DESIGN FEATURES Versatile Ring Tone Generator Finds Uses in Motor Drivers and Amplifiers by Dale Eagar Overview The LT1684 was specifically designed for OEM telephone equipment. Its function is to interface between the digital control logic and the high voltage analog phone line. When used in the design of a phone system, the LT1684 allows software to control the frequency, voltage and cadence of the ring signal. Because of similarities of application to telephone systems, the LT1684 ring tone chip finds itself at home in motor drives, digital input amplified speakers, alarm systems and sine wave UPS systems. ISOLATION BARRIER 5V VDIGITAL DIGITAL STUFF + HIGH VOLTAGE PWM –HIGH VOLTAGE DIGITAL GROUND The Circuit The LT1684 provides the tool set to easily implement a digital, pulse width modulated (PWM) signal to DCcoupled voltage converter (at high currents), while providing isolation, switching frequency filtering and output protection. Figure 1 is a system-level diagram of the LT1684 in action. By controlling a pair of external MOSFETs, the LT1684 utilizes their inherent robustness while pro- viding control of the output voltage and current. In its telecom application circuit, the LT1684 provides up to ±240V of smooth, clean output at up to 200mA of output current. Higher output voltages are obtained by cascoding MOSFETs, while higher currents are readily achieved by using the LT1166 MOSFET automatic bias generator chip as a companion. DC ISOLATION P1 µC P2 C2 100pF +100V C5 6.8nF R1 10k R7 100k GATE + IN A R2 10k R8 100Ω FB1 LIM + OUT ATREF BGOUT LT1684 R3 3k D1 1N4001 + C6 100pF RING-TONE OUTPUT – COMP1 R6 5.1k AMPIN R5 300k C4 4700pF COMP2 LIM – V– R5 C4 D2 1N5817 R3 R9 100Ω GATE– C3 1µF Figure 2 is the schematic of the LT1684 implementing a digital-PWMto-ringing-telephone converter. This is something like a high power, high voltage, isolated, output filtered D/A converter. Like its DAC counterpart, the LT1684 has a precision reference, switches and an output amplifier. Unlike its DAC counterpart, it includes post-conversion ripple filtering, isolation and a robust high voltage output. In addition to the isolation, filtering and amplification, the LT1684 provides the gate-bias control and gate voltage protection for the two external MOSFETs. Providing such a plethora of functions from a single, monolithic IC requires the use of a somewhat tricky circuit. This circuit C9 0.1µF C7 20pF R4 2k Q1 IRF610 V+ IN B HIGH VOLTAGE GROUND Figure 1. The LT1684 uses differential pulse width modulation to provide isolation for digitally controlled analog power solutions. Introducing the LT1684 C1 100pF HIGH VOLTAGE LOAD LT1684 STUFF C8 6.8nF R10 100k R4 – C3 + Q2 IRF9610 –100V FB1: FERRONICS FMB1601 (716) 388-1020 Figure 2. Typical LT1684 digital-PWM-to-ringing-telephone converter application 28 1684 TA01 Figure 3. The basic 2nd order lowpass MFB filter, as implemented by the LT1684 Linear Technology Magazine • September 1999 DESIGN FEATURES 47Ω 100Ω 2N3906 120V 100V 2N3906 100Ω 100k 100Ω 6800pF MTP2N50E IRF230 1nF 1 11 1000pF 10k 14 PWM IN FB1 GATE+ V+ IN A COMP1 1 1000pF 10k 13 3k 2k OUT LT1684 ATREF BGOUT COMP2 5.1k 12 – 0.1µF 6 6800pF ILIM+ 7 1k 9 180µH 8 0.22Ω LT1166 2k 2 VOUT VIN 3 1µF 6 1µF 0.22Ω 7 3 V– 5 100Ω 8 3.9k 100pF 2 4 GATE SENSE+ 10 – LIM AMPIN 470pF 300k LIM+ IN B VTOP ILIM– 20pF 1µH 5kW LOAD 0.22Ω 1k 180µH SENSE– VBOTTOM MTP2N50E 5 4 1nF 100Ω IRF9240 100k 2N3904 –100V 2N3904 –120V FB1: FERRONICS FMB1601 100Ω 47Ω TYPICAL POWER SLICE (1 OF 13 IN PARALLEL) (716) 388-1020 Figure 4. 5kW PWM-to-analog converter nents, in fact, form the 2nd order MFB filter shown in Figure 3. The values chosen for these components in Figure 2 implement a 2nd order Butterworth MFB lowpass filter with a cutoff frequency of 100Hz and a DC gain of 100. These were chosen to provide ±80V of output swing with PWM duty factors of 10% to 90%, while filtering the 10kHz PWM ripple to meet telephone specifications. is arrived at by applying a circuit transformation to a simple filter circuit. This transformation is performed on the basic 2nd order lowpass multiple feedback filter (MFB) circuit and ends up looking somewhat like the filter/amplifier shown in Figure 3. The filter/amplifier components in Figure 2 are R3–R5 and C3 and C4. Looking backwards through the circuit transformation, these compo- 1V 5V TRIANGLE IN 10kHz 5V C1 100pF 0 –1V + LT1671 – ANALOG IN R1 10k 14 RTERM 120Ω LT1784 LT1684 1 RS485 3kHz LPF C2 100pF IN A IN B R2 10k –5V 0.8V 0 –0.8V Figure 5. A remote, isolated, analog input amplifier using a robust RS485 driver and a terminated, twisted-pair line Linear Technology Magazine • September 1999 Stealing the LT1684 for Use In Other Applications The LT1684, used as shown in Figure 1, outputs a ring signal that meets Belcore specification. This means we can ring a phone 22,000 feet away. The LT1684 fits another role, where 22,000 feet of separation would be a nice minimum. This is the application of the LT1684 in the scaleable power amplifier, as detailed in Figure 4. This amplifier can be used to drive motors, simulate the power company in sine wave UPS systems and operate large audio drivers. Because of its scaleable nature, this design can be used at any power level. The circuit in Figure 4 is shown implementing a 5000W bits-to-decibels converter. When this converter is implemented with the appropriate audio drivers and enclosures, the output sound pressure level can be significant—so significant, in fact, that the author suggests giving it a wide berth of at least 22,000 feet. continued on page 35 29 DESIGN IDEAS SMBus Controlled CCFL Power Supply by Jim Williams Figure 1 shows a cold cathode fluorescent lamp (CCFL) power supply that is controlled via the popular SMBus interface. The LT1786 CCFL switching regulator receives the SMBus instruction. The IC converts this instruction to a current, which appears at the IOUT pin. This current, routed to the ICCFL pin, provides a set point for switching regulator operation. The resultant duty cycle at the VSW pin pulls current through L2. L2, acting as a switched current sink, drives a resonant Royer converter composed of Q1–Q2, C1 and L1. The high voltage sine wave produced at D1 BAT85 1 2 3 CCFL PGND CCFL VSW ICCFL BULB 16 1 Local historians can’t be certain, but this may be the only IC pin ever named after a person. L1 = COILTRONICS CTX210605 BAT LT1786F 13 CCFL VC ROYER 5 12 6 7 8 AGND VCC SHDN IOUT SMBSUS SCL ADR SDA 11 2 + R2 220k + C4 2.2µF 3V ≤ VCC ≤ 6.5V 10 9 L2 = COILTRONICS CTX100-4 *DO NOT SUBSTITUTE COMPONENTS 1 4 COILTRONICS (561) 241-7876 5 + C3B 2.2µF 35V C3A 2.2µF 35V BAT 8V TO 28V R1 750Ω C1* 0.068µF R3 100k Q2* TO SMBus HOST C1 MUST BE A LOW LOSS CAPACITOR, C1 = WIMA MKI OR MKP-20 (914) 347-2474 = PANASONIC ECH-U (201) 348-7522 (516) 543-7100 6 L1 3 14 DIO C2 27pF 3kV C5 1000pF 15 4 C7, 1µF Q1, Q2 = ZETEX ZTX849 References: 1. Williams, Jim. Linear Technology Application Note 65: A Fourth Generation of LCD Backlight Technology. November 1995. 2. LT1786F Data Sheet. Linear Technology Corp. 1998. LAMP 10 SHUTDOWN instruction codes with attendant RMS lamp current. Detailed information on circuit operation and measurement techniques appears in the references below. L2’s secondary drives the floating lamp. Current flow into the Royer converter is monitored by the IC at pin 13 (“Royer” in Figure 1).1 Royer current correlates tightly with lamp current, which, in turn, is proportional to intensity. The IC compares the Royer current to the SMBus-derived current, closing a lamp-intensity control loop. The SMBus permits wide-range regulated lamp-intensity control and allows complete IC shutdown. Optimal display and lamp characteristics permit 90% efficiency. The circuit is calibrated by correlating SMBus Q1* L2 100µH 0µA TO 50µA ICCFL CURRENT GIVES 0mA TO 6mA LAMP CURRENT FOR A TYPICAL DISPLAY. D1 1N5818 FOR ADDITIONAL CCFL/LCD CONTRAST APPLICATION CIRCUITS, REFER TO THE LT1182/83/84/84F DATA SHEET OR ROHM 2SC5001 (800) 955-7646 Figure 1. 90% efficient floating CCFL with 2-wire SMBus lamp-current control Ring Tone, continued from page 29 Analog Inputs Welcome The scaleable amplification system detailed in Figure 4 can be driven with analog inputs while still maintaining full isolation. Such a system is detailed in Figure 5, where the analog input is filtered (to prevent ailiasing) and converted to PWM. Figure 5 goes on to show the use of an Linear Technology Magazine • September 1999 Conclusion RS485 differential driver to drive a twisted pair line. The receiver end of the twisted pair line is terminated with a resistor and put across the isolation barrier. This provides very good ESD protection on both ends of the line. The LT1684 is useful in a wide variety of applications. The LT1684 is a highly integrated solution for use in any system that requires digital control of high output voltage or high output power. 35