Datasheet LTC7151S (Analog Devices) - 9

LTC7151S
OPERATION Main Control Loop
The “S” in LTC7151S refers to the second generation The LTC7151S is a current mode monolithic 15A step- Silent Switcher 2 technology. The IC has integrated down regulator. In normal operation, the internal top ceramic capacitors for VIN and BOOST to keep all the power MOSFET is turned on for a fixed interval deter- fast AC current loops small, thus improving the EMI mined by a one-shot timer (OST). When the top power performance. Furthermore, it allows for faster switching MOSFET turns off, the bottom power MOSFET turns on edges which greatly improves efficiency at high switch- until the current comparator, I ing frequencies. CMP, trips, restarting the one-shot timer and initiating the next cycle. Inductor cur-
RUN Threshold
rent is determined by sensing the voltage drop across the bottom power MOSFET when it is on. The voltage on Pulling the RUN pin to ground forces the LTC7151S into the ITH pin sets the comparator threshold correspond- its shutdown state. Bringing the RUN pin to above 0.6V ing to the inductor valley current. The error amplifier, EA, will turn on the internal reference only, while keeping the adjusts this ITH voltage by comparing the feedback signal, power MOSFETs off. Further increasing the RUN voltage VFB, with an internal 0.5V reference. If the load current above the RUN rising threshold (nominally 1.2V) turns on increases, it causes a drop in the feedback voltage rela- the entire chip. The accurate 1.2V RUN threshold allows tive to the internal reference, the ITH voltage then rises the user to program the SVIN under voltage lockout until the average inductor current matches that of the threshold by placing a resistor divider from SVIN. load current.
INTV
At low load currents, the inductor current can drop to zero
CC Regulator
and become negative. In discontinuous mode (DCM), this An internal low dropout (LDO) regulator produces the 3.3V is detected by the current reversal comparator, I supply that powers the drivers and internal bias circuitry. REV, which then shuts off the bottom power MOSFET. Both power The INTVCC must be bypassed to ground with a minimum MOSFETs will remain off with the output capacitor supply- of a 4.7µF ceramic capacitor. Good bypassing is neces- ing the load current until the I sary to supply the high transient currents required by the TH voltage rises above zero current level to initiate the next cycle. If continuous mode power MOSFET gate drivers. Applications with high input of operation is desired, simply float the MODE/SYNC pin voltage and high switching frequency will experience an or tie it to INTV increase in die temperature due to the higher power dis- CC. sipation across the LDO. In such cases, if there’s another The operating frequency is determined by the value of the 5V or 3.3V supply rail available, consider using that to RT resistor, which programs the current for the internal drive the SV oscillator. An internal phase-lock loop servos the oscilla- IN pin to lower the power dissipation across the internal LDO. tor frequency to an external clock signal if one is pres- ent on the MODE/SYNC pin. Another internal phase-lock
VIN Overvoltage Protection
loop servos the switching regulator on-time to track the internal oscillator to force a constant switching frequency. In order to protect the internal power MOSFET devices against transient voltage spikes, the LTC7151S constantly Overvoltage and undervoltage comparators OV and UV monitors the PVIN pin for an overvoltage condition. When pull the PGOOD output low if the output feedback volt- the PVIN rises above 24.5V, the regulator suspends opera- age, VFB, exits a ±7.5% window around the regulation tion by shutting off both power MOSFETs. Once PVIN drops point. Continuous operation is forced during OV and UV below 21.5V, the regulator immediately resumes normal conditions except during start-up when the TRACK pin is operation. During an overvoltage event, the internal soft- ramping up to 0.5V. start voltage is clamped to a voltage slightly higher than the feedback voltage, thus the soft-start feature will be present upon exiting an overvoltage condition. Rev. 0 For more information www.analog.com 9 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Typical Applications Package Photo Typical Application Related Parts