Datasheet AD5227 (Analog Devices) - 10

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AD5227 THEORY OF OPERATION
The AD5227 is a 64-position 3-terminal digitally controlled The end-to-end resistance, RAB, has 64 contact points accessed potentiometer device. It presets to a midscale at system power- by the wiper terminal, plus the B terminal contact, assuming on. When CS is enabled, changing the resistance settings is that RWB is used (see Figure 25). Clocking the CLK input steps, achieved by clocking the CLK pin. It is negative-edge triggered, RWB by one step. The direction is determined by the state of and the direction of stepping is determined by the state of the U/D pin. The change of RWB can be determined by the number U/D input. When the wiper reaches the maximum or the of clock pulses, provided that the AD5227 has not reached its minimum setting, additional CLK pulses do not change the maximum or minimum scale. ΔRWB can, therefore, be wiper setting. approximated as
VDD
⎛ R ⎞ ΔR = ±⎜CP× AB + R ⎟ WB W (1)
AD5227
⎝ 64 ⎠
CS A
where:
6-BIT UP/DOWN U/D CONTROL W LOGIC
CP is the number of clock pulses.
CLK B
RAB is the end-to-end resistance. RW is the wiper resistance contributed by the on-resistance of
GND
the internal switch.
POR WIPER MIDSCALE REGISTER
Since in the lowest end of the resistor string a finite wiper 04419-0-026 resistance is present, care should be taken to limit the current Figure 24. Functional Block Diagram flow between W and B in this state to a maximum pulse current of no more than 20 mA. Otherwise, degradation or possible destruction of the internal switches can occur.
A RS
Similar to the mechanical potentiometer, the resistance of the RDAC between the Wiper W and Terminal A also produces a
D0 R D1 S
digitally controlled complementary resistance, RWA. When these
D2 D3
terminals are used, the B terminal can be opened or shorted to
RS D4
W. Similarly, ΔR
D5
WA can be approximated as
W RDAC RW
⎛ RAB ⎞
UP/DOWN
ΔR = ± WA ⎜(64 −CP) + W R ⎟ (2)
CTRL AND DECODE
⎝ 64 ⎠
R
027
S B
0- Equations 1 and 2 do not apply when CP = 0.
RS = RAB/64
04419- Figure 25. AD5227 Equivalent RDAC Circuit The typical distribution of the resistance tolerance from device to device is process lot dependent. It is possible to have ±20%
PROGRAMMING THE DIGITAL POTENTIOMETERS
tolerance.
Rheostat Operation Potentiometer Mode Operation
If only the W-to-B or W-to-A terminals are used as variable resistors, the unused terminal can be opened or shorted with W. If all three terminals are used, the operation is called This operation is called rheostat mode and is shown in Figure 26. potentiometer mode. The most common configuration is the voltage divider operation as shown in Figure 27.
A A A VI A W W W VC B B B
04419-0-028
W
Figure 26. Rheostat Mode Configuration
B
04419-0-029 Figure 27. Potentiometer Mode Configuration Rev. B | Page 10 of 16 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY ELECTRICAL CHARACTERISTICS INTERFACE TIMING DIAGRAMS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION PROGRAMMING THE DIGITAL POTENTIOMETERS Rheostat Operation Potentiometer Mode Operation DIGITAL INTERFACE TERMINAL VOLTAGE OPERATION RANGE POWER-UP AND POWER-DOWN SEQUENCES LAYOUT AND POWER SUPPLY BIASING APPLICATIONS MANUAL CONTROL WITH TOGGLE AND PUSHBUTTON SWITCHES MANUAL CONTROL WITH ROTARY ENCODER ADJUSTABLE LED DRIVER ADJUSTABLE CURRENT SOURCE FOR LED DRIVER ADJUSTABLE HIGH POWER LED DRIVER AUTOMATIC LCD PANEL BACKLIGHT CONTROL 6-BIT CONTROLLER CONSTANT BIAS WITH SUPPLY TO RETAIN RESISTANCE SETTING OUTLINE DIMENSIONS ORDERING GUIDE