Datasheet LTC7150S, LTC7150S-4 (Analog Devices) - 10

LTC7150S/LTC7150S-4
OPERATION Main Control Loop
The “S” in LTC7150S refers to the second generation The LTC7150S is a current mode monolithic 20A step- Silent Switcher technology. The IC has integrated ceramic down regulator. In normal operation, the internal top power capacitors for VIN and BOOST to keep all the fast AC cur- MOSFET is turned on for a fixed interval determined by a rent loops small, thus improving the EMI performance. one-shot timer, OST. When the top power MOSFET turns Furthermore, it allows for faster switching edges which off, the bottom power MOSFET turns on until the current greatly improves efficiency at high switching frequencies. comparator, ICMP, trips, restarting the one-shot timer and
RUN Threshold
initiating the next cycle. Inductor current is determined by sensing the voltage drop across the bottom power Pulling the RUN pin to ground forces the LTC7150S into MOSFET when it is on. The voltage on the ITH pin sets its shutdown state. Bringing the RUN pin to above 0.6V the comparator threshold corresponding to the inductor will turn on the internal reference only, while keeping the valley current. The error amplifier, EA, adjusts this ITH power MOSFETs off. Further increasing the RUN voltage voltage by comparing the feedback signal, VFB, with an above the RUN rising threshold (nominally 1.2V) turns on internal 0.6V reference. If the load current increases, it the entire chip. The accurate 1.2V RUN threshold allows causes a drop in the feedback voltage relative to the inter- the user to program the SVIN under voltage lockout nal reference, the ITH voltage then rises until the average threshold by placing a resistor divider from SVIN. inductor current matches that of the 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 LTC7150S 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 ±8% window around the regulation point. tion by shutting off both power MOSFETs. Once PVIN drops Continuous operation is forced during OV and UV condi- below 21.5V, the regulator immediately resumes normal tions except during start-up when the TRACK pin is ramp- operation. During an overvoltage event, the internal soft- ing up to 0.6V. 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. B 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Typical Applications Package Photo Revision History Typical Application Related Parts