Datasheet LT3519, LT3519-1, LT3519-2 (Analog Devices) - 8

LT3519/LT3519-1/LT3519-2
OPERATION
The LT3519/LT3519-1/LT3519-2 are constant frequency, voltage, the switch current will increase; if FB voltage is current mode regulators with an internal power switch and higher than the reference voltage, the switch demand Schottky. Operation can be best understood by referring current will decrease. The LED current sense feedback to the Block Diagram. At the start of each oscillator cycle, interacts with the FB voltage feedback so that FB will not the SR latch is set, which turns on the Q1 power switch. exceed the internal reference and the voltage between ISP A voltage proportional to the switch current is added to and ISN will not exceed the threshold set by the CTRL pin. a stabilizing ramp and the resulting sum is fed into the For accurate current or voltage regulation, it is necessary positive terminal of the PWM comparator, A4. When this to be sure that under normal operating conditions the voltage exceeds the level at the negative input of A4, the appropriate loop is dominant. To deactivate the voltage SR latch is reset, turning off the power switch. The level loop entirely, FB can be connected to GND. To deactivate at the negative input of A4 is set by the error amplifi er A3. the LED current loop entirely, the ISP and ISN should be A3 has two inputs, one from the voltage feedback loop and tied together and the CTRL input tied to VREF . the other one from the current loop. Whichever feedback When the FB input exceeds a voltage about 60mV lower input is lower takes precedence to set the VC node voltage, than the FB regulation voltage, the pull-down driver on and forces the converter into either a constant-current or the OPENLED pin is activated. This function provides a a constant-voltage mode. status indicator that the load may be disconnected and The LT3519/LT3519-1/LT3519-2 are designed to transi- the constant-voltage feedback loop is taking control of tion cleanly between these two modes of operation. The the switching regulator. current sense amplifi er senses the voltage across RSENSE Dimming of the LED array is accomplished by pulsing the and provides an ×4 pre-gain to amplifi er A1. The output current using the PWM pin. When the PWM pin is low, of A1 is simply an amplifi ed version of the difference be- switching is disabled and the error amplifi er is turned off tween the voltage across RSENSE and the lower of VCTRL so that it does not drive the V or 1.1V. In this manner, the error amplifi er sets the correct C node. Also, all internal loads on the V peak switch current level to regulate the current through C node are disabled so that the charge state of the V R C node will be saved on the internal compensation SENSE. If the error amplifi er’s output increases, more capacitor. This feature reduces transient recovery time. current is delivered to the output; if it decreases, less When the PWM input again transitions high, the demand current is delivered. The current regulated in RSENSE can current for the switch returns to the value just before PWM be adjusted by changing the input voltage VCTRL. The FB last transitioned low. To further reduce transient recovery voltage loop is implemented by the amplifi er A2. When time, an external MOSFET should be used to disconnect the voltage loop dominates, the VC node voltage is set by the LED array current loop when PWM is low, stopping the amplifi ed difference of the internal reference of 1.22V C and the FB pin. If FB voltage is lower than the reference OUT from discharging. 3519fa 8