lt1681 Linear Technology Corporation, lt1681 Datasheet
lt1681
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lt1681 Summary of contents
Page 1
... Switch phase is also con- trolled during synchronized operation to accommodate mul- tiple converter systems. Internal logic guarantees 50% maxi- mum duty cycle operation to prevent transformer saturation. The LT1681 incorporates a soft-start feature that provides a controlled increase in supplied current during start-up and after an undervoltage lockout or overvoltage/overcurrent event. ...
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... Pin ........................................ – 85mA to 10mA REF FSET Pin ............................................. – 2mA to 5mA All Other Inputs .................................. – 2mA to 2mA Operating Ambient Temperature Range LT1681E (Note 4) .............................. – LT1681I ............................................. – Storage Temperature Range ................ – 150 C Lead Temperature (Soldering, 10 sec)................. 300 C ELECTRICAL CHARACTERISTICS ...
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... Measured at I Input MAX Measured at I Input MAX Measured at BG Output 50mV Overdrive Measured at FSET Pin f 500kHz (Note 3) OSC FSET Charging FSET Positive-Going Edge Negative-Going Edge f = Oscillator Free-Run Frequency OSC LT1681 = MIN TYP MAX UNITS 4.85 5 5.10 V 4.80 5. 1.242 1.250 1 ...
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... Note 4: The LT1681E is guaranteed to meet performance specifications from Specifications over the – operating temperature range are assured by design, characterization and correlation with statistical process controls. For guaranteed performance to specifications over the – range, the LT1681I is available. I Supply Current CC ...
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... TEMPERATURE ( C) 1681 G08 Soft-Start Output Current vs Temperature – 125 –40 TEMPERATURE ( C) 1681 G11 LT1681 UVLO I Supply Current CC vs Temperature 1 0.8 0.6 0.5 –55 – 125 TEMPERATURE ( C) 1681 G06 Error Amp Reference vs Temperature 1.260 1.255 1.250 1.245 1.240 – ...
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... FSET (Pin 6): Oscillator Timing Pin. Connect a resistor (R ) from the 5V FSET (C ) from this pin to ground. FSET The LT1681 oscillator operates by monitoring the voltage charged via R FSET FSET pin reaches 2.5V, the oscillator rapidly discharges the capacitor with an average current of 0.8mA. Once the –15 ...
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... If the synchroniza- tion function is not desired, this pin may be shorted to ground. The LT1681 internal oscillator drives a toggle flip-flop that assures 50% duty cycle operation during oscillator free- run. The oscillator, therefore, runs at twice the operating frequency of the converter. The SYNC input decoder ...
Page 8
... The pin is typically connected to the primary bottom-side switch source and monitors switch current via a ground-refer- enced current sense resistor. If the pin voltage exceeds 360mV, LT1681 switching function is disabled in 130ns. Exceeding the I threshold also triggers a soft-start MAX reset, resulting in a graceful recovery from a current runaway event ...
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... W BLOCK DIAGRA LT1681 1681f 9 ...
Page 10
... The LT1681 contains circuitry for detecting internal and system faults. Detection of a fault triggers a “general fault condition” or GFC. When a GFC is detected, the LT1681 disables switching and discharges the soft-start capaci- tor. When the GFC subsides, the LT1681 initiates a start- up cycle via the soft-start circuitry to assure a graceful recovery ...
Page 11
... The error amplifier is generally configured as an integrator and is used to create the dominant pole for the main converter feedback loop. The LT1681 error amplifier is a true high gain voltage amplifier. The amplifier noninverting input is internally referenced to 1.25V; the inverting input is the ...
Page 12
... If this pin is + externally forced low for any reason, external current 1.25V limiting is required to prevent damage to the LT1681. Continuous source current from the FSET pin should not exceed 1mA. Putting a 2k resistor in series with any low 1681 F01 impedance pull-down device will assure proper function and protect the IC from damage ...
Page 13
... SYNC SYSTEM CLOCK (INTERNAL) Figure 4. Oscillator/SYNC Waveforms It is also possible to run the LT1681 in a SYNC-only mode by disabling the oscillator completely. Connecting a resis- tor divider from the 5V pin to the FSET pin, forcing a REF voltage within the charge range of 1.5V to 2.5V, will allow the oscillator to follow the SYNC input exclusively with no provision for free-run ...
Page 14
... SGND, to which sensitive circuits such as the error amp reference and the current sense circuits, as well as the local 5V supply, are referred. By virtue of the topologies REF used in LT1681 applications, the large currents from the primary switches, as well as the switch drive transients pass through the sense resistor to ground ...
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... U TYPICAL APPLICATIO S • • • • LT1681 1681f 15 ...
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... LT1681 U TYPICAL APPLICATIO S • • • 16 1681f ...
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... U TYPICAL APPLICATIO S • • • LT1681 1681f 17 ...
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... LT1681/LTC1698 Isolated 5V/10A Converter Efficiency vs Load Current 100 95 36V 90 48V 85 72V CURRENT (A) 1681 TA04 LT1681/LTC1698 Isolated 3.3V/20A Converter Efficiency vs Load Current 100 95 36V 90 48V 72V CURRENT (A) 1681 TA05 10 20 ...
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... NOTE 0.093 – 0.104 (2.362 – 2.642) 0 – 8 TYP 0.050 (1.270) BSC 0.014 – 0.019 (0.356 – 0.482) TYP LT1681 0.496 – 0.512 0.394 – 0.419 (10.007 – 10.643 0.037 – 0.045 (0.940 – ...
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... Power Good Output; Minimum Input/Output Capacitors; 3.5V V Synchronizable 150kHz to 300kHz, V High Side Current Sense 60V Input Use with the LT1681, Isolated Power Supplies, Contains Voltage Margining, Optocoupler Driver, Synchronization Circuit with the Primary Side Up to 42A Output; 0.925V 42A Output; VRM 8.4; 1.3V V Burst Mode 3 ...