Power Supply Considerations for Atmel® Capacitive-touch ICs 1. Introduction This application note discusses the dedicated power supply arrangement for Atmel capacitive touch-sensitive integrated circuits (QT™ ICs). 2. Why Do I Need a Special Power Supply? QT ICs derive their internal reference from the Vdd power supply. It is therefore important to ensure that Vdd is stable and free of electrical noise. The proprietary drift compensation algorithms of the QT IC are designed to overcome long-term drift. Nevertheless, transient spikes of ±10 mV can cause erratic behaviour such as false key activations, stuck keys, and loss of touch detection. A simple and inexpensive regulator circuit eliminates these potential problems. Power Supply Considerations for QT ICs Application Note QTAN0015 3. General Recommendations Do: • Use a dedicated regulator to power the QT IC. • Place the regulator as close as possible to the QT IC. • Use a GROUND plane under the regulator and under the Vss pins of the QT IC. • For PCBs with a GROUND plane layer, extend the plane under communications circuits, any oscillator or crystal, and also decoupling capacitors. • Place a 0.1 µF decoupling capacitor in close physical proximity to each Vdd pin. Don’t: • Share Vdd with other logic circuitry or light-emitting diodes (LED). • Use long power and GROUND traces between the regulator and the QT IC. • Extend the GROUND plane beneath the sense pins of the QT IC, Cs capacitors, Rs resistors, or any sense line traces. 4. Low Drop-out (LDO) Regulator QT ICs are optimized for low power operation. To conserve power, the device mostly operates in an ultra-low-power sleep mode in which minimal current (µA) is drawn. Periodically, the QT IC wakes up to perform sampling and communication routines that consume mA of current. The poor load step response of some LDO regulators is well-known, often resulting in significant output transients when the load increases from µA to mA, causing erratic sensor operation. Care must be taken in the selection and use of LDO regulators. Figure 4-1 shows the circuit for a Seiko S-817 series LDO regulator. 10703A–AT42–10/08 Figure 4-1. LDO Regulator Circuit Vdd S-817 Vunreg VIN C1 C2 VOUT GND 0.1 µF 2.2 µF C3 C4 0.1 µF 2.2 µF The Seiko S-817 regulator has a good load step response and reliable performance when used with QT ICs. Low quiescent current makes the S-817 regulator especially suitable for battery-driven applications. C2 and C3 should be good quality ceramic capacitors, placed as close as possible to the regulator. An additional ceramic capacitor should be placed close to the Vdd pin on the QT IC. 5. Standard Linear Regulator Figure 5-1 shows the circuit for an LM78L05 linear regulator. Figure 5-1. Linear Regulator Circuit (LM78L05) Vdd LM78L05 Vunreg VIN C1 2.2 µF C2 VOUT GND 0.1 µF C3 0.1 µF C4 2.2 µF C2 and C3 should be good quality ceramic capacitors, placed as close as possible to the regulator. An additional ceramic capacitor should be placed close to the Vdd pin on the QT IC. 6. Direct Battery Power The internal resistance of a battery causes the voltage to drop as the load increases, so load variations result in voltage transients. As the battery ages and discharges, its internal resistance increases and the transient effects worsen. To overcome these characteristics, QT ICs should be powered from an LDO regulator connected between the QT IC and the battery. Exceptionally, a QT IC can be safely powered directly from the battery, but this can only be done in applications in which the battery load remains relatively constant. Particularly on older batteries, a delta value of >10 mV can be caused merely by flashing an LED or driving a small relay coil. 7. Associated Publications The following document published by Atmel’s Touch Technology Division is also of interest: • QTAN0017 – Checking the Effects of External Noise on Atmel Capacitive-touch ICs 2 Power Supply Considerations for QT ICs 10703A–AT42–10/08 Power Supply Considerations for QT ICs Notes 3 10703A–AT42–10/08 Headquarters International Atmel Corporation 2325 Orchard Parkway San Jose, CA 95131 USA Tel: 1(408) 441-0311 Fax: 1(408) 487-2600 Atmel Asia Unit 01-05 & 16, 19/F BEA Tower, Millennium City 5 418 Kwun Tong Road Kwun Tong Kowloon Hong Kong Tel: (852) 2245-6100 Fax: (852) 2722-1369 Atmel Europe Le Krebs 8, Rue Jean-Pierre Timbaud BP 309 78054 Saint-Quentin-enYvelines Cedex France Tel: (33) 1-30-60-70-00 Fax: (33) 1-30-60-71-11 Atmel Japan 9F, Tonetsu Shinkawa Bldg. 1-24-8 Shinkawa Chuo-ku, Tokyo 104-0033 Japan Tel: (81) 3-3523-3551 Fax: (81) 3-3523-7581 Technical Support [email protected] Sales Contact [email protected] Touch Technology Division 1 Mitchell Point Ensign Way Hamble Southampton Hampshire SO31 4RF United Kingdom Tel: (44) 23-8056-5600 Fax: (44) 23-8045-3939 Product Contact Web Site www.atmel.com Literature Requests www.atmel.com/literature Disclaimer: The information in this document is provided in connection with Atmel products. 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