Datasheet LT3596 (Analog Devices) - 10

LT3596 operaTion The LT3596 uses a constant-frequency, peak current mode Activation of the adaptive loop is set by the status of the control scheme to provide excellent line and load regula- PWMn pins. If any channel’s PWM pin is low, then the buck tion. Operation is best understood by referring to the Block regulator output ascends to an external y programmed out- Diagram (Figure 1). The bias, start-up, reference, oscil ator, put voltage. This voltage is always set above the maximum TSET amplifier and the buck regulator are shared by the voltage drop of the LEDs. This guarantees that the buck three LED current sources. The conversion and control, output voltage is high enough to immediately supply the PWM dimming logic, LED fault detection, and LED drive LED current once the strings are reactivated. As soon as circuitry are identical for all three current sources. all of the PWM pins transition high, the output voltage of Enable and undervoltage lockout (UVLO) are both con- the buck drops until the adaptive loop regulates the output trolled by a single pin. If the EN/UVLO pin falls below with about 1V across the LED current sinks. This scheme 1.51V (typical), an accurate comparator turns off the LED optimizes the efficiency for the system since the output drivers and the buck regulator. If the pin continues to fall voltage regulates to the minimum voltage required for all to less than 0.4V, the part enters a low quiescent current three LED strings. shutdown mode.
LED Current
The LT3596 contains three constant-current sink LED Each LED string current is individually programmed to drivers. These drivers sink up to 100mA with 1.5% match- a maximum of 100mA with 1.5% matching accuracy ing accuracy between LED strings. The LED strings are between the strings. An external resistor on the I powered from the buck converter. SETn pin programs the maximum current for each string. The The buck converter contains an adaptive loop that adjusts CTRLn pin is used for analog dimming. Digital dimming the output voltage based on LED string voltage to ensure is programmed using the PWMn pin. A dimming ratio of maximum efficiency. External compensation and soft-start 10,000:1 is achievable at a frequency of 100Hz. components are not required, minimizing component count and simplifying board layout. An external resistor
Fault Protection and Reporting
programs the buck’s switching frequency between 200kHz The LT3596 features diagnostic circuitry that protects and 1MHz. The frequency can also be synchronized to an the system in the event that a LEDn pin is shorted to an external clock using the SYNC pin. undesirable voltage. The LT3596 detects when the LEDn voltage exceeds 12V or is within 1.2V of V
Step-Down Adaptive Control
OUT when the LED string is sinking current. If either faulted condition occurs, Adaptive control of the output voltage maximizes system the channel is disabled until the fault is removed. The fault efficiency. This control scheme regulates the output voltage is reported on FAULT until the fault has cleared. to the minimum that ensures all three LED strings turn The LT3596 also offers open-LED detection and report- on. This accounts for the variation in the forward voltage ing. If a LED string is opened and no current flows in the of the LEDs, and minimizes the power dissipation across string, then a fault is reported on FAULT. A fault is also the internal current sources. reported if the internal die temperature reaches the TSET programmed derating limit. LED faults are only reported if the respective PWM signal is high. 3596fa 0 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Revision History Typical Application Related Parts