Datasheet LTC1698 (Analog Devices) - 9

LTC1698
U OPERATIO (Refer to Block Diagram)
catch MOSFETs Q3 and Q4 are off. As soon as transistors forcing the error amplifier reference voltage to move Q1 and Q2 turn on, the flux in the power transformer T1 linearly by ±5%. The internal RMARGIN resistor converts forces the body diodes of Q3 and Q4 to conduct, and the the MARGIN voltage to a current and linearly controls the whole circuit starts like a conventional forward converter. offset of the error amplifier. Connecting the MARGIN pin At the same time, the LTC1698 VDD potential ramps up to 3.3V increases the VFB voltage by 5%, and connecting quickly through the VDD bias circuitry. Once the VDD the MARGIN pin to 0V reduces VFB by 5%. With the voltage exceeds 4.0V, the LTC1698 enables its drivers and MARGIN pin floating, the VFB voltage is regulated to the enters synchronous operation. internal bandgap voltage. The pulse transformer T2 synchronizes the primary and The current limit transconductance amplifier ILIM provides secondary MOSFET drivers. In a typical conversion cycle, the secondary side average current limit function. The the primary MOSFETs Q1 and Q2 turn on simultaneously. average voltage drops across the RSECSEN resistor is SG goes low and generates a negative spike at the LTC1698 sensed and compared to the – 25mV threshold set by the SYNC input through the pulse transformer. The LTC1698 internal ILIM amplifier. Once ILIM detects high output forces FG to turn on and CG to turn off. Power is delivered current, the current amplifier output pulls high, overrides to the load through the transformer T1 and the inductor L1. the error amplifier, injects more current into the photo At the beginning of the next phase in which Q1 and Q2 turn diode and forces a lower duty cycle. An RC network off, SG goes high, SYNC sees a positive spike, the MOSFET connected to the ICOMP pin is used to stabilize the second- Q3 shuts off, Q4 conducts and allows continuous current ary current limit loop. Alternatively, if only overcurrent to flow through the inductor L1. The capacitor COUT filters fault protection is required, ICOMP can float. the switching waveform to provide a steady DC output If under abnormal conditions the feedback path is broken, voltage for the load. OVPIN provides another route for overvoltage fault pro- The LTC1698 error amplifier ERR senses the output volt- tection. If the voltage at OVPIN is higher than the bandgap age through an external resistor divider and regulates the voltage, the OVP comparator forces OPTODRV high im- VFB pin potential to the 1.233V internal bandgap voltage. mediately. A simple external RC filter prevents a momen- An external RC network across the VFB and VCOMP pins tary overshoot at OVPIN from triggering the OVP frequency compensates the error amplifier feedback. The comparator. Short OVPIN to ground if this pin is not used. opto driver amplifies the voltage difference between the The LTC1698 provides an open-drain PWRGD output. If VCOMP pin and the bandgap potential, driving the external V optocoupler diode with an inverting gain of 5. The FB is less than 94% of its nominal value for more than 1ms, the PWRGD comparator pulls the PWRGD pin low. optocoupler feeds the amplified output error signal to the If V primary controller and closes the forward converter volt- FB is higher than 94% of its nominal value for more than 2ms, the transistor M age feedback loop. Under start-up conditions, the internal PWRGD shuts off, and an external resistor pulls the PWRGD pin high. diode across the LTC1698 error amplifier clamps the V The LTC1698 provides an auxiliary 3.3V logic power COMP pin. This speeds up the opto driver recovery time by reducing the negative slew rate excursion at the COMP pin. supply. This auxiliary power supply is externally compen- sated with a minimum 0.1µF bypass capacitor. It supplies The forward converter output voltage can be easily ad- up to 10mA of current to any external devices. justed. The potential at the MARGIN pin is capable of 1698f 9