Application Note: AN-157 Increased LITELINK™ III Transmit Power AN-157-R04 www.ixysic.com 1 Application Note: AN-157 1. Introduction Some applications, such as voice computer telephony, require higher power transmission from the host equipment to the telephone network. This application note describes changes to the standard LITELINK III application circuits to provide the transmit gain needed for a particular application. R NTF < 5. Use the values from step 4 in the following formulas to solve for the new RNTX and RHTX values: Note: The recommended maximum drive level into the line from LITELINK III is 6 mA peak, which is equivalent to about +10 dBm into a 600 Ω load. Datasheet specifications for maximum line-side current draw (IDDL) do not apply with higher-thanspecified output power. Minimum line operating current is defined as follows: 0.625V + 5.5mA I MIN = -----------------------------R ZTX || R ZNT Where RZNT is the resistive component of ZZNT. Higher transmit signal levels may require a higher dc bias level on the loop to meet peak voltage requirements and avoid signal compression. The value of RDCS2 may be reduced to increase the dc loop bias level if necessary. See AN-158, LITELINK III Application Circuit Calculations for more information. R NTF -----------AV ( (• ( (• R ZNT R NTX = 1 + --------------2R ZTX R ZNT R HTX = 1 + -----------R ZTX R NTF ------------2 200kΩ These calculations result in a transmit (4-wire to 2wire) gain of AV, and a receive gain (2-wire to 4-wire) of 1/AV. If necessary, the receive loss can be compensated with either the programmable input gain of a CODEC or with a discrete op-amp gain stage between the LITELINK RX output and the host system. 3. Examples 3.1 PBX Example To meet +3.18 dBm into 900 Ω, use the following calculations: 2. Transmit Gain Design Procedure 1. Determine the peak transmit level needed for the application. 2. If necessary, convert the required peak transmit power into a voltage level. For example, 0 dBm into 600 Ω = 1.1 VP. 3. Calculate the required linear voltage gain AV by dividing the peak transmit voltage level from step 2 by 1.1. 4. Modify the following application circuit component values: +3.18 dBm into 900 Ω = 1.935 VP. 1.935 V/1.1 V = 1.76. The calculations work out as follows: Z ZNT < Z ZNT ⁄ A V AN-157-R04 www.ixysic.com R ZNT = 453 ⁄ 1.76 = 256.9 R NTF = 499kΩ ⁄ 1.76 = 282938 The closest standard resistor values are 255 Ω for RZNT and 280 kΩ for RNTF. Use these values to find for RNTX and RHTX as follows: 2 Application Note: AN-157 255 R NTX = 1 + -----------6640 ( ( • 140000 = 145.38kΩ ( ( • 200000 = 215.36kΩ 255 R HTX = 1 + -----------3320 The closest standard resistor values are 147 kΩ for RNTX, and 215 kΩ for RHTX. 3.2 +3 dBm into 600 Ohm Transmit Power Example The following application circuit uses component values determined by the design procedure above for +3 dBm transmit power into 600 Ω. Peak transmit power of +3 dBm into 600 Ω = 1.55 VP . 1.55/1.1 = 1.4. The calculations work out as follows: R ZNT = 301 ⁄ 1.4 = 215 R NTF = 499kΩ ⁄ 1.4 = 356429 The closest standard resistor values are 215 Ω for RZNT and 357 kΩ for RNTF. Use these values to find for RNTX and RHTX as follows: ( ( •178500 = 184.28kΩ ( ( • 200000 = 212.95kΩ 215 R NTX = 1 + -----------6640 215 R HTX = 1 + -----------3320 Standard resistor values have been substituted in the circuit in Figure 1. AN-157-R04 www.ixysic.com 3 Application Note: AN-157 Figure 1. +3 dBm Application Circuit 3.3 or 5 V R23² 10 C1 1 FB1 600 Ω 200 mA C16 10 C9 0.1 A U1 LITELINK A 1 R1 (RTX) 80.6K 1% 2 C13 0.1 3 TX- C2 0.1 TX+ 4 5 6 7 8 OH VDD REFL TXSM TXF TX- ZTX TX+ ZNT TX TXSL MODE BR- GND NTS OH GAT 9 RING 10 CID RING CID C14 0.1 11 RX12 RX+ C4 0.1 13 SNP+ 14 SNP- RXRX+ 15 RXF 16 RX A C10 0.01 500V BR- NTF DCS1 DCS2 ZDC BRRPB RXS VDDL R2 (RRXF) 130K 1% 32 31 R5 (RTXF) 60.4K 1% C15 0.01 500V 30 29 28 27 25 24 R12 (RNTF) 357K 1% 22 R15 (RDCS2) 1.69M 1% 21 20 R16 (RZDC) 8.2 1% 19 R76 (RHNTF) 200K 1% 18 R21 (RDCS1B) 6.2 M 1% C12 (CDCS) 0.027 C21 (CGAT) 100 pF BR- R20 (RVDDL) 2 BR- + DB1 17 R4 (RPB) 68.1 1% BR- R18 (RZTX) 3.32 K 1% R8 (RHTX) 215K 1% TIP - SP1¹ 1 BR- C18 15 pF³ BR- R10 (RZNT) 215 1% BR- 2 RING NOTE: Unless otherwise noted, all resistors are in Ohms, 5%. All capacitors are in microFarads. C7 (CSNP-) 220pF 2000V R3 (RSNPD) 1.5M 1% R22 (RDCS1A) 6.8 M 1% R14 (RGAT) 47 26 23 BR- Q1 CPC5602C R13 (RNTS) 1M 1% R75 (RNTX) 182K 1% R6 (RSNP-2) 1.8M 1/10W 1% R44 (RSNP-1) 1.8M 1/10W 1% R7 (RSNP+2) C8 (CSNP+) 1.8M 1/10W 1% 220pF 2000V R45 (RSNP+1) 1.8M 1/10W 1% 4. Receive Gain 5. LITELINK Design Resources This application note addresses LITELINK III transmit gain. It must be noted, however, that increasing LITELINK III transmit gain decreases receive gain by a corresponding amount. For instance, increasing transmit power to +3 dBm results in receive gain of -3 dB. This difference can be accounted for by adding a 6 dB gain block between the RX+/RX- pins and the host circuitry, but can be adjusted using CODEC gain (for CODEC applications). 5.1 Design Resources www.ixysic.com has a wealth of information useful for designing with LITELINK, including application notes and reference designs that already meet all applicable regulatory requirements. LITELINK data sheets also contains additional application and design information. See the following links: LITELINK datasheets and reference designs Application note AN-117 Customize Caller-ID Gain and Ring Detect Voltage Threshold AN-157-R04 www.ixysic.com 4 Application Note: AN-157 Application note AN-146, Guidelines for Effective LITELINK Designs Application note AN-152 LITELINK II to LITELINK III Design Conversion Application note AN-155 Understanding LITELINK Display Feature Signal Routing and Applications For additional information please visit our website at: www.ixysic.com IXYS Integrated Circuits Division makes no representations or warranties with respect to the accuracy or completeness of the contents of this publication and reser t descriptions at any time without notice. Neither circuit patent licenses nor indemnity are expressed or implied. 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Specification: AN-157-R04 ©Copyright 2014, IXYS Integrated Circuits Division All rights reserved. Printed in USA. 4/14/2014 AN-157-R04 www.ixysic.com 5