Datasheet LT3844 (Analog Devices) - 9

LT3844
operaTion (Refer to Functional Diagram)
The LT3844 is a PWM controller with a constant frequency,
VCC/Boosted Supply
current mode control architecture. It is designed for low An internal V to medium power, switching regulator applications. Its CC regulator provides VIN derived gate-drive power for start-up under all operating conditions with high operating voltage capability allows it to step up MOSFET gate charge loads up to 90nC. The regulator can or down input voltages up to 60V without the need for operate continuously in applications with V a transformer. The LT3844 is used in nonsynchronous IN voltages up to 60V, provided the power dissipation of the regulator applications, meaning that a freewheeling rectifier diode does not exceed 250mW. The power dissipation is cal- (D1 of Function Diagram) is used instead of a bottom culated as follows: side MOSFET. For circuit operation, please refer to the Functional Diagram of the IC and Typical Application on Pd(REG) = (VIN – 8V) • fSW • QG the front page of the data sheet. The LT3800 is a similar where QG is the MOSFET gate charge. part that uses synchronous rectification, replacing the In applications where these conditions are exceeded, V diode with a MOSFET in a step-down application. CC must be derived from an external source after start-up.
Main Control Loop
Maximum continuous regulator power dissipation may be exceeded for short duration V During normal operation, the external N-channel MOSFET IN transients. switch is turned on at the beginning of each cycle. The For higher converter efficiency and less power dissipa- switch stays on until the current in the inductor exceeds tion in the IC, VCC can also be supplied from an external a current threshold set by the DC control voltage, V supply such as the converter output. When an external C, which is the output of the voltage control loop. The voltage supply back drives the internal VCC regulator through an control loop monitors the output voltage, via the V external diode and the V FB pin CC voltage is pulled to a diode voltage, and compares it to an internal 1.231V reference. above its regulation voltage, the internal regulator is dis- It increases the current threshold when the V abled and goes into a low current mode. V FB voltage CC is the bias is below the reference voltage and decreases the current supply for most of the internal IC functions and is also threshold when the V used to charge the bootstrapped capacitor (C FB voltage is above the reference BOOST) via an voltage. For instance, when an increase in the load current external diode. The external MOSFET switch is biased from occurs, the output voltage drops causing the V the bootstrapped capacitor. While the external MOSFET FB volt- age to drop relative to the 1.231V reference. The voltage switch is off, an internal BJT switch, whose collector is control loop senses the drop and increases the current connected to the SW pin and emitter is connected to the threshold. The peak inductor current is increased until the PGND pin, is turned on to pull the SW node to PGND and average inductor current equals the new load current and recharge the bootstrap capacitor. The switch stays on until the output voltage returns to regulation. either the start of the next cycle or until the bootstrapped capacitor is fully charged.
Current Limit/Short-Circuit
The inductor current is measured with a series sense
MOSFET Driver
resistor (see the Typical Application on the front page). The LT3844 contains a high speed boosted driver to turn When the voltage across the sense resistor reaches the on and off an external N-channel MOSFET switch. The maximum current sense threshold, typically 100mV, the MOSFET driver derives its power from the boost capacitor TG MOSFET driver is disabled for the remainder of that which is referenced to the SW pin and the source of the cycle. If the maximum current sense threshold is still ex- MOSFET. The driver provides a large pulse of current to turn ceeded at the beginning of the next cycle, the entire cycle on the MOSFET fast to minimize transition times. Multiple is skipped. Cycle skipping keeps the inductor currents to a MOSFETs can be paralleled for higher current operation. reasonable value during a short-circuit, particularly when VIN is high. Setting the sense resistor value is discussed in the “Application Information” section. 3844fc For more information www.linear.com/LT3844 9 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Pin Configuration Typical Performance Characteristics Pin Functions Functional Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts