® PWS727 PWS728 Isolated, Unregulated DC/DC CONVERTERS FEATURES APPLICATIONS ● 100% TESTED FOR HIGH VOLTAGE BREAKDOWN ● INDUSTRIAL PROCESS CONTROL ● GROUND LOOP ELIMINATION ● COMPACT (28-pin DIP) ● 5V OR 15V INPUT OPTIONS ● SYNCHRONIZABLE (TTL) ● POINT-OF-USE ANALOG POWER DESCRIPTION The PWS727 is a DC/DC converter which uses minimal PC board space and converts a single input 10 to 18VDC to bipolar voltages of the same value as the input. The PWS728 converts a 4.7 to 6VDC to bipolar voltages three times the value of the input. The converters are capable of providing ±15mA (PWS727) or ±12mA (PWS728) at rated voltage and up to ±30mA. The PWS727 and PWS728 use a high-frequency (800kHz nominal) surface mount oscillator that makes it possible to minimize the transformer size. The transformer is composed of a split bobbin isolation transformer using a ferrite core and is encapsulated in a plastic package, allowing a higher isolation voltage rating. The design minimizes high frequency radiated noise on the output by using a ground plane directly under the high frequency components. * Use a 10V zener on PWS728 to prevent input spikes overstressing the diode bridge. International Airport Industrial Park • Mailing Address: PO Box 11400 Tel: (602) 746-1111 • Twx: 910-952-1111 • Cable: BBRCORP • • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706 Telex: 066-6491 • FAX: (602) 889-1510 • Immediate Product Info: (800) 548-6132 © PDS-987A 1989 Burr-Brown Corporation 1 Printed in U.S.A. June, 1992 PWS727/728 SPECIFICATIONS ELECTRICAL At VIN =15VDC, Output Load = ±15mA (PWS727) and TA = +25°C unless otherwise noted. Or VIN = 5VDC, Output Load = ±12mA (PWS728) and TA = +25°C unless otherwise noted PWS727 PARAMETER ISOLATION Voltage Rated Continuous AC 60Hz 100% Test(1) Barrier Impedance Leakage Current at 60Hz CONDITIONS MIN 60Hz, 1s, <5pC PD 750 1200 Sensitivity Balance Ripple Voltage Output Switching Noise Output Voltage Temp Coefficient Sync Sync Frequency(2) MAX MIN TYP MAX UNITS * Vrms Vrms Ω || pF µArms * * 1012 || 8 1 VISO = 240Vrms, 60Hz INPUT Rated Voltage Voltage Range Current Current Ripple Current Limit TTLIN, IIH IIL VIH VIL Frequency Range TTLOUT, IOL OUTPUT Rated Output Voltage Current Regulation (single ended load change) TYP PWS728 * * 1.5 15 10 55 2.5 5 18 65 * * * * 15 V V mA mAp-p mAp-p nA µA V V MHz mA * * 12 * 30 VDC mA 4.5 6 130 110 7.5 250 10 * * –1 * 2.0 * 1 1.6 .8 2.5 15 14.25 15.0 15 15.75 30 +IO = 7.5 –15mA, –IO = 15mA VIN = 10 to 18 V 45 135 1.13 mV/mA 3.8 15 At Switching Frequency >800kHz TEMPERATURE RANGE Specification Operation Storage 10 20 .05 Loading 725 800 0 –40 –40 * * V/V mV mVpp mVpp V/°C kΩ || pF kHz * * * °C °C °C 25 * * * 20 || 40 875 * 70 85 85 * * * * * Specifications the same as PWS727. NOTE: (1) Tested at 1.6 x rated, fail on 5pC partial discharge leakage current on 5 successive pulses. (2) Nominal with pin 17 grounded. ORDERING INFORMATION PIN CONFIGURATION Basic Model Number _________________________________ PWS727 PWS728 ABSOLUTE MAXIMUM RATINGS Supply Voltage ..................................................................................... 18V Continuous Isolation Voltage ....................................................... 750Vrms Junction Temperature .................................................................... +150°C Storage Temperature ......................................................................... 85°C Lead Temperature (Solder, 10s) ...................................................... 300°C Output Short to Common ......................................................... Continuous Max Load, Sum of Both Outputs ....................................................... 60mA PWS727/728 2 NC 1 28 +VO Sync 2 27 Output Gnd NC 3 26 –VO NC 4 25 NC NC 11 18 NC VD 12 17 TTLIN Input Gnd 13 16 TTLOUT +VIN 14 15 Enable MECHANICAL 28-Pin Double — Wide DIP DIM A B C G H K L INCHES MIN MAX 1.440 1.460 .690 .710 .390 .410 .100 BASIC .020 BASIC .190 .210 .600 BASIC MILLIMETERS MIN MAX 36.58 37.08 17.53 18.03 9.91 10.41 2.54 BASIC 0.51 BASIC 4.83 5.33 15.24 BASIC NOTE: Leads in true position within 0.01" (0.25mm) R at MMC at seating plane. TYPICAL PERFORMANCE CURVES TA = +25°C, VIN = 15VDC, ILOAD = ±15mA (PWS727), or VIN = 5VDC, output load = ±12mA (PWS728) unless otherwise noted. 3 PWS727/728 TYPICAL PERFORMANCE CURVES (CONT) THEORY OF OPERATION The PWS727 and PWS728 are composed of the PWS750 building blocks which are assembled along with some standard components to build an isolated push pull DC/DC converter. TA = +25°C, VIN = 15VDC, ILOAD = ±15mA (PWS727), or VIN = 5VDC, output load = ±12mA (PWS728) unless otherwise noted. PWS727 AND PWS728 PIN FUNCTIONS TTLIN is used to optionally control the frequency of the oscillator with an external TTL level frequency source. The input frequency must be twice the desired driver frequency, since there is an internal divide by 2 circuit to produce a 50% duty cycle output. The input duty cycle can vary from 12% to 95%, (see the Typical Performance Curves). When in the free running mode the TTLIN pin must be tied to ground. TTLOUT is used to synchronize the outputs of multiple PWS727s and/or PWS728s to minimize beat frequency problems if desired. A standard open collector output is provided, therefore a 330 to 3.3kΩ resistor will be necessary, depending on the amount of stray capacitance on the TTLIN line. A maximum of 8 PWS727s or PWS728s can be connected without the use of an external TTL buffer. Connect the TTLOUT of a master unit to the TTLIN of the slave units. An Enable pin is provided so that the driver can be shut down to minimize power use if required. A TTL low applied to the pin will shut down the driver within one cycle (1.25µs). A TTL high will enable driver outputs within one cycle. The TTLOUT will still have an 800kHz signal when a master driver is disabled, so other synchronized drivers will not be shut down. The pin can be left open for normal operation. The +VIN pin supplies power to the converter. The VD pin connects the power to the transformer through the internal overcurrent sense resister. The other end of the overcurrent sense resister is tied to +VIN. A 0.3µF bypass capacitor must be connected to the VD pin to reduce the ripple current through the shunt resistor, otherwise false current limit conditions can occur due to ripple voltage peaks. During overload conditions the output drive shuts off for approximately 80µs, then turns back on for 20µs, resulting in a 25% power up duty cycle. If the overload condition still exists, then the output will shut off again. When the fault or the excessive load is removed, the converter resumes normal operation. The Sync pin can be used to synchronize the internal oscillator of an ISO120 to the operating frequency of the converter. The Sync pin is connected directly to the secondary of the transformer, so an 800kHz square wave of twice the output magnitude is present. Minimum pc board trace length should be used to minimize loading the transformer. When making the connection to the ISO120, a simple frequency divider circuit (see Figure 3) is necessary to match the 400kHz nominal frequency required by the ISO120. If this function is not used, leave the pin disconnected. PWS727/728 4 12-bit linearity, 20µs conversion time is possible with this circuit. NOTE: See page 276 of the Burr-Brown Handbook of Linear Applications for details of the counter operation. FIGURE 1. Isolated Integrated A/D Conversion System Using Ratiometric Counting and Microprocessor Interface, Operating from a Single 5V Supply. FIGURE 2. Regulated Output Using the Enable Pin to Cycle the Converter Output On and Off. 5 PWS727/728 FIGURE 3. The PWS727 or PWS728 Can Be Used to Synchronize the ISO120. The Output Sync Must be Divided by 2 and the TTLOUT Must Be Divided by 4. PWS727/728 6 HIGH VOLTAGE TESTING Burr Brown Corporation has adopted a partial discharge test criterion that is similar to the German VDE0884 optocoupler standard. This method requires that less than 5pC partial discharge across the isolation barrier with 1200 Vrms 60Hz applied. This criterion confirms transient overvoltage (1.6 x 750 Vrms) protection without damage to the PWS727 or PWS728. Lifetest results verify the absence of high-voltage breakdown under continuous rated voltage and maximum temperature. BASIC OPERATION PC BOARD LAYOUT CONSIDERATIONS These converters have low output ripple so that standard layout methods are all that is necessary for good circuit performance. Bypass capacitors should be placed close to each device powered by the converters. To minimize the noise pickup by sensitive (12 bit resolution) circuits, a 10Ω resistor can be placed in series with the isolated power traces. The resistor in conjunction with the bypass capacitor acts as a low pass filter, to minimize isolation amplifier PSRR related ripple voltage feedthrough. A π filter can also be used at the input (+VIN) to minimize input supply ripple current. The minimum AC barrier voltage that initiates partial discharge above 5pC is defined as the “inception voltage”. Decreasing the barrier voltage to a lower level is required before partial discharge ceases and is known as “extinction voltage”. We have developed a package insulation system to yield an inception voltage greater than 1200 Vrms so that transient voltages below this level will not damage the isolation barrier. The extinction voltage is above 750 Vrms so that even overvoltage-induced partial discharge will cease once the barrier voltage is reduced to the rated value. Previous high voltage test methods relied on applying a large enough overvoltage (above rating) to break down marginal devices, but not so high as to damage good ones. Our partial discharge testing gives us more confidence in barrier reliability than breakdown/no breakdown criteria. 5V OPERATION With 5V operation (PWS728) the transformer winding ratio is 3:1, therefore generating much greater currents in the primary. The input ripple will be larger, so an input π filter will be necessary to isolate the supply noise from the rest of the circuit. For example, when the output load is ± 15mA the input current will be approximately 120mA. OUTPUT CURRENT RATING The PWS727 and PWS728 contain soft start circuitry to protect the FETs from high inrush currents during turn on. The internal input current limit is 250mA peak to prevent thermal overload of the MOSFETs. The maximum output current rating is ±30mA. Total current, which can be drawn from each isolation channel, is the total of the of the power being drawn from both the +V and –V outputs. For example, if one output is not used, then maximum current can be drawn from the other output. In all cases the maximum current that can be drawn from any individual channel is: (+IOUT) + (–IOUT) < 60mA It should be noted that many analog circuit functions do not simultaneously draw equal current from both the positive and negative supplies. The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support devices and/or systems. 7 PWS727/728 This datasheet has been download from: www.datasheetcatalog.com Datasheets for electronics components.