Datasheet LTC1624 (Analog Devices) - 5

LTC1624
U U U PIN FUNCTIONS TG (Pin 6):
High Current Gate Drive for Top N-Channel swing at this pin is from INTVCC to VIN + INTVCC in step- MOSFET. This is the output of a floating driver with a down applications. In non step-down topologies the volt- voltage swing equal to INTVCC superimposed on the age at this pin is constant and equal to INTVCC if SW = 0V. switch node voltage SW.
VIN (Pin 8):
Main Supply Pin and the (+) Input to the
BOOST (Pin 7):
Supply to Topside Floating Driver. The Current Comparator. Must be closely decoupled to ground. bootstrap capacitor CB is returned to this pin. Voltage
U OPERATIO (Refer to Functional Diagram) Main Control Loop
implemented by ramping the voltage on the ITH/RUN pin The LTC1624 uses a constant frequency, current mode from 1.19V to its 2.4V maximum (see Applications Infor- architecture. During normal operation, the top MOSFET is mation section). turned on each cycle when the oscillator sets the RS latch Comparator OV guards against transient output over- and turned off when the main current comparator I1 resets shoots >7.5% by turning off the top MOSFET and keeping the RS latch. The peak inductor current at which I1 resets it off until the fault is removed. the RS latch is controlled by the voltage on the ITH/RUN pin, which is the output of error amplifier EA. The VFB pin,
Low Current Operation
described in the pin functions, allows EA to receive an The LTC1624 is capable of Burst Mode operation in which output feedback voltage from an external resistive divider. the external MOSFET operates intermittently based on When the load current increases, it causes a slight load demand. The transition to low current operation decrease in VFB relative to the 1.19V reference, which in begins when comparator B detects when the ITH/RUN turn causes the ITH/RUN voltage to increase until the voltage is below 1.5V. If the voltage across RSENSE does average inductor current matches the new load current. not exceed the offset of I2 (approximately 20mV) for one While the top MOSFET is off, the internal bottom MOSFET full cycle, then on following cycles the top and internal is turned on for approximately 300ns to 400ns to recharge bottom drives are disabled. This continues until the ITH the bootstrap capacitor CB. voltage exceeds 1.5V, which causes drive to be returned to The top MOSFET driver is biased from the floating boot- the TG pin on the next cycle. strap capacitor CB that is recharged during each off cycle.
INTVCC Power/Boost Supply
The dropout detector counts the number of oscillator cycles that the top MOSFET remains on and periodically Power for the top and internal bottom MOSFET drivers is forces a brief off period to allow C derived from V B to recharge. IN. An internal regulator supplies INTVCC power. To power the top driver in step-down applications The main control loop is shut down by pulling the ITH/RUN an internal high voltage diode recharges the bootstrap pin below its 1.19V clamp voltage. Releasing ITH/RUN capacitor CB during each off cycle from the INTVCC supply. allows an internal 2.5µA current source to charge com- A small internal N-channel MOSFET pulls the switch node pensation capacitor CC. When the ITH/RUN pin voltage (SW) to ground each cycle after the top MOSFET has reaches 0.8V the main control loop is enabled with the ITH/ turned off ensuring the bootstrap capacitor is kept fully RUN voltage pulled up by the error amp. Soft start can be charged. 5