lt3837 Linear Technology Corporation, lt3837 Datasheet
lt3837
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lt3837 Summary of contents
Page 1
... A typical application is 10W to 60W with the part powered from a DC supply. The LT3837 is a current mode controller that regulates an output voltage based on sensing the secondary voltage via a transformer winding during fl yback. This allows for tight output regulation without the use of an optoisolator, improving dynamic response and reliability ...
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... LT3837EFE#TRPBF LEAD BASED FINISH TAPE AND REEL LLT3837EFE LT3837EFE#TR Consult LTC Marketing for parts specifi ed with wider operating temperature ranges. For more information on lead free part marking, go to: For more information on tape and reel specifi cations, go to: ELECTRICAL CHARACTERISTICS temperature range, otherwise specifi ...
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... UVLO V = 1.3V UVLO Note 3: The LT3837E is guaranteed to meet performance specifi cations from 0°C to 85°C. Specifi cations over the –40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. Note calculated from the ambient temperature T ...
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... LT3837 TYPICAL PERFORMANCE CHARACTERISTICS V Shutdown Current CC vs Temperature UVLO 14V –50 – 100 TEMPERATURE (°C) 3837 G02 SENSE Fault Voltage vs Temperature 220 + SENSE = V SENSE – WITH 215 SENSE 210 205 200 195 ...
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... FALL TIME 40 30 RISE TIME 125 CAPACITANCE (nF) LT3837 Feedback Amplifi er Voltage Gain vs Temperature 1700 1650 1600 1550 1500 1450 1400 1350 1300 1250 1200 1150 1100 –50 – 100 125 TEMPERATURE (° ...
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... LT3837 TYPICAL PERFORMANCE CHARACTERISTICS PG Delay Time vs Temperature 300 250 R = 27.4k PGDLY 200 150 R = 16.7k PGDLY 100 50 0 –50 –30 – TEMPERATURE (°C) PIN FUNCTIONS SG (Pin 1): Synchronous gate drive output. This pin pro- vides an output signal for a secondary-side synchronous switch. Large dynamic currents may fl ow during voltage transitions ...
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... GND (Exposed Pad, Pin 17): This is the ground connec- at its C tion for both signal ground and gate driver grounds. This GND should be connected to the PCB ground plane for electrical contact and rated thermal performance. Careful attention must be paid to ground layout. See Applications Information for details. LT3837 3837fa 7 ...
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... LT3837 BLOCK DIAGRAM 1.25V REFERENCE ( DISABLE + 0.8V – UVLO 10 OSC 7 OSCILLATOR SYNC PGDLY 15 ENDLY 4 8 CLAMPS 0.7 + 1.3 – 1.25V 3V INTERNAL REGULATOR S R – UVLO + COMPARATOR TSD CURRENT TRIP SLOPE COMPENSATION SET ENABLE LOGIC BLOCK PGATE SGATE – ERROR AMP ...
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... FLYBACK FEEDBACK AMPLIFIER LT3837 FEEDBACK AMP 1.25V – + COLLAPSE DETECT ENABLE TIMING DIAGRAM PRIMARY SIDE MOSFET DRAIN VOLTAGE PG VOLTAGE SG VOLTAGE V FLBK – 3837 FFA FLBK MIN ENABLE ON ENABLE DELAY FEEDBACK AMPLIFIER ENABLED ...
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... This precludes the need of an optoisolator in isolated designs greatly improving dynamic response and reliability. The LT3837 has a unique feedback amplifi er that samples a transformer winding voltage during the fl yback period and uses that voltage to control output voltage. ...
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... These include effective transconductance and V node slew rate. C Load Compensation Theory The LT3837 uses the fl yback pulse to obtain information about the isolated output voltage. An error source is caused by transformer secondary current fl ow through the synchronous MOSFET R ances of the transformer secondary and output capacitor. ...
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... LT3837 OPERATION LOAD COMP – CMP CMP Figure 1. Load Compensation Diagram Figure 1 shows the block diagram of the load compensa- tion function. Switch current is converted to voltage by the external sense resistor, averaged and lowpass fi ltered by ...
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... R2 ) ⎤ ⎤ ⎥ − ⎥ ⎦ LT3837 instead • – SENSE = K 1 • • • ESR FLYBK V IN • PRIMARY SECONDARY • LT3837 Figure 2 C OUT 3837 F10 3837fa 13 ...
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... LT3837 APPLICATIONS INFORMATION Transformer Design Transformer design/specifi cation is the most critical part of a successful application of the LT3837. The following sections provide basic information about designing the transformer and potential tradeoffs. If you need help, the LTC Applications group is available to assist in the choice and/or design of the transformer. ...
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... MIN N V • MAX ( ) + + OUT ( ) • 355 = = 200 kHz • • LT3837 , is selected based • • Eff ( ) MIN • X • P MAX OUT • μ H 3837fa 15 ...
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... LT3837 APPLICATIONS INFORMATION Optimization might show that a more effi cient solution is obtained at higher peak current but lower inductance and the associated winding series resistance. A simple spreadsheet program is useful for looking at tradeoffs. Transformer Core Selection Once L is known, the type of transformer is selected. ...
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... R compensation (from step 2). Setting Frequency The switching frequency of the LT3837 is set by an external capacitor connected between the OSC pin and ground. Recommended values are between 200pF and 33pF , yielding switching frequencies between 50kHz and 250kHz. Figure 3 shows the nominal relationship between external capacitance and switching frequency ...
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... The enable delay time should be made long enough to ignore the “irrelevant” portion of the fl yback waveform at light load. Even though the LT3837 has a robust gate drive, the gate transition-time slows with very large MOSFETs. Increase delay time is as required when using such MOSFETs. ...
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... PGDLY good starting point is 27k. Soft-Start Functions The LT3837 contains an optional soft-start function that is enabled by connecting an external capacitor between the SFST pin and ground. Internal circuitry prevents the control voltage at the V pin from exceeding that on the SFST pin. C There is an initial pull-up circuit to quickly bring the SFST voltage to approximately 0.8V. From there it charges to approximately 2.8V with a 20μ ...
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... Figure 5. V Compensation Network C In further contrast to traditional current mode switchers, V pin ripple is generally not an issue with the LT3837. C The dynamic nature of the clamped feedback amplifi er forms an effective track/hold type response, whereby the V voltage changes during the fl yback pulse, but is then C “ ...
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... So divide the load current by (1 – DC). If the output load current is relatively constant, the feedback resistive divider is used to compensate for these losses. Otherwise, use the LT3837 load compensation circuitry ) (see Load Compensation). + ...
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... LT3837 APPLICATIONS INFORMATION where N refl ects the turns ratio of that secondary-to-pri- SP mary winding the primary-side leakage inductance LKG and C is the primary-side capacitance (mostly from the the primary-side power MOSFET). A snubber OSS may be added to reduce the leakage inductance spike as discussed earlier ...
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... The LT3837 gate drives will clamp the max gate voltage to roughly 7.5V, so you can safely use MOSFETs with max V of 10V or larger. ...
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... PC board. Parasitic inductance due to poor layout can signifi cantly impact ripple. Refer to the PC Board Layout section for more details. IC Thermal Considerations Take care to ensure that the LT3837 junction temperature ) does not exceed 125°C. Power is computed from the aver- = Ω ...
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... APPLICATIONS INFORMATION PC Board Layout Considerations In order to minimize switching noise and improve output load regulation, connect the GND pin of the LT3837 directly to the ground terminal of the V CC the bottom terminal of the current sense resistor, the ground terminal of the input capacitor, and the ground plane (multiple vias) ...
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... Isolated Converter 1/2 T1 • 470pF 39Ω 3/4 BAS70 20Ω • Si4896DY SENSE SG PG 8mΩ – LT3837 SENSE V C ENDLY OSC GND SFST C CMP CMP 3.3nF 150k 20k 1.37k 47pF 0.1μF 1% T1: EFD20-3F3 (L PIN 32AWG PIN 25AWG FOIL ...
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... BSC 4. RECOMMENDED MINIMUM PCB METAL SIZE FOR EXPOSED PAD ATTACHMENT MILLIMETERS (INCHES) *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.150mm (.006") PER SIDE LT3837 4.90 – 5.10* (.193 – .201) 3.58 (.141) 16 1514 6.40 2.94 (.252) ( ...
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... LT3837 RELATED PARTS PART NUMBER DESCRIPTION LT3825 Isolated Synchronous Flyback Controller with Wide Input Supply Range LT1424-5 Isolated Flyback Switching Regulator LT1424-9 Isolated Flyback Switching Regulator LT1425 Isolated Flyback Switching Regulator LTC1698 Isolated Secondary Synchronous Rectifi er Controller LT1725 General Purpose High Power Isolated Flyback Controller ...