474 toc10
f1 = 848MHz
MAX1474
Miniature Electronically Trimmable Capacitor
While EN is asserted, the internal counter counts posi-
tive edges of DAT. The number of sequential pulses on
DAT determines the capacitance setting. As EN is de-
asserted, the counter value is latched into the capaci-
tance 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 dia-
gram.
Q Enhancement Circuit
Since the capacitors have the multiplexer series resis-
tance 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 5Ωseries resistance. As in Figure 2, an
impedance transformer-like circuit is needed to over-
come this low Q. With this circuit, a much higher Q is
achievable using high-Q external capacitors. This con-
figuration 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 applica-
tions, 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 sec-
ond 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 manufactur-
ing 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.
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 appli-
cations where only differential capacitance matters.
_______________________________________________________________________________________ 5
NUMBER OF
PULSES ON
DAT
TYPICAL
VALUES
(CM = GND)
(pF)
TYPICAL
VALUES
( D I F F ER EN T IA L )
(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
14 9.57 5.18
15 9.77 5.52
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
Table 1. Capacitance Values
MAX1474
Miniature Electronically Trimmable Capacitor
6 _______________________________________________________________________________________
EN
DAT
tRESET
tSETUP
tDAT_HIGH
tSETUP
tLOAD
COUNT
Figure 1. Digital Timing Diagram
12pF
35pF
CIN: 9.3 TO
9.6pF
C: 6.41 TO
13.33pF
Figure 2. Q Enhancement Circuit
Test Circuits/Timing Diagrams
CAPACITANCE METER
CP
0.01µF
CM
f = 1MHz
DATEN
2MΩ
VDD
VGEN
VSS
MAX1474
Figure 3a. Capacitance vs. Voltage (Single Ended)
MAX1474
Miniature Electronically Trimmable Capacitor
CAPACITANCE METER
CP
0.01µF
CM
DATEN
2MΩ
VDD
VGEN1
VGEN1 + VGEN2 = VDD
0.01µF
VGEN1
VSS
MAX1474
Figure 3b. Capacitance vs. Voltage (Differential)
CFIXED
MAX1474
MAX1474
Figure 5b. Increasing Capacitance Range and Resolution
CS
CM
CR
MAX1474
Figure 4. Electronic Offset Trimming
CP CM
DATEN
VDD VSS
MAX1474
CP CM
DATEN
VDD VSS
MAX1474
Figure 5a. Increasing Capacitance Range
Test Circuits/Timing Diagrams (continued)
_______________________________________________________________________________________ 7
Chip Information
TRANSISTOR COUNT: 634
MAX1474
Miniature Electronically Trimmable Capacitor
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.
Package Information
SC70, 6L.EPS
