AND9149/D NCN5120 - Driving Relays in an Efficient Way http://onsemi.com APPLICATION NOTE INTRODUCTION because these relays are not operated at a very high rate this approach works perfectly. NCN5120 is a receiver−transmitter IC suitable for use in KNX® twisted pair networks (KNX TP1−256). It supports the connection of actuators, sensors, microcontrollers, switches or other applications in a building network. NCN5120 handles the transmission and reception of data on the bus. It generates from the unregulated bus voltage stabilized voltages for its own power needs as well as to power external devices, for example, a microcontroller. NCN5120 assures safe coupling to and decoupling from the bus. Bus monitoring warns the external microcontroller for loss of power so that critical data can be stored in time. NCN5120 has several voltage regulators to generate different voltages. One of these voltage regulators is a 20 V low drop linear regulator typically used for driving relays. Because NCN5120 has a high efficient adjustable DC−DC converter, driving of relays can be done in a more efficient way. This application note describes how to use the adjustable DC−DC converter for driving relays in an efficient way under all operating conditions. NCN5120 RELAY APPLICATION The 20 V LDO is not an efficient regulator to be used. NCN5120 has an adjustable DC−DC converter which is high efficient (DC2). One could decide to use DC2 to drive the relays. However, due to some limitations of NCN5120 this DC2 voltage could drop too much under certain situations (mainly when the capacitor needs to be charged). This would result in a too low voltage to drive the relays. Also, to store as much as possible energy in the big storage capacitor it’s advised to have an as high as possible voltage. Figure 2 gives the schematic for driving relays with DC2 and assures a stable voltage under all situations. Figure 3 displays only the DC2 part (which gives a more clear view on the operation). With exception of DC2, the schematic as given in Figure 2 is very basic (see also NCN5120 datasheet). The connection between the KNX bus (see connector A and B) and NCN5120 makes use of all the standard components as given in the datasheet. A standard 16 MHz crystal is used and a clock signal is supplied back to the microcontroller (uC CLK). It’s not mandatory to work in this way. The relay driving principle will also work when the microcontroller supplies the clock to NCN5120. RESETB and SAVEB are supplied to the microcontroller for monitoring of the status. A standard UART interface is used as communication between microcontroller and NCN5120 but any of the other interfaces will also do. DC1 is used as normal. FANIN/WAKE−pin is pulled to ground. Although this is not necessary, it’s advised to do this. When FANIN/WAKE−pin is pulled to ground, more power can be taken from the KNX bus to recharge the big buffer capacitor faster (C13 in Figure 2). V20V is not used in Figure 2 because we use DC2 to drive the relays. Table 1 gives the components list for all required DC2 components. All components from Figure 2 which cannot be found back in Table 1 can be found back in the NCN5120 datasheet. STANDARD RELAY APPLICATION Figure 1 gives the block diagram of the relay driving principle as used in KNX relay actuators today. A 20 V LDO is used to power the relays. Because 12 V relays are generally used, this 20 V needs to be converted down to 12 V. To minimize the required power to drive the relays, bistable relays are used which are controlled by H−bridges. The H−bridges itself are controlled by a microcontroller (microcontroller not displayed in Figure 1). Although bistable relays are used still a relatively high current is required to change the state of the relays. This current cannot be requested from the 20 V LDO because this would result in a too high current taken from the KNX bus. To make sure not too much current is sourced from the KNX bus a current limit circuit is added. A big storage capacitor (10 mF or higher) is used to store the energy required to drive the relays. Every time the relays are activated the voltage on the big storage capacitor will drop. Recharging of this capacitor will be relatively slow (due to the current limiter) but © Semiconductor Components Industries, LLC, 2013 August, 2013 − Rev. 0 1 Publication Order Number: AND9149/D AND9149/D Optional Current Limiter 20 V LDO H−bridge Relay 1 20 V to H−bridge Relay 2 12 V H−bridge Relay 3 H−bridge Relay n Figure 1. Relay Block Diagram RESETb SAVEb uC CLK C8 C9 3.3 3.3 VSSD 31 XCLK 32 XTAL2 XSEL 33 SAVEB XTAL1 35 36 RESETB 37 38 TESTOUT 39 34 25 10 21 VDD1M VDD1 VSS1 B VSW1 C4 SCK/UC2 SDO/TXD CSB/UC1 TREQ MODE2 MODE1 NC NC 3.3 C 10 R8 R6 R7 C 12 L2 T1 R5 D3 V2 R4 C 13 Figure 2. Application Example for Driving Relays http://onsemi.com 2 TxD SDI/RXD R2 L1 GND VDDD 20 22 19 23 9 18 8 17 24 16 7 VDD2MV C3 6 11 V20V 26 NCN5120 15 VFILT 5 VIN D2 CEQ2 27 VDD2MC C2 28 4 VSW2 CEQ1 3 14 CAV VBUS1 29 13 C1 VCC2 C6 30 VSS2 CCP VCC 3.3 2 VDD2 A 39 VDDA TXO V2 1 12 VSSA VBUS2 R1 D1 FANIN/WAKE X1 C5 RxD AND9149/D R6 VFILT VDD2MC VDD2 VIN V2 D3 L2 VSW2 NCN5120 R7 C12 C 13 T1 R4 VDD2MV R8 R5 Figure 3. DC2 Schematic for Driving Relays Table 1. DC2 BOM 1. 2. 3. 4. Comp. Value Tolerance C12 56 pF ±10% Remarks Notes 1 C13 12 mF ±20% 2, 4 D3 MBR1H100SFT3G L2 220 mF ±10% Coils Electronic DA54NP−221K R4 39 kW ±5% 0.0625 W R5 10 kW ±5% 0.0625 W 4 R6 2.7 W ±1% 0.0625 W 3, 4 R7, R8 100 kW ±1% 0.0625 W T1 2N7002LT1G ON Semiconductor 4 ON Semiconductor It’s advised to take this value as close as possible to the input capacitance of T1. The value of this capacitor will depend on the required energy for driving the relays. R2 (see Figure 2) should be lower than R6! See below for defining the value. R4 and R5 can be calculated as next: R4 + R5 R VDD2M − V2 = output voltage when relays are active (advised value is 13 V or 14 V) − tdel = 0.15 (0.25 worst case) The capacitor C13 can be calculated as next: V 2 * 3.3 3.3 R 5 ) R VDD2M The current limiting resistor R6 can be calculated as next: R6 + Ǹǒ 2 L 2 V V OC I FILT 1 T 1 ) V FILT Ǔ C 13 + * V FILT L t del n i relay t act V drop where: − n = number of relays that are switched simultaneously − irelay = current required by one relay (V2 / Rrelay gives a good estimation) − tact = activation time of the relays − Vdrop = the allowed drop on VOUT A calculator is provided on the ON Semiconductor website as a guide for defining the optimum values (http://www.onsemi.com/PowerSolutions/supportDoc.do?t ype=tools&rpn=NCN5120). 2 2 where: − VOC = 0.15 (0.165 worst case) − IFILT = the maximum VFILT current − VFILT = VFILT voltage when relays are active (advised value is 12 V) − T = 4 (5 worst case) − L2 = inductance of DC2 (in mH) http://onsemi.com 3 AND9149/D KNX and the KNX logos are registered trademarks of KNX and its suppliers or licensors. 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