LT1425 Linear Technology, LT1425 Datasheet
LT1425
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LT1425 Summary of contents
Page 1
... By utilizing current mode switching techniques, it provides excellent AC and DC line regulation. The LT1425 has a number of features not found on other switching regulator ICs. Its unique control circuitry can maintain regulation well into discontinuous mode in most applications ...
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... Duty Cycle = 50%, – Duty Cycle = 80% Duty Cycle = Minimum U W INFORMATION ORDER PART TOP VIEW NUMBER 16 GND 15 SHDN 14 R OCOMP LT1425CS 13 R CCOMP LT1425IS PGND 9 GND S PACKAGE = open 1.4V, unless otherwise specified. C MIN TYP MAX = 3.000k ...
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... Open CONDITIONS Note 2: Feedback amplifier transconductance is R Note 3: Voltage gain is R Switch Current Limit vs Duty Cycle 2 1.5 1.0 0 100 DUTY CYCLE (%) 1425 G02 LT1425 = 1.4V, unless otherwise specified. C MIN TYP MAX UNITS 0.45 1.5 2.2 320 450 40 2.8 3.1 7.0 9 0.4 0.9 1.3 referred. ...
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... LT1425 W U TYPICAL PERFORMANCE CHARACTERISTICS Feedback Amplifier Output Current vs R Pin Voltage REF 125 C 40 – –20 –40 –60 –80 1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40 R NODE VOLTAGE (V) REF 1425 G04 Switching Frequency vs Temperature 300 295 290 285 280 275 270 265 50 100 125 –50 – ...
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... See Applications Information for more details. SHDN (Pin 15): Shutdown. This pin is used to turn off the regulator and reduce V microamperes. The SHDN pin can be left floating when unused. and the V REF C LT1425 pin low. C input current to a few tens ...
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... LT1425 W BLOCK DIAGRAM V IN 2.6V SHDN REGULATOR 285kHz SYNC OSCILLATOR W FLYBACK ERROR A PLIFIER DIAGRA FLYBACK ERROR AMPLIFIER LOGIC LOAD COMP COMPENSATION SGND V C GND IS OMITTED FOR CLARITY • • FXD ...
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... ON FLYBACK AMP STATE U OPERATION The LT1425 is a current mode switching regulator IC that has been designed specifically for the isolated flyback topology. The special problem normally encountered in such circuits is that information relating to the output voltage on the isolated secondary side of the transformer must be communicated to the primary side in order to maintain regulation ...
Page 8
... LT1425. The capaci- tor connected to the R pin is also external. CCOMP The LT1425 operates much the same as traditional current mode switchers, the major difference being a different type of error amplifier which derives its feedback informa- tion from the flyback pulse. Due to space constraints, this discussion will not reiterate the basics of current mode switcher/controllers and isolated flyback converters ...
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... These include effective transconductance and V LOAD COMPENSATION THEORY The LT1425 uses the flyback pulse to obtain information about the isolated output voltage. A potential error source is caused by transformer secondary current flow through the real life nonzero impedances of the output rectifier, transformer secondary and output capacitor ...
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... LT1425 U OPERATION may be minimized by the use of the load compensation function. To implement the load compensation function, a voltage is developed that is proportional to average output switch current. This voltage is then impressed across the external R resistor and the resulting current is then sub- OCOMP tracted from the R node ...
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... R REF R should be approximately 3k. This is because the REF LT1425 is trimmed and specified using this value the impedance of R varies considerably from 3k, REF additional errors will result. However, a variation several percent perfectly acceptable. This yields ...
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... R short to ground. TRANSFORMER DESIGN CONSIDERATIONS Transformer specification and design is perhaps the most critical part of applying the LT1425 successfully. In addi- tion to the usual list of caveats dealing with high frequency isolated power supply transformer design, the following information should prove useful. ...
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... SW network that connects to V ence. The LT1425, which senses the output voltage in both a dynamic and an isolated manner, exhibits additional potential error sources to contend with. Some of these errors are proportional to output voltage, others are fixed in an absolute millivolt sense ...
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... They relate to the minimum switch ON time and the minimum enable time. Discontinu- ous mode operation will be assumed in the following theoretical derivations. As outlined in the Operation section, the LT1425 utilizes a minimum output switch ON time, t combined with expected V yield an expression for minimum delivered power. ...
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... This general treatment allows these expressions to be used when the transformer turns ratio is nonunity.) MAXIMUM LOAD/SHORT-CIRCUIT CONSIDERATIONS The LT1425 is a current mode controller. It uses the V node voltage as an input to a current comparator which turns off the output switch on a cycle-by-cycle basis as this peak current is reached ...
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... In further contrast to traditional current mode switchers, V pin ripple is generally not an issue with the LT1425. The C dynamic nature of the clamped feedback amplifier forms an effective track/hold type response, whereby the V voltage changes during the flyback pulse, but is then “ ...
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... The “– 48V to 5V Isolated Telecom Supply” uses an external cascoded 200V MOSFET to extend the LT1425’s 35V maximum switch voltage limit. The input voltage range (– 36V to – 72V) also exceeds the LT1425’s 20V ...
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... GND GND 0.1 F PIN TURNS BIFILAR 34AWG PIN TURNS 40AWG PIN TURNS 40AWG PIN TURNS BIFILAR 34AWG – 48V to 5V Isolated Telecom Supply BAV21 5 T1 LT1425 BAV21 1 16 GND GND SHDN 30.1k 3 ...
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... TYP 0.014 – 0.019 (0.355 – 0.483) 0.386 – 0.394* (9.804 – 10.008 0.150 – 0.157** (3.810 – 3.988 0.004 – 0.010 (0.101 – 0.254) 0.050 (1.270) TYP LT1425 S16 0695 19 ...
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... MUR120 SGND PGND 9 GND GND 9.3k 0 COMMENTS Built-In Isolated Regulation Without Optoisolator Up to 200kbps Data Rate, UL Listed Isolated Flyback Mode for Higher Currents Uses Ultrasmall Magnetics 8-Pin Fixed Voltage Version of LT1425 330pF 100 MBRS340T3 220 F 220 F 10V 10V ...