Datasheet LT3724 (Analog Devices) - 9

LT3724
OPERATIONS (Refer to Functional Diagram)
The LT3724 is a PWM controller with a constant frequency,
VCC/Boosted Supply
current mode control architecture. It is designed for low to medium power, switching regulator applications. Its An internal VCC regulator provides VIN derived gate-drive high operating voltage capability allows it to step-up power for start-up under all operating conditions with or down input voltages up to 60V without the need for MOSFET gate charge loads up to 90nC. The regulator can a transformer. The LT3724 is used in nonsynchronous operate continuously in applications with VIN voltages applications, meaning that a freewheeling rectifier diode up to 60V, provided the VIN voltage and/or MOSFET gate (D1 of Function Diagram) is used instead of a bottom charge currents do not create excessive power dissipa- side MOSFET. For circuit operation, please refer to the tion in the IC. Safe operating conditions for continuous Functional Diagram of the IC and Typical Application on regulator use are shown in Figure 1. In applications where the front page of the data sheet. The LT3800 is a similar these conditions are exceeded, VCC must be derived from part that uses synchronous rectification, replacing the an external source after start-up. The LT3724 regulator diode with a MOSFET in a step-down application. can, however, be used for “full time” use in applications where short-duration VIN transients exceed allowable
Main Control Loop
continuous voltages. During normal operation, the external N-channel MOSFET 70 switch is turned on at the beginning of each cycle. The 60 switch stays on until the current in the inductor exceeds a current threshold set by the DC control voltage, VC, the 50 output of the voltage control loop. The voltage control loop (V) 40 monitors the output voltage, via the VFB pin voltage, and V IN compares it to an internal 1.231V reference. It increases 30 the current threshold when the VFB voltage is below the SAFE reference voltage and decreases the current threshold 20 OPERATING AREA when the VFB voltage is above the reference voltage. For 10 instance, when an increase in the load current occurs, 0 20 40 60 80 100 MOSFET TOTAL GATE CHARGE (nC) the output voltage drops causing the VFB voltage to drop 3724 F01 relative to the 1.231V reference. The voltage control loop senses the drop and increases the current threshold. The
Figure 1. VCC Regulator Continuous Operating Conditions
peak inductor current is increased until the average induc- For higher converter efficiency and less power dissipa- tor current equals the new load current and the output tion in the IC, V voltage returns to regulation. CC can also be supplied from an external supply such as the converter output. When an external supply back drives the internal V
Current Limit/Short-Circuit
CC regulator through an external diode and the VCC voltage is pulled to a diode The inductor current is measured with a series sense above its regulation voltage, the internal regulator is dis- resistor (see the Typical Application on the front page). abled and goes into a low current mode. VCC is the bias When the voltage across the sense resistor reaches the supply for most of the internal IC functions and is also maximum current sense threshold, typically 150mV, the used to charge the bootstrapped capacitor (CBOOST) via an TG MOSFET driver is disabled for the remainder of that external diode. The external MOSFET switch is biased from cycle. If the maximum current sense threshold is still ex- the bootstrapped capacitor. While the external MOSFET ceeded at the beginning of the next cycle, the entire cycle switch is off, an internal BJT switch, whose collector is is skipped. Cycle skipping keeps the inductor currents to connected to the SW pin and emitter is connected to the a controlled value during a short-circuit, particularly when PGND pin, is turned on to pull the SW node to PGND and VIN is high. Setting the sense resistor value is discussed recharge the bootstrap capacitor. The switch stays on until in the “Application Information” section. 3724fd 9