Datasheet LT3509 (Analog Devices) - 8

LT3509
operaTion Overview
that when the voltage at FB reaches 0.8V, the main current The LT3509 is a dual, constant frequency, current mode comparator threshold will fall and reduce the peak inductor switching regulator with internal power switches. The two current and hence the average current, until it matches the independent channels share a common voltage reference load current. By making current the controlled variable in and oscillator and operate in phase. The switching frequency the loop, the inductor impedance is effectively removed is set by a single resistor and can also be synchronized to from the transfer function and the compensation network an external clock. Operation can be best understood by is simplified. The main current comparator threshold is referring to the Block Diagram (Figure 1). reduced by the slope compensation signal to eliminate sub-harmonic oscillations at duty cycles >50%.
Startup and Shutdown Current Limiting
When the RUN/SS[1,2] pins are pulled low (<0.4V) the associated regulator channel is shut down. If both channels Current mode control provides cycle-by-cycle current are shut down, the common circuitry also enters a low limiting by means of a clamp on the maximum current that current state. When the RUN/SS pins exceed approximately can be provided by the switch. A comparator monitors the 0.8V, the common circuitry and the associated regulator current flowing through the catch diode via the DA pin. are enabled but the output current is limited. From 0.8V This comparator delays switching if the diode current is up to 2.0V the current limit increases until it reaches the higher than 0.95A (typical). This current level is indicative full value. The RUN/SS pins also incorporate a 1µA pull- of a fault condition such as a shorted output with a high up to approximately 3V, so the regulator will run if they input voltage. Switching will only resume once the diode are left open. A capacitor to ground will cause a current current has fallen below the 0.95A limit. This way the DA limited soft-start to occur at power-up. In the case of comparator regulates the valley current of the inductor to undervoltage, overvoltage or overtemperature conditions 0.95A during a short circuit. This will ensure the part will the internal circuitry will pull the RUN/SS pins down with survive a short-circuit event. a current of approximately 250µA. Thus a new soft-start
Over and Undervoltage Shutdown
cycle will occur when the fault condition ends. A basic undervoltage lockout prevents switching if VIN
Voltage and Current Regulation
is below 3.3V (typical). The overvoltage shutdown stops The power switches are controlled by a current-mode the part from switching when VIN is greater than 38.5V regulator architecture. The power switch is turned on at (typical). This protects the device and its load during the beginning of each clock cycle and turned off by the momentary overvoltage events. After the input voltage main current comparator. The inductor current will ramp falls below 38.5V, the part initiates a soft start sequence up while the switch is on until it reaches the peak current and resumes switching. threshold. The current at which it turns off is determined
BOOST Circuit
by the error amp and the internal compensation network. When the switch turns off, the current in the inductor To ensure best efficiency and minimum dropout voltage will cause the SW pin to fall rapidly until the catch diode, the output transistor base drive is boosted above VIN by D1, conducts. The voltage applied to the inductor will the external boost capacitors (C4). When the SW pin is now reverse and the current will linearly fall. The resistor low the capacitors are charged via the BOOST diodes and divider, R1 and R2, sets the desired output voltage such the supply on BD. 3509fd 8 For more information www.linear.com/LT3509 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Related Parts