LT1725 Linear Technology Corporation, LT1725 Datasheet
LT1725
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LT1725 Summary of contents
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... Its gate drive capability coupled with a suitable external MOSFET can deliver load power up to tens of watts. The LT1725 has a number of features not found on most other switching regulator ICs. By utilizing current mode switching techniques, it provides excellent AC and DC line regulation ...
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... Pin Voltage .................................................... 2V SENSE FB Pin Current ..................................................... ±2mA Operating Junction Temperature Range LT1725C .............................................. 0°C to 100°C LT1725I ........................................... –40°C TO 125°C Storage Temperature Range ................ – 65°C to 150°C Lead Temperature (Soldering, 10 sec)................ 300°C ELECTRICAL CHARACTERISTICS temperature range, otherwise specifications are at T SYMBOL ...
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... Duty Cycle = Min Duty Cycle ≤ 30% Duty Cycle ≤ 30% Duty Cycle = 80 100pF OSCAP (Note 50k tON R = 50k ENDLY R = 50k MENAB (Note 1.2V UVLO V = 1.3V UVLO I = 1mA LOAD LT1725 = 1.4V unless otherwise noted. C MIN TYP MAX ● 11.5 12.1 ● 11.0 11.8 ● 0.3 0.45 ● 0.6 1.0 ● 1.2 2 0.90 1.12 1.25 ● ...
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... LT1725 W U TYPICAL PERFOR A CE CHARACTERISTICS V Turn-On Voltage vs CC Temperature 16.00 15.75 15.50 15.25 15.00 14.75 14.50 14.25 –50 – 100 125 TEMPERATURE (°C) 1725 G01 Supply Current vs Temperature – 100 125 –25 TEMPERATURE (°C) 1725 G04 GATE SINK 1.0 0 125° ...
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... T = 125°C A 600 400 200 1.25 1.30 1.35 1.40 1725 G13 2.5 V(SFST 2.0 1.5 1.0 0 100 125 1725 G15 LT1725 Enable Delay Time vs Temperature 275 250 225 200 175 150 125 100 125 –50 – TEMPERATURE (°C) 1725 G11 Feedback Amplifier Transconductance vs Temperature –50 –25 ...
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... LT1725 CTIO S PGND (Pin 1): The power ground pin carries the GATE node discharge current. This is typically a current spike of several hundred mA with a duration of tens of nanosec- onds. It should be connected directly to a good quality ground plane. I (Pin 2): Pin to measure switch current with exter- SENSE nal sense resistor ...
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... BIAS OSCAP OSC FB FDBK V (Pin 15): Supply voltage for the LT1725. Bypass this CC pin to ground with 1µF or more. GATE (Pin 16): This is the gate drive to the external power MOSFET switch and has large dynamic currents flowing through it. Keep the trace to the MOSFET as short as possible to minimize electromagnetic radiation and volt- age spikes. A series resistance of 5Ω ...
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... LT1725 DIAGRA V SW VOLTAGE V IN GND SWITCH OFF STATE MINIMUM t ON FLYBACK AMP STATE W FLYBACK ERROR A PLIFIER OFF ENABLE DELAY DISABLED MINIMUM ENABLE TIME T1 D1 • • • FXD ...
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... U OPERATIO The LT1725 is a current mode switcher controller IC designed specifically for the isolated flyback topology. The Block Diagram shows an overall view of the system. Many of the blocks are similar to those found in traditional designs, including: Internal Bias Regulator, Oscillator, Logic, Current Amplifier and Comparator, Driver and Out- put Switch ...
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... These include effective transconductance and V LOAD COMPENSATION THEORY The LT1725 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, ...
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... K1 = dimensionless variable related to V efficiency as above R = external sense resistor SENSE R = uncompensated output impedance OUT The practical aspects of applying this equation to deter- mine an appropriate value for the R in the Applications Information section. LT1725 . This voltage is then CMPC resistor by op amp OCMP ⎞ R SENSE • • ...
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... APPLICATIO S I FOR ATIO TRANSFORMER DESIGN CONSIDERATIONS Transformer specification and design is perhaps the most critical part of applying the LT1725 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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... 1• ⎝ ⎜ ⎠ ⎟ OUT may therefore be theoretically OCMP OUT , V and the measured (differential) effi- IN OUT SENSE . OCMP pin to ground. (Disconnect the OCMP and connect the requisite 0.1µF CMPC LT1725 to IN CMPC (straight line as indicated 1725fa 13 ...
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... OCMP effective compensation. SELECTING OSCILLATOR CAPACITOR VALUE The switching frequency of the LT1725 is set by an external capacitor connected between the OSCAP pin and ground. Recommended values are between 200pF and 33pF, yielding switching frequencies between 50kHz and 250kHz ...
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... For example, a peak switch current of 10A requires a sense resistor of 0.025Ω. Note that the instantaneous peak power in the sense resistor is 2.5W, and it must be rated accord- ingly. The LT1725 has only a single sense line to this re- FLYBACK WAVEFORM WITH LARGE LEAKAGE sistor. Therefore, any parasitic resistance in the ground side ...
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... SENSE L P PARASITIC INDUCTANCE 1725 F05 Figure 5 SOFT-START FUNCTION The LT1725 contains an optional soft-start function that is enabled by connecting an explicit external capacitor be- tween the SFST pin and ground. Internal circuitry prevents the control voltage at the V pin from exceeding that on the C SFST pin. 16 ...
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... Figure 7). Operation is as follows: CC “Trickle charge” resistor R1 is connected to V supplies a small current, typically on the order of a single mA, to charge C1. At first, the LT1725 is off and draws only its start-up current. After some time, the voltage reaches the V turn-on threshold ...
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... In further contrast to traditional current mode switchers, V pin ripple is generally not an issue with the LT1725. 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 “held” ...
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... These relate to the mini- mum switch on time and the minimum enable time. Discontinuous mode operation will be assumed in the following theoretical derivations. As outlined in the Operation section, the LT1725 utilizes a minimum output switch on time, t combined with expected V and switching frequency to IN yield an expression for minimum delivered power ...
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... U U APPLICATIO S I FOR ATIO MAXIMUM LOAD/SHORT-CIRCUIT CONSIDERATIONS The LT1725 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. The internal clamp on the V node, nominally 2 ...
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... W U GATE DRIVE RESISTOR CONSIDERATIONS The gate drive circuitry internal to the LT1725 has been designed to have as low an output impedance as practi- cally possible—only a few ohms. A strong L/C resonance is potentially presented by the inductance of the path leading to the gate of the power MOSFET and its overall gate capacitance ...
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... LT1725 itself. TM Capacitor C7 sets the switching frequency at approxi- mately 200kHz. Optimal load compensation for the trans- former and secondary circuit components is set by resistor R8. Output voltage regulation and overall efficiency are shown in the accompanying graphs ...
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... R2: IRC LR2010. 0.1Ω, 1/2W current sense resistor R3: 34.0k, 1% resistor R4: 3.01k, 1% resistor R5, R6, R7: 51k, 5% resistor R8: 6.2k, 5% resistor R9: 51Ω, 5% resistor R10: 18Ω, 5% resistor R11: 150Ω, 1/4W, 5% resistor R12: 5.1Ω, 5% resistor R13a, R13b: 1.5k, 1/2W, 5% resistor R14: 820k, 5% resistor R15: 33k, 5% resistor LT1725 Application Efficiency 48V 15V OUT ...
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... R13 and R14 sets the undervoltage lockout threshold at about 32V, with a hysteresis band of about 2V. The soft-start and 3V D2 BAS16 R10 R1 1.5µF 22Ω 47k + C1 C10 15µF 1µ GATE LT1725 2 I SENSE MENAB R R SGND PGND OCMP CMPC 51k 51k 2.7k 0.1µ ...
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... C10: 1µF, 25V, Z5U ceramic capacitor R1: 47k, 1/4W, 5% resistor R2: Panasonic type ERJ-14RSJ. 0.18Ω, 1/4W, 5% resistor R3: 35.7k, 1% resistor R4: 3.01k, 1% resistor R5, R6, R7: 51k, 5% resistor R8: 2.7k, 5% resistor R9: 18Ω, 5% resistor R10: 22Ω, 5% resistor R11: 51Ω, 1W, 5% resistor R12: 68Ω, 5% resistor R13: 820k, 5% resistor R14: 33k, 5% resistor LT1725 1725fa 25 ...
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... LT1725 PACKAGE DESCRIPTIO .254 MIN .0165 ± .0015 RECOMMENDED SOLDER PAD LAYOUT .007 – .0098 (0.178 – 0.249) .016 – .050 (0.406 – 1.270) NOTE: 1. CONTROLLING DIMENSION: INCHES INCHES 2. DIMENSIONS ARE IN (MILLIMETERS) 3. DRAWING NOT TO SCALE *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" ...
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... N .053 – .069 (1.346 – 1.752) 0° – 8° TYP .014 – .019 (0.355 – 0.483) TYP LT1725 .386 – .394 (9.804 – 10.008) NOTE .150 – .157 (3.810 – 3.988) ...
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... LT1725 RELATED PARTS PART NUMBER DESCRIPTION LT1424-5 Isolated Flyback Switching Regulator LT1424-9 Isolated Flyback Switching Regulator LT1425 Isolated Flyback Switching Regulator LT1533 Ultralow Noise 1A Switching Regulator LTC1693 High Speed Single/Dual N-Channel MOSFET Drivers LTC1698 Secondary Synchronous Rectifier Controller LT1737 High Power Isolated Flyback Controller ...