Application Note MSAN-151 Implementation Details for the MH88615 SLIC Contents Issue 2 March 1997 1. Overview 1. Overview 2. Power Up Sequence 3. Signalling 3.1 Ringing 3.2 Power Denial 3.3 Switch Hook Detection 4. Line Current Feed and Battery Voltage 5. 5.0 Ringing Voltage Amplifier 5.1 Ringing Oscillator 5.2 Ringing Supply Voltages 6. Protection Circuit 7. 7.0 Audio Interface 7.1 Receive Signal 7.2 Transmit Signal 8. Design Example 9. Additional Reference Material The MH88615 is a subscriber line interface circuit (SLIC) which provides the interface between a telephone and a codec. The functions provided by the MH88615 include 2-4 Wire conversion, constant current line feed, on board ringing amplifier, power denial, signalling and control. Different variants are provided to meet different line impedances. This application note is intended to assist the user in implementing an analogue line interface circuit. Please refer to the MH88615 datasheet for parametric details. Typical applications include Pair-Gain systems, Internet Surfboards, Terminal Adapters and Multiplexers. A basic application is shown in Figure 1. 2. Power Up Sequence When powering up the MH88615 the user should ensure that the VEE supply rail is connected before, or at the same time as, the VDD supply rail. If the user were to power up the VDD supply rail first it is possible to cause one of the operational amplifiers on the hybrid to latch up. This is a non-destructive condition. If it were to happen simply powering down the hybrid and re-applying the power correctly will overcome the latch-up condition. If the user cannot guarantee that the correct power up sequence will be followed then by adding two Schottky diodes in their application, as shown in Figure 2, the circuit will not latch-up. 3. Signalling The MH88615 provides control signals for Ringing and Power Denial and provides indication of Switch Hook status. A-279 MSAN-151 Application Note 3.1 Ringing 3.2 Power Denial Ringing is enabled by setting RC (pin 11) to logic 1. When the subscriber set goes off-hook, the DC loop current will be detected within a maximum of 200 ms and the SHK output will go to a logic 1. Ringing is not automatically disabled on ring trip. A suitable circuit which automatically disables ringing is shown in Figure 3. This circuit will ensure that the Ringing Control signal is taken low within 200ms of receiving a Switch Hook signal. It will also fulfill the criterion that the RC signal must not be taken high again within 400ms of receiving the SHK signal to allow DC loop current to stabalize. The battery voltage may be effectively isolated from the loop driver circuit under the control of the Power Denial (PD) pin (pin 12). This pin should be set to a logic 1 to enable Power Denial. The resulting loop current is negligible and power consumption is minimized. This function is useful for disabling a loop which may have a ground fault. Note that the offhook state cannot be detected with power denial applied. +5V VDCRI 8 VDD 4 1 TIP RC DCRI TIP PD Protection Circuit 11 SD0 12 SD1 15 VR VR 2 RING RING VX 5 18 VX LPGND CAP + 17 SD3 C1 MH88615 MT896x 6 C2 VBAT R1 VBAT VR 17-68Hz C1 1uF 20% 16V C2 10nF 20% 16V R1 470k Ω 10% Protection Circuit See section 6.0 and Figures 10 & 11 20 SHK 14 RV VREF VEE 10 16 AGND 9 -5V Figure 1 - Basic Application Circuit A-280 VREF MSAN-151 Application Note 3.3 Switch Hook Detection The SHK output (pin 14) is set to logic 1 when the DC loop current exceeds the internally set threshold of typically 10mA indicating that the subscriber set has gone off-hook. Dial pulses can be detected by monitoring the interruption rate at the SHK pin. These pulses may need to be debounced by the system software. during ringing and must be switched out during pulse dialling if dial pulse detection is required. This may be achieved using either a transistor (see Figure 4), relay or sense drive output of a CODEC (see Figure 1). If only DTMF signaling is required, the capacitor may be left permanently connected. Once SHK goes high Ring Control should be kept low for a minimum of 400ms to allow the DC control loop sufficient time to stabilize. For switch hook detection during ringing a 1µF capacitor will provide adequate attenuation of ringing frequencies. This capacitor must be switched in VDD 1N5817 Schottky Diode AGND 1N5817 Schottky Diode VEE Figure 2 - Latch-up Protection A small-signal diode Switch Hook Detect SHK, Pin 14 10kΩ Ringing Control RC, Pin 11 470kΩ A small-signal pnp transistor (e.g. 2N2907A) Ringing Control Signal from System Controller (e.g. MT896x SD0) 10kΩ 1µ F 10k Ω Figure 3 - Ring Trip Circuit A-281 MSAN-151 Application Note 4. Line Current Feed and Battery Voltage VREF may be adjusted to supply loop currents outside the recommended 18-30mA range, although performance is not guaranteed. The MH88615 employs a complex feedback circuit to supply a constant current feed to the line. The loop current may be programmed by applying a DC reference voltage to VREF (pin 16). The loop current can be found using the following equation: It should be noted that above 35mA excessive heat dissipation and clipping of the audio signal may occur. The loop current control fails below loop currents of 12mA. If the loop length is too long, the voltage drop across the combination of line and telephone can prevent the Tip and Ring drivers from supplying the maximum desired loop current. Under these conditions the Tip and Ring drivers become saturated and the audio transmission performance deteriorates. ILOOP = -(0.52 VBAT+ 4.24 VREF) mA For example if VREF is connected to ground and Vbat is -48V, ILOOP = -(0.52 x -48 + 4.24 x 0) mA ILOOP = -(-24.96) ≈ 25 mA If a value for VREF other than 0V is required then a solution is shown in Figure 5. R1 and R2 are chosen to produce the correct voltage and this is buffered by the op-amp. The op-amp provides the low impedance source. So, for example, to generate VREF = -2.5V the values R1 = 300kΩ and R2 = 100kΩ could be used. VREF should be supplied from a low impedance source. 1 µF 17 + A small-signal pnp transistor (e.g. 2N2907A) 470kΩ 10nF CAP MH88615 2.2kΩ Ringing Control RC, Pin 11 Figure 4 - Ringing Filter Circuit +5V R1 + VREF (pin 16) General purpose op-amp (e.g. LM358) R2 100nF -5V Figure 5 - V ref Generation A-282 MSAN-151 Application Note 5. Ringing Voltage Amplifier 5.1 Ringing Oscillator The open circuit output Ringing Voltage is controlled by varying the input voltage at RV, Figure 7 shows a simple Wien Bridge oscillator circuit which may be evaluated for use as a ringing oscillator. The oscillator frequency for this circuit is given by: Vtf-rf = (VR x 60) Vrms Fosc = 1 2π x 68 x 103 x Cring Vren= Vtf-rf X Zren Zren + 200 (assuming zero line length) Thus if a 75Vrms signal is required at Vtf-rf then the input level should be 1.25Vrms. See Figure 9. Do not use excessive ringing input signals which cause clipping and saturation in the ringing voltage amplifier as this interferes with correct ring trip detection and may cause excessive heat dissipation. Frequency (Hz) Cring (nF) 19.5 120 23.4 100 34.4 68 49.8 47 Table 1 - Possible values for Cring Capacitor values are all from the E24 range. The output amplitude is controlled by Ra such that Do not use square waves for ringing input signals as this causes incorrect ring trip detection. Sine wave drive is strongly recommended. However, if necessary, a TTL square wave can be suitably conditioned using the filter shown in Figure 6. Ra ≈ Vo x 500 x 103 2.5 Where Vo is the required input voltage to RV (pin 20). The input to the ringing section, RV (pin 20), must be ground referenced with a low resistance DC path to ground. Any DC offset in the input signal will result in a corresponding shift in the output voltage (multiplied by 60). This may result in clipping of the ringing signal. The input voltage RV must be chosen so that the ringing output is not driven into saturation. The input impedance at this pin is typically 5.5kΩ. The input to RV can be AC coupled using a series 1µF capacitor followed by a 1kΩ resistor to ground. This circuit will not be suitable for all applications especially if low total harmonic distortion of the ringing signal is essential to the application. There are other oscillator circuits, including variations on the one given here, which may also be considered. 5.2 Ringing Supply Voltages During ringing (Ringing Control pin 11 is set to logic 1), the MH88615 uses both the VBAT and the VDCRI TTL square wave 510 Ω To RV (pin 20) + 22µF 1.5kΩ -5V Figure 6 - Suggested Square Wave Filter A-283 MSAN-151 Application Note 71kΩ Ra +5V 33kΩ - U1a 500kΩ 68kΩ + + -5V Cring U1b Cring 68kΩ U1 - e.g. LM358 general purpose op-amp. Figure 7 - Suggested Ringing Oscillator supplies to power the output stage. The maximum open circuit voltage swing that can be accommodated is given by: Vtf-rf = (0.602 VDCRI - 0.6 VBAT -2.0) Vrms. See Figure 8. Vtf-rf is the voltage across the Tip and Ring feed stages and is not accessible externally (see Figure 9). This voltage is fed to line via 2 x 100Ω resistors. To determine the actual Tip-Ring voltage available under worst case load, the maximum line length and maximum Ringer Equivalent Number (R.E.N) must also be taken into account. Rloop is the resistance of the telephone loop. Typical loops have a resistance of 168Ω per km (154Ω. per 3000ft). The typical loop capacitance is 50nF per km (46nF per 3000ft) and so at ringing frequencies (17Hz-68Hz) the capacitive reactance of the line may be ignored. ZLOAD is the ringing load presented by however many instruments are connected to the line. This value is country dependent but a REN of 1 is typically between 7kΩ and 8kΩ. See Figure 9. A-284 6. Protection Circuit If the SLIC is to be used in an exposed or "offpremise" application it will usually be required to withstand certain levels of voltage surge and AC power line contact test conditions, which are specified by the PTT in each country (examples of these are CCITT K20 and Bellcore GR-1089). In practice these conditions originate from lightning strikes or fractured overhead power cables collapsing across telephone lines. It is the customers responsibility to determine these requirements and implement protection in their system. Typically a system will require Primary Protection such as a gas discharge tube at the Main Distribution Frame (MDF) and secondary Protection which usually consists of a series element for overcurrent protection and a shunt element for overvoltage suppression. See Figure 10. MSAN-151 Application Note 100 90 Ringing Voltage (Vrms) 80 70 Vbat=-48V 60 50 Vbat=-28V 40 30 20 10 0 0 60 40 20 100 80 120 DC Supply Voltage Vdcri (Vdc) Figure 8 - Maximum Ringing Voltage Vtf-rf Tip Driver 100Ω Rloop/2 1 Zload VREN Vtf-rf 100Ω 2 Rloop/2 Ring Driver MH88615 ZREN = (ZLoad + RLoop/2 + RLoop/2) Figure 9 - Maximum Ringing Voltage Vtf-rf A-285 MSAN-151 Application Note F1 TIP TIP Z1 MH88615 RING RING F2 F1,2 = Fuse or P.T.C. Z1 = Solid State Transient Suppressor e.g.Fold back diode, M.O.V., transzorb diodes. Figure 10 - Typical Secondary Protection Circuit 7. Audio Interface 8. Design Example The receive direction originates on the 4-wire side (VR), is converted to 2-wire and sent to be received by the telephone. This is an example of how to design a SLIC interface using the MH88615. In the transmit direction, the signal is sent from the telephone (2-wire) to the MH88615 where it is converted to 4-wire and transmitted over the TX line. The SLIC interface must be capable of driving a REN of 5 (1 REN = 7kΩ) up to 7km. The loop current must be set to 25mA. Design Procedure 7.1 Receive Signal 1. Determine the Battery Voltage: The input to this section, VR (pin 15), must be AC coupled (use 220nF capacitors) or DC coupled and ground referenced with a 0V offset. Any DC offsets will result in an error in the loop current. If the AC signal is not ground referenced there will be an error in the loop current. Under these circumstances the signal must not be DC coupled. The input impedance is typically 100kΩ. Assuming standard 0.5mm telephony cabling the loop resistance is 7 x 168Ω = 1200Ω approximately. Tip-Ring Voltage is 1500 x 0.025 = 37.5V. The voltage drop in the feed resistors is 2 x 100 x 0.025 = 5V. This gives Vtf-rf = 42.5V. The transistors in the driver need approximately 2.5V collector-emitter bias, so choosing a battery voltage of -48V is appropriate. 7.2 Transmit Signal 2. Determine the Reference Voltage setting. The output of this section, VX (pin 18), is DC coupled. The output impedance is typically 10Ω. A-286 The desired loop current is 25mA. With a battery Voltage of -48V VREF should be connected directly to ground. MSAN-151 Application Note Vtf-rf 200Ω 1200Ω RFEED RLINE ~ 1400Ω 40Vrms ZLOAD 10 µ F Figure 11 - Ringing Equivalent Circuit 3. Determine the Ringing Voltage output requirement. 9. Additional Reference Material MH88615 Data Sheet. Assuming that each telephone requires 40V to energize the bell, then VLOAD = 40V. As the load is 5 telephones, each with an impedance of 7kΩ in parallel, then the total RLOAD is 1400Ω. The equivalent circuit is shown in Figure 11. Vtf-rf = 2800 x 40/1400 = 80Vrms MSAN-131 Subscriber Line Interface for Digital Switching Systems. Glossary of Telecommunications Terms (see section G1 of Zarlink Telecom Components Data Book issue 10). 4. Determine the Ringing Power Supply requirement. As a ringing voltage of 80Vrms is required, with a battery voltage of -48V, from Figure 8 the minimum VDCRI is 90V approximately. Choosing 96V DC for VDCRI ensures additional headroom and may be generated from -48V by using a voltage triple circuit. 5. Determine the Ringing Voltage input. RV=Vtf-rf /60 RV = 80/60 = 1.33 Vrms 6. This design can be implemented using the schematic of a Basic Application Circuit shown in Figure 1. Use VBAT = -48V VREF = 0V VDCRI = 90V VR = 1.33V A-287 MSAN-151 Notes: A-288 Application Note For more information about all Zarlink products visit our Web Site at www.zarlink.com Information relating to products and services furnished herein by Zarlink Semiconductor Inc. trading as Zarlink Semiconductor or its subsidiaries (collectively “Zarlink”) is believed to be reliable. However, Zarlink assumes no liability for errors that may appear in this publication, or for liability otherwise arising from the application or use of any such information, product or service or for any infringement of patents or other intellectual property rights owned by third parties which may result from such application or use. Neither the supply of such information or purchase of product or service conveys any license, either express or implied, under patents or other intellectual property rights owned by Zarlink or licensed from third parties by Zarlink, whatsoever. Purchasers of products are also hereby notified that the use of product in certain ways or in combination with Zarlink, or non-Zarlink furnished goods or services may infringe patents or other intellectual property rights owned by Zarlink. This publication is issued to provide information only and (unless agreed by Zarlink in writing) may not be used, applied or reproduced for any purpose nor form part of any order or contract nor to be regarded as a representation relating to the products or services concerned. The products, their specifications, services and other information appearing in this publication are subject to change by Zarlink without notice. No warranty or guarantee express or implied is made regarding the capability, performance or suitability of any product or service. Information concerning possible methods of use is provided as a guide only and does not constitute any guarantee that such methods of use will be satisfactory in a specific piece of equipment. It is the user’s responsibility to fully determine the performance and suitability of any equipment using such information and to ensure that any publication or data used is up to date and has not been superseded. Manufacturing does not necessarily include testing of all functions or parameters. These products are not suitable for use in any medical products whose failure to perform may result in significant injury or death to the user. All products and materials are sold and services provided subject to Zarlink’s conditions of sale which are available on request. Purchase of Zarlink s I2C components conveys a licence under the Philips I2C Patent rights to use these components in and I2C System, provided that the system conforms to the I2C Standard Specification as defined by Philips. Zarlink and the Zarlink Semiconductor logo are trademarks of Zarlink Semiconductor Inc. Copyright 2001, Zarlink Semiconductor Inc. All Rights Reserved. TECHNICAL DOCUMENTATION - NOT FOR RESALE For more information about all Zarlink products visit our Web Site at www.zarlink.com Information relating to products and services furnished herein by Zarlink Semiconductor Inc. or its subsidiaries (collectively “Zarlink”) is believed to be reliable. However, Zarlink assumes no liability for errors that may appear in this publication, or for liability otherwise arising from the application or use of any such information, product or service or for any infringement of patents or other intellectual property rights owned by third parties which may result from such application or use. Neither the supply of such information or purchase of product or service conveys any license, either express or implied, under patents or other intellectual property rights owned by Zarlink or licensed from third parties by Zarlink, whatsoever. Purchasers of products are also hereby notified that the use of product in certain ways or in combination with Zarlink, or non-Zarlink furnished goods or services may infringe patents or other intellectual property rights owned by Zarlink. This publication is issued to provide information only and (unless agreed by Zarlink in writing) may not be used, applied or reproduced for any purpose nor form part of any order or contract nor to be regarded as a representation relating to the products or services concerned. The products, their specifications, services and other information appearing in this publication are subject to change by Zarlink without notice. No warranty or guarantee express or implied is made regarding the capability, performance or suitability of any product or service. Information concerning possible methods of use is provided as a guide only and does not constitute any guarantee that such methods of use will be satisfactory in a specific piece of equipment. It is the user’s responsibility to fully determine the performance and suitability of any equipment using such information and to ensure that any publication or data used is up to date and has not been superseded. Manufacturing does not necessarily include testing of all functions or parameters. These products are not suitable for use in any medical products whose failure to perform may result in significant injury or death to the user. All products and materials are sold and services provided subject to Zarlink’s conditions of sale which are available on request. Purchase of Zarlink’s I2C components conveys a licence under the Philips I2C Patent rights to use these components in and I2C System, provided that the system conforms to the I2C Standard Specification as defined by Philips. Zarlink, ZL and the Zarlink Semiconductor logo are trademarks of Zarlink Semiconductor Inc. Copyright Zarlink Semiconductor Inc. All Rights Reserved. TECHNICAL DOCUMENTATION - NOT FOR RESALE