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LT3509IDE-TRPBF Datasheet(PDF) 8 Page - Linear Technology |
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LT3509IDE-TRPBF Datasheet(HTML) 8 Page - Linear Technology |
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8 / 24 page  LT3509 8 3509f OPERATION Overview The LT3509 is a dual, constant frequency, current mode switching regulator with internal power switches. The two independent channels share a common voltage reference andoscillatorandoperateinphase.Theswitchingfrequency is set by a single resistor and can also be synchronized to an external clock. Operation can be best understood by referring to the Block Diagram (Figure 1). Startup and Shutdown When the RUN/SS[1,2] pins are pulled low (<0.3V) the associated regulator channel is shut-down. If both channels are shut down, the common circuitry also enters a low current state. When the RUN/SS pins exceed approximately 0.7V, the common circuitry and the associated regulator are enabled but the output current is limited. From 0.7V up to 2.0V the current limit increases until it reaches the full value. The RUN/SS pins also incorporate a 1μA pull-up to approximately 3V, so the regulator will run if they are left open. A capacitor to ground will cause a current limited soft-start to occur at power-up. In the case of undervoltage, overvoltage or over-temperature conditions the internal circuitry will pull the RUN/SS pins down with a current of approximately 250μA. Thus a new soft-start cycle will occur when the fault condition ends. Voltage and Current Regulation The power switches are controlled by a current-mode regulator architecture. The power switch is turned on at the beginning of each clock cycle and turned off by the Main Current Comparator. The inductor current will ramp up while the switch is on until it reaches the peak current threshold. The current at which it turns off is determined by the Error Amp and the internal compensation network. When the switch turns off, the current in the inductor will cause the SW pin to fall rapidly until the catch diode, D1, conducts. The voltage applied to the inductor will now reverse and the current will linearly fall. The resistor divider, R1 and R2, sets the desired output voltage such that when the voltage at FB reaches 0.8V, the Main Current Comparator threshold will fall and reduce the peak inductor current and hence the average current, until it matches the load current. By making current the controlled variable in the loop, the inductor impedance is effectively removed from the transfer function and the compensation network is simplified. The Main Current Comparator threshold is reduced by the slope compensation signal to eliminate sub-harmonic oscillations at duty cycles >50%. Current Limiting Current mode control provides cycle by cycle current limiting by means of a clamp on the maximum current that can be provided by the switch. A comparator monitors the current flowing through the catch diode via the DA pin. This comparator delays switching if the diode current is higher than 0.95A (typical). This current level is indicative of a fault condition such as a shorted output with a high input voltage. Switching will only resume once the diode current has fallen below the 0.95A limit. This way the DA comparator regulates the valley current of the inductor to 0.95A during a short circuit. This will ensure the part will survive a short circuit event. Over and Under Voltage Shutdown A basic under voltage lockout prevents switching if VIN is below 3.3V (typical). The overvoltage shutdown stops the part from switching when VIN is greater than 38.5V (typical). This protects the device and its load during momentary overvoltage events. After the input voltage falls below 38.5V, the part initiates a soft start sequence and resumes switching. BOOST Circuit To ensure best efficiency and minimum dropout voltage the output transistor base drive is boosted above VIN by the external boost capacitors (C4). When the SW pin is low the capacitors are charged via the BOOST diodes and the supply on BD. |
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