Remotely Adjustable Voltage Regulators using XDCPs ® Application Note April 21, 2005 AN1154.0 Author: Applications Staff This note shows how an X9312 can be used to implement a remote, adjustable regulator. The LM317 is probably the most common adjustable three- terminal voltage regulator in use today. Often it would be desirable to adjust these regulators remotely, but extending the adjustment rheostat any distance introduces noise and instability. Intersil’s X9312 XDCP can provide a simple solution. By using this XDCP, the voltage output of the LM317 can be set using a simple 3wire digital interface over practically any distance. Figure 1 shows the LM317’s usual adjustment resistor replaced with the 10kΩ version of the X9312. The pushbutton control circuitry would be located remotely (on a separate board). The adjustment range can be set remotely to fit an application by changing R7, just remember not to exceed the VH or VL maximum limit of +15 volts. 1 The standard formula for output voltage naturally still applies: R pot V out = V ref + V ref ------------ + I adj ( R ref ) R ref where Vref = 1.25V Rref = R7 Rpot = 0 to 10kΩ value for U1 in 99 steps. Iadj = approx. 50µA of current from the adjustment pin of the LM317 Figure 1 shows the control circuitry connected via a 10-pin connector. This allows an LM317 voltage regulator to be set and then for the control circuitry to be removed. In this configuration, it’s a good idea to include a pull-up resistor on all of the control lines (R9, R10, and R11) to assure that inadvertant XDCP changes do not occur. CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 1-888-352-6832 | Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2005. All Rights Reserved All other trademarks mentioned are the property of their respective owners. Application Note 1154 VR1 LM31 7T 3. 5mm PL UG 3 VI N VOUT 1 ADJ C6 10 0UF 3. 5mm J ACK 1. 25 V T O 1 0V WALL MOUNTED 12 V TO 1 5V POWER CONVERTER 2 R7 1. 3K R8 2. 7K C5 1 0UF RET URN HEAT SI NK U1 +5V 3 5 6 VR2 3 2 I N OUT 1 VH VW VL I NC U/ D CS 1 2 7 + 5V X9C103 GND P1/J1 C7 0 . 1UF LM7 8L0 5ACZ 1 3 5 7 9 PUSHBUTTON CONTROLLER R11 10K R9 10K + 5V +5 V ++ ++ ++ ++ ++ 2 4 6 8 10 R10 1 0K J1/P1 +5V CR1 1N4 148 R4 1 0K U2B R5 1 0K 1 3 5 7 9 4 6 5 S1 DECREASE R2 2 7K U2A 7 4HC132 C2 1 000 PF ++ ++ ++ ++ ++ 2 4 6 8 10 1 +5V 3 2 S2 I NCREASE 7 4HC132 CR2 1 N414 8 R6 47 0 U2C U2D 10 8 C8 0. 1UF C9 0 . 1UF DS1 13 9 11 12 R3 2 7K C3 1 000 PF LED 74 HC132 7 4HC13 2 C1 1UF R1 2 20K FIGURE 1. 1.25V to 10V DC Supply Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that the Application Note or Technical Brief is current before proceeding. For information regarding Intersil Corporation and its products, see www.intersil.com 2 April 21, 2005