Datasheet LTC3703 (Analog Devices) - 9

LTC3703
operaTion (Refer to Functional Diagram)
drop in the feedback voltage relative to the reference. The COMP voltage then rises, increasing the duty ratio until the output feedback voltage again matches the reference VOUT 50mV/DIV voltage. In normal operation, the top MOSFET is turned on when the RS latch is set by the on-chip oscillator and VIN 20V/DIV is turned off when the PWM comparator trips and resets the latch. The PWM comparator trips at the proper duty I ratio by comparing the error amplifier output (after being L 2A/DIV “compensated” by the line feedforward multiplier) to a sawtooth waveform generated by the oscillator. When the 20µs/DIV 3703 F02 VOUT = 12V top MOSFET is turned off, the bottom MOSFET is turned ILOAD = 1A 25V TO 60V VIN STEP on until the next cycle begins or, if pulse-skip mode op- eration is enabled, until the inductor current reverses as
Figure 2. Line Transient Performance
determined by the reverse current comparator. MAX and MIN comparators ensure that the output never exceed
Strong Gate Drivers
±5% of nominal value by monitoring VFB and forcing the The LTC3703 contains very low impedance drivers capable output back into regulation quickly by either keeping the top of supplying amps of current to slew large MOSFET gates MOSFET off or forcing maximum duty cycle. The operation quickly. This minimizes transition losses and allows paral- of its other features—fast transient response, outstanding leling MOSFETs for higher current applications. A 100V line regulation, strong gate drivers, short-circuit protection floating high side driver drives the topside MOSFET and and shutdown/soft-start—are described below. a low side driver drives the bottom side MOSFET (see Figure 3). They can be powered from either a separate
Fast Transient Response
DC supply or a voltage derived from the input or output The LTC3703 uses a fast 25MHz op amp as an error ampli- voltage (see MOSFET Driver Supplies section). The bottom fier. This allows the compensation network to be optimized side driver is supplied directly from the DRVCC pin. The for better load transient response. The high bandwidth of top MOSFET drivers are biased from floating bootstrap the amplifier, along with high switching frequencies and capacitor, CB, which normally is recharged during each off low value inductors, allow very high loop crossover fre- cycle through an external diode from DRVCC when the top quencies. The 800mV internal reference allows regulated MOSFET turns off. In pulse-skip mode operation, where output voltages as low as 800mV without external level it is possible that the bottom MOSFET will be off for an shifting amplifiers. extended period of time, an internal counter guarantees that the bottom MOSFET is turned on at least once every
Line Feedforward Compensation
10 cycles for 10% of the period to refresh the bootstrap The LTC3703 achieves outstanding line transient response capacitor. An undervoltage lockout keeps the LTC3703 using a patented feedforward correction scheme. With shut down unless this voltage is above 8.7V. this circuit the duty cycle is adjusted instantaneously to The bottom driver has an additional feature that helps changes in input voltage, thereby avoiding unacceptable minimize the possibility of external MOSFET shoot-through. overshoot or undershoot. It has the added advantage of When the top MOSFET turns on, the switch node dV/dt making the DC loop gain independent of input voltage. pulls up the bottom MOSFET’s internal gate through the Figure 2 shows how large transient steps at the input have Miller capacitance, even when the bottom driver is holding little effect on the output voltage. the gate terminal at ground. If the gate is pulled up high enough, shoot-through between the topside and bottom 3703fc 9 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts