Datasheet LT3597 (Analog Devices) - 10

LT3597
operaTion
The LT3597 uses a constant frequency, internally com- is 1V across the LED current sink. This scheme ensures pensated peak current mode control scheme. Operation the best efficiency for each LED channel. Since each LED is best understood by referring to the Block Diagram in string is independently driven from a separate buck chan- Figure 1. nel, efficiency is optimized for all three strings even if the Enable and undervoltage lockout (UVLO) are both con- number of LEDs is mismatched between the channels. trolled by a single pin. If the pin falls below 1.51V, an Another benefit of this regulation method is that the LT3597 accurate comparator turns off the LED drivers and buck starts up with 10,000:1 dimming even if the PWM1-3 pulse regulators. If the pin continues to fall to less than 0.4V, width is 1µs. Since VOUT starts up even if PWM1-3 is low, the part enters shutdown and consumes less than 2µA. the part achieves high dimming ratios with narrow pulse The LT3597 contains three constant current sink LED widths within a couple of PWM1-3 clock cycles. drivers. Each of the three LED strings is powered from
LED Current
a dedicated adaptive buck converter in order to achieve maximum efficiency. The frequency of the buck regulators Each LED string current can be individually programmed is programmed from 200kHz to 1MHz using an external up to a maximum of 100mA with a 2% matching accuracy resistor. The frequency can also be synchronized to an between the strings. An external resistor on the ISET1-3 pin external clock using the SYNC pin. programs the max current for that string. The CTRL1-3 pin can be used for analog dimming. Digital PWM can be Internal buck compensation and soft-start requires few programmed using the PWM1-3 pin. A dimming ratio of external components and permits simple board layout. A 10,000:1 can be achieved at a frequency of 100Hz. high-side switch current limit protects the internal switch during its on time, while a low side current limit prevents
Fault Protection and Reporting
the switch from turning on in the case of excessive off phase current. The LT3597 protects against both open LED and shorted LED conditions. If the LED1-3 pin voltage exceeds 12V
Step-Down Adaptive Control
while the LED string is sinking current, or if the LED1-3 pin voltage is within 1.25V of V Adaptive control of the output voltages achieves superior OUT1-3 pin voltage, the channel is disabled until the fault is removed. If the LED system efficiency. When a given channel’s PWM pin is string opens, the LT3597 will limit the output to the voltage low, the respective buck regulator output will go to a set by the FB resistor divider. programmable high output voltage. In this case the buck will enter into a pulse-skipping mode since there is no load The LT3597 reports a fault on the FAULT pin if any of the connected. This guarantees that the buck output voltage LED strings is open or shorted. LED faults are only reported is high enough to immediately supply the LED current if the respective string PWM signal is high. A fault is also once the string is reactivated. As soon as PWM goes reported if the internal die temperature reaches the TSET high, the output voltage of the buck will drop until there programmed derating limit. 3597fa 10 Document Outline Features Applications Description Typical Application 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