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LTC1698 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1698 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 24 page  9 LTC1698 1698f catch MOSFETs Q3 and Q4 are off. As soon as transistors Q1 and Q2 turn on, the flux in the power transformer T1 forces the body diodes of Q3 and Q4 to conduct, and the whole circuit starts like a conventional forward converter. At the same time, the LTC1698 VDD potential ramps up quickly through the VDD bias circuitry. Once the VDD voltage exceeds 4.0V, the LTC1698 enables its drivers and enters synchronous operation. The pulse transformer T2 synchronizes the primary and secondary MOSFET drivers. In a typical conversion cycle, the primary MOSFETs Q1 and Q2 turn on simultaneously. SG goes low and generates a negative spike at the LTC1698 SYNC input through the pulse transformer. The LTC1698 forces FG to turn on and CG to turn off. Power is delivered to the load through the transformer T1 and the inductor L1. At the beginning of the next phase in which Q1 and Q2 turn off, SG goes high, SYNC sees a positive spike, the MOSFET Q3 shuts off, Q4 conducts and allows continuous current to flow through the inductor L1. The capacitor COUT filters the switching waveform to provide a steady DC output voltage for the load. The LTC1698 error amplifier ERR senses the output volt- age through an external resistor divider and regulates the VFB pin potential to the 1.233V internal bandgap voltage. An external RC network across the VFB and VCOMP pins frequency compensates the error amplifier feedback. The opto driver amplifies the voltage difference between the VCOMP pin and the bandgap potential, driving the external optocoupler diode with an inverting gain of 5. The optocoupler feeds the amplified output error signal to the primary controller and closes the forward converter volt- age feedback loop. Under start-up conditions, the internal diode across the LTC1698 error amplifier clamps the VCOMP pin. This speeds up the opto driver recovery time by reducing the negative slew rate excursion at the COMP pin. The forward converter output voltage can be easily ad- justed. The potential at the MARGIN pin is capable of forcing the error amplifier reference voltage to move linearly by ±5%. The internal RMARGIN resistor converts the MARGIN voltage to a current and linearly controls the offset of the error amplifier. Connecting the MARGIN pin to 3.3V increases the VFB voltage by 5%, and connecting the MARGIN pin to 0V reduces VFB by 5%. With the MARGIN pin floating, the VFB voltage is regulated to the internal bandgap voltage. The current limit transconductance amplifier ILIM provides the secondary side average current limit function. The average voltage drops across the RSECSEN resistor is sensed and compared to the – 25mV threshold set by the internal ILIM amplifier. Once ILIM detects high output current, the current amplifier output pulls high, overrides the error amplifier, injects more current into the photo diode and forces a lower duty cycle. An RC network connected to the ICOMP pin is used to stabilize the second- ary current limit loop. Alternatively, if only overcurrent fault protection is required, ICOMP can float. If under abnormal conditions the feedback path is broken, OVPIN provides another route for overvoltage fault pro- tection. If the voltage at OVPIN is higher than the bandgap voltage, the OVP comparator forces OPTODRV high im- mediately. A simple external RC filter prevents a momen- tary overshoot at OVPIN from triggering the OVP comparator. Short OVPIN to ground if this pin is not used. The LTC1698 provides an open-drain PWRGD output. If VFB is less than 94% of its nominal value for more than 1ms, the PWRGD comparator pulls the PWRGD pin low. If VFB is higher than 94% of its nominal value for more than 2ms, the transistor MPWRGD shuts off, and an external resistor pulls the PWRGD pin high. The LTC1698 provides an auxiliary 3.3V logic power supply. This auxiliary power supply is externally compen- sated with a minimum 0.1 µF bypass capacitor. It supplies up to 10mA of current to any external devices. OPERATIO (Refer to Block Diagram) |
Similar Part No. - LTC1698_15 |
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Similar Description - LTC1698_15 |
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