LT3597
10
3597fa
operaTion
The LT3597 uses a constant frequency, internally com-
pensated peak current mode control scheme. Operation
is best understood by referring to the Block Diagram in
Figure 1.
Enable and undervoltage lockout (UVLO) are both con-
trolled by a single pin. If the pin falls below 1.51V, an
accurate comparator turns off the LED drivers and buck
regulators. If the pin continues to fall to less than 0.4V,
the part enters shutdown and consumes less than 2µA.
The LT3597 contains three constant current sink LED
drivers. Each of the three LED strings is powered from
a dedicated adaptive buck converter in order to achieve
maximum efficiency. The frequency of the buck regulators
is programmed from 200kHz to 1MHz using an external
resistor. The frequency can also be synchronized to an
external clock using the SYNC pin.
Internal buck compensation and soft-start requires few
external components and permits simple board layout. A
high-side switch current limit protects the internal switch
during its on time, while a low side current limit prevents
the switch from turning on in the case of excessive off
phase current.
Step-Down Adaptive Control
Adaptive control of the output voltages achieves superior
system efficiency. When a given channel’s PWM pin is
low, the respective buck regulator output will go to a
programmable high output voltage. In this case the buck
will enter into a pulse-skipping mode since there is no load
connected. This guarantees that the buck output voltage
is high enough to immediately supply the LED current
once the string is reactivated. As soon as PWM goes
high, the output voltage of the buck will drop until there
is 1V across the LED current sink. This scheme ensures
the best efficiency for each LED channel. Since each LED
string is independently driven from a separate buck chan-
nel, efficiency is optimized for all three strings even if the
number of LEDs is mismatched between the channels.
Another benefit of this regulation method is that the LT3597
starts up with 10,000:1 dimming even if the PWM1-3 pulse
width is 1µs. Since V
OUT
starts up even if PWM1-3 is low,
the part achieves high dimming ratios with narrow pulse
widths within a couple of PWM1-3 clock cycles.
LED Current
Each LED string current can be individually programmed
up to a maximum of 100mA with a 2% matching accuracy
between the strings. An external resistor on the I
SET1-3
pin
programs the max current for that string. The CTRL1-3
pin can be used for analog dimming. Digital PWM can be
programmed using the PWM1-3 pin. A dimming ratio of
10,000:1 can be achieved at a frequency of 100Hz.
Fault Protection and Reporting
The LT3597 protects against both open LED and shorted
LED conditions. If the LED1-3 pin voltage exceeds 12V
while the LED string is sinking current, or if the LED1-3
pin voltage is within 1.25V of V
OUT1-3
pin voltage, the
channel is disabled until the fault is removed. If the LED
string opens, the LT3597 will limit the output to the voltage
set by the FB resistor divider.
The LT3597 reports a fault on the FAULT pin if any of the
LED strings is open or shorted. LED faults are only reported
if the respective string PWM signal is high. A fault is also
reported if the internal die temperature reaches the T
SET
programmed derating limit.
