Datasheet LTC3822 (Analog Devices) - 8

LTC3822
OPERATION (Refer to Functional Diagram) Main Control Loop
EAMP compares the feedback signal VFB to the internal soft-start ramp (instead of the 0.6V reference), which rises The LTC3822 uses a constant frequency, current mode linearly from 0V to 0.6V in about 650μs. This allows the architecture. During normal operation, the top external output voltage to rise smoothly from 0V to its fi nal value N-channel power MOSFET is turned on when the clock while maintaining control of the inductor current. sets the RS latch, and is turned off when the current comparator (ICMP) resets the latch. The peak inductor
Light Load Operation
current at which ICMP resets the RS latch is determined by the voltage on the I LTC3822 operates discontinuously at low load currents. TH pin, which is driven by the output of the error amplifi er (EAMP). The V The reverse current comparator RICMP senses the FB pin receives the output voltage feedback signal from an external resistor drain-to-source voltage of the bottom external N-channel divider. This feedback signal is compared to the internal MOSFET. This MOSFET is turned off when the inductor 0.6V reference voltage by the EAMP. When the load cur- current reaches zero. Under certain operating conditions rent increases, it causes a slight decrease in V brief inductor current reversal may cause continuous FB relative to the 0.6V reference, which in turn causes the I switching operation. TH voltage to increase until the average inductor current matches the new load current. While the top N-channel MOSFET is off,
Short-Circuit Protection
the bottom N-channel MOSFET is turned on until either The LTC3822 monitors VFB to detect a short-circuit on the inductor current starts to reverse, as indicated by the VOUT . When VFB is near ground, switching frequency is current reversal comparator RICMP, or the beginning of reduced to prevent the inductor current from running the next cycle. away. The oscillator frequency will progressively return to normal when VFB rises above ground. This feature is
Shutdown and Soft-Start (RUN Pin)
disabled during startup. The LTC3822 is shut down by pulling the RUN pin low. In shutdown, all controller functions are disabled and the chip
Output Overvoltage Protection
draws only 7.5μA. The TG and BG outputs are held low As further protection, the overvoltage comparator (OVP) (off) in shutdown. Releasing the RUN pin allows an internal guards against transient overshoots, as well as other more 0.7μA current source to pull up the RUN pin to VIN. The serious conditions that may overvoltage the output. When controller is enabled when the RUN pin reaches 1.1V. the feedback voltage on the VFB pin has risen 13.33% The start-up of V above the reference voltage of 0.6V, the external top-side OUT is controlled by the LTC3822’s internal soft-start. During soft-start, the error amplifi er MOSFET is turned off and the bottom MOSFET is turned on until the overvoltage is cleared. 3822fa 8