Rainbow Electronics MAX5439 - Manual (Page 7)

Detailed Description
Digital Interface Operation
The MAX5436–MAX5439 use a 3-wire SPI/QSPI/
MICROWIRE-compatible serial data interface to control
the wiper position. This write-only interface contains
three inputs: chip select (CS), data in (DIN), and serial
clock (SCLK). When CS is taken low, data from DIN is
synchronously loaded into the serial shift register on the
rising edge of each SCLK pulse (Figure 2). The 8-bit
data word requires 8 clock pulses to input the serial
data. Note that the first bit of the data word, D7, is
unused and should be ignored. Therefore, the second
rising edge of SCLK loads the MSB. After all the data
bits have been shifted in, they are latched into the
potentiometer control register when CS transitions from
low to high, the wiper position is then updated. Note
that if CS is not kept low during the entire data stream,
the data will be corrupted and the device will need to
be reloaded.
Applications Information
Power-Up Sequencing
The MAX5436–MAX5439 have been designed so that
any of the supplies can turn on first without causing any
unwanted crowbar currents to flow. Note that both digital
and analog supplies are required to power up the wiper
and uncommitted amplifier (MAX5437/MAX5439 only).
Adjustable Gain Amplifier
The MAX5436/MAX5438 are used with the MAX427 to
make a digitally adjustable gain circuit as shown in
Figure 4. The normal feedback resistor is replaced with
the MAX5436/MAX5438 in a variable-resistor configura-
tion so that the gain of the circuit can be digitally con-
trolled. The MAX5437/MAX5439 can use the internal
high-voltage amplifier to make this digitally adjustable
gain circuit.
LCD Biasing Control Applications
The MAX5436–MAX5439 are ideal for LCDs that require
separate voltage for contrast control in addition to the
main supply voltage. Figure 5a shows the MAX5436–
MAX5439 being used for LCD contrast control along with
the MAX629, which provides the LCD supply voltage. A
similar circuit with an additional buffer circuit is shown in
Figure 5b.
MAX5436–MAX5439
±15V, 128-Tap, Low-Drift Digital Potentiometers
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Pin Description
PIN
MAX5436/
MAX5438
MAX5437/
MAX5439
NAME DESCRIPTION
1 1 SCLK Serial Clock Input
2 2 DIN Serial Data Input
3 3 CS Chip Select
— 4 SHDN Amplifier Shutdown
4 5 GND Analog and Digital Ground
5 6 V SS Negative Analog Supply. Bypass with a 0.1µF capacitor to GND.
— 7 OUT Amplifier Output
— 8 IN- Amplifier Negative Input
— 9 IN+ Amplifier Positive Input
6 10 V DD Positive Analog Supply. Bypass with a 0.1µF capacitor to GND.
7 11 H High Terminal of Resistor
8 12 W Wiper Terminal of Resistor
9 13 L Low Terminal of Resistor
10 14 V CC Digital Supply. Bypass with a 0.1µF capacitor to GND.