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78253 SERIES
Application Notes
The 78253 series of converter transformers are specifically designed for use with the MAX253 chip set to provide isolated power supplies. The 5V version can supply 1W and the 3.3V version can supply 500mW. A centre tapped secondary winding allows for full bridge, half bridge or voltage doubling. The surface mount devices are fully compatible with CECC00802 to 280C which allows them to be placed and reflowed with IC's thus reducing time and cost in production.
TEST CIRCUIT
All characterisation curves are derived from tests using the test circuit shown in figure 1 featuring a MAX253 driver IC, a pair of ZDX6 fast rectifier diodes and a 10F tantalum output capacitor. Figure 1: Test Circuit
APPLICATIONS
RECTIFICATION CIRCUITS The 78253 series are designed to provide a 5V output from either 3.3V or 5V supply to the MAX253 IC. Other output configurations can be derived to produce positive and negative 5V outputs or voltage doubled output (for application circuits see www.maxim-ic.com). Under rectification schemes other than the standard single rail 5V output, the total power drawn from the circuit must not exceed 1W for a 5V input supply and 500mW for a 3.3V input supply. DIODE SELECTION The switching circuit can operate at a relatively high frequency (maximum 500kHz), hence, high speed rectifiers ar e recommended. If operating at low load levels (less than 50% maximum output current) low cost parts can be used even though they exhibit a high voltage drop. This is because at low output load the transformer output voltage is quite high and these lower cost parts will bring the voltage into regulation. When operating at high load demands, high efficiency, low drop Schottky diodes ar e recommended, the final choice is left to the designer. It must be noted that different component combinations will produce different characteristic curves to those shown in this data sheet. INPUT FILTERING The MAX253 IC driver and 78253 series transformer form a switching converter typically operating at either 200kHz (FS = LOW) or 350kHz (FS = HIGH). The circuit can therefore introduce some switching noise into the supply line feeding the MAX253. At C&D Technologies (NCL) we recommend filtering this supply locally via a capacitor and inductor filter at the MAX253 to reduce the noise introduced to other circuits operating from the same supply rail (see figure 2). OUTPUT FILTERING The output ripple from the rectifier circuit can be reduced further by a series inductor and second filter capacitor forming a pi-filter with the original smoothing capacitor (see figure 3). The values shown reduce ripple to less than
Figure 2: Input Filtering
Figure 3: Output Filtering
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78253 SERIES
Application Notes
10mV at full load, further reductions can be achieved by using larger capacitor values. The largest recommended capacitor for the first stage smoothing is 22F, up to 47F can be used if required after the series inductor. The output filter capacitors should exhibit low effective equivalent series resistance (ESR) at the switching frequency. Tantalum or ceramic capacitors are the recommended choice of dielectric. TRANSFORMER ISOLATED ADC The circuit in figure 4 offers a very low quiescent current for an isolated interface by using logic compatible tranfomer isolator/translator devices (1635). The isolators require some additional logic to interface with the ADC and a 3.3V logic controller, a few 74HC IC's and two delay lines are all that is required to implement full control over the ADC and its output. Figure 4: Transformer Isolated ADC
Figure 5: Typical Characteristics (Voltage Curves)
VOLTAGE CURVES
10 8 6 4 2 0 0 20 40 60 80 100 120 Output Current (mA) FS=Low FS=High 78253/35(M) 10 8 6 4 2 0 0 100 200 300 Output Current (mA) 400 78253/55(M) FS=Low/High 10 8 6 4 2 0 0 20 40 60 80 Output Current (mA) 100 FS=High FS=Low 78253/35(M)V 10 8 6 4 2 0 0 100 200 300 Output Current (mA) 400 FS=High FS=Low 78253/55(M)V
EFFICIENCY CURVES
100 80 60 40 20 0 0 20 40 60 80 100 120 Output Current (mA) 78253/35(M) FS=High FS=Low 100 80 60 40 20 0 0 100 200 300 Output Current (mA) 400 FS=Low/High 78253/55(M) 100 80 60 40 20 0 0 20 40 60 80 Output Current (mA) 100 FS=Low 78253/35(M)V FS=High 100 80 60 40 20 0 0 100 200 300 Output Current (mA) 400 FS=High 78253/55(M)V FS=Low
VOLTAGE DEVIATION
110 105 100 95 90 110 110 110
78253/35(M) FS=High
78253/55(M)
105 100
FS=High FS=Low
105 100 95 90
78253/35(M)V FS=High
78253/55(M)V
105
FS=Low FS=Low
100
FS=Low
FS=High
95 90
95 90
-50
-25
0 25 50 75 100 Temperature (C)
-50
-25
0 25 50 75 100 Temperature (C)
-50
-25
0 25 50 75 100 Temperature (C)
-50
-25
0 25 50 75 100 Temperature (C)
C&D Technologies (NCL) Limited reserve the right to alter or improve the specification, internal design or manufacturing process at any time, without notice. Please check with your supplier or visit our web site to ensure that you have the current and complete specification for your product before use. (c) C&D Technologies (NCL) Limited 2000 AN005.1 No part of this publication may be copied, transmitted or stored in a retrieval system or reproduced in any way including, but not limited to, photography, photocopy, magnetic or other recording means, without prior written permission from C&D Technologies (NCL) Limited. Instructions for use are available from www.dc-dc.com
C&D Technologies (NCL) Ltd Tanners Drive, Blakelands North Milton Keynes MK14 5BU, England Tel: +44 (0)1908 615232 Fax:+44 (0)1908 617545 email: info@cdtechno-ncl.com www: http://www.dc-dc.com
C&D Technologies (NCL), Inc. 8917 Glenwood Avenue, Raleigh NC 27612, USA Tel: +1 (919) 571-9405 Fax: +1 (919) 571-9262 email: info@us.cdtechno-ncl.com

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