Datasheet LT3518 (Analog Devices) - 8

LT3518
operation
The LT3518 is a constant frequency, current mode regula- more current is delivered to the output; if it decreases, tor with an internal power switch. Operation can be best less current is delivered. The current regulated in RSENSE understood by referring to the Block Diagram in Figure 1. can be adjusted by changing the input voltage VCTRL. At the start of each oscillator cycle, the SR latch is set, The current sense amplifier provides rail-to-rail current which turns on the Q1 power switch. A voltage proportional sense operation. The FB voltage loop is implemented by to the switch current is added to a stabilizing ramp and the the amplifier A2. When the voltage loop dominates, the resulting sum is fed into the positive terminal of the PWM error amplifier and the amplifier A2 regulate the FB pin to comparator, A4. When this voltage exceeds the level at the 1.01V (constant-voltage mode). negative input of A4, the SR latch is reset, turning off the Dimming of the LED array is accomplished by pulsing the power switch. The level at the negative input of A4 is set LED current using the PWM pin. When the PWM pin is by the error amplifier A3. A3 has two inputs, one from the low, switching is disabled and the error amplifier is turned voltage feedback loop and the other one from the current off so that it does not drive the VC pin. Also, all internal loop. Whichever feedback input is lower takes precedence, loads on the VC pin are disabled so that the charge state and forces the converter into either constant-current or of the VC pin will be saved on the external compensation constant-voltage mode. The LT3518 is designed to transi- capacitor. This feature reduces transient recovery time. tion cleanly between these two modes of operation. The When the PWM input again transitions high, the demand current sense amplifier senses the voltage across RSENSE current for the switch returns to the value just before and provides a pre-gain to amplifier A1. The output of A1 PWM last transitioned low. To further reduce transient is simply an amplified version of the difference between recovery time, an external PMOS is used to disconnect the voltage across RSENSE and the lower of VCTRL/10 the LED array current loop when PWM is low, stopping or 100mV. In this manner, the error amplifier sets the C correct peak switch current level to regulate the current FILT from discharging. through RSENSE. If the error amplifier’s output increases, 3518ff 8 For more information www.linear.com/LT3518 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts