Datasheet LTC1625 (Analog Devices) - 8

LTC1625
U OPERATIO Main Control Loop
will attempt to turn on the top MOSFET continuously (‘’dropout’’). A dropout counter detects this condition and The LTC1625 is a constant frequency, current mode forces the top MOSFET to turn off for about 500ns every controller for DC/DC step-down converters. In normal tenth cycle to recharge the bootstrap capacitor. operation, the top MOSFET is turned on when the RS latch is set by the on-chip oscillator and is turned off when the An overvoltage comparator OV guards against transient current comparator I1 resets the latch. While the top overshoots and other conditions that may overvoltage the MOSFET is turned off, the bottom MOSFET is turned on output. In this case, the top MOSFET is turned off and the until either the inductor current reverses, as determined bottom MOSFET is turned on until the overvoltage condi- by the current reversal comparator I2, or the next cycle tion is cleared. begins. Inductor current is measured by sensing the VDS Foldback current limiting for an output shorted to ground potential across the conducting MOSFET. The output of is provided by a transconductance amplifer CL. As V the appropriate sense amplifier (TA or BA) is selected by FB drops below 0.6V, the buffered I the switch logic and applied to the current comparator. TH input to the current comparator is gradually pulled down to a 0.95V clamp. The voltage on the ITH pin sets the comparator threshold This reduces peak inductor current to about one fifth of its corresponding to peak inductor current. The error ampli- maximum value. fier EA adjusts this voltage by comparing the feedback signal VFB from the output voltage with the internal 1.19V
Low Current Operation
reference. The VPROG pin selects whether the feedback voltage is taken directly from the V The LTC1625 is capable of Burst Mode operation at low OSENSE pin or is derived from an on-chip resistive divider. When the load current load currents. If the error amplifier drives the ITH voltage increases, it causes a drop in the feedback voltage relative below 0.95V, the buffered ITH input to the current com- to the reference. The I parator will remain clamped at 0.95V. The inductor current TH voltage then rises until the average inductor current again matches the load current. peak is then held at approximately 30mV/RDS(ON)(TOP). If ITH then drops below 0.5V, the Burst Mode comparator B The internal oscillator can be synchronized to an external will turn off both MOSFETs. The load current will be clock applied to the SYNC pin and can lock to a frequency supplied solely by the output capacitor until ITH rises between 100% and 150% of its nominal 150kHz rate. above the 50mV hysteresis of the comparator and switch- When the SYNC pin is left open, it is pulled low internally ing is resumed. Burst Mode operation is disabled by and the oscillator runs at its normal rate. If this pin is taken comparator F when the FCB pin is brought below 1.19V. above 1.2V, the oscillator will run at its maximum 225kHz This forces continuous operation and can assist second- rate. ary winding regulation. Pulling the RUN/SS pin low forces the controller into its shutdown state and turns off both MOSFETs. Releasing
INTVCC/EXTVCC Power
the RUN/SS pin allows an internal 3µA current source to Power for the top and bottom MOSFET drivers and most charge up an external soft start capacitor CSS. When this of the internal circuitry of the LTC1625 is derived from the voltage reaches 1.4V, the controller begins switching, but INTVCC pin. When the EXTVCC pin is left open, an internal with the ITH voltage clamped at approximately 0.8V. As 5.2V low dropout regulator supplies the INTVCC power CSS continues to charge, the clamp is raised until full range from VIN. If EXTVCC is raised above 4.7V, the internal operation is restored. regulator is turned off and an internal switch connects The top MOSFET driver is powered from a floating boot- EXTVCC to INTVCC. This allows a high efficiency source, strap capacitor C such as the primary or a secondary output of the converter B. This capacitor is normally recharged from INTV itself, to provide the INTV CC through a diode DB when the top MOSFET is CC power. turned off. As VIN decreases towards VOUT, the converter 8