BB PWS728

®
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
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