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LT3598
APPLICATIONS INFORMATION
The SYNC pin should be grounded if the clock sync-
hronization feature is not used. When the SYNC pin is
grounded, the internal oscillator generates switching
frequency to the converter.
Soft-Start and Switching Frequency Foldback
For many applications, it is necessary to minimize the
inrush current at start-up. The LT3598’s soft-start circuit
signi?cantly reduces the start-up current spike and output
voltage overshoot. Before the SS pin voltage reaches 1V,
the switching frequency will also fold back proportional
to the SS pin voltage. A typical value for the soft-start
capacitor is 10nF.
OPENLED FLAG
The OPENLED pin is an open-collector output and needs
an external resistor tied to a supply (see Figure 11). If any
LED string is open during normal operation, the OPENLED
pin will be pulled down.
LT3598
R1
OPENLED
3598 F11
Figure 11. OPENLED Connection
The OPENLED ?ag is only activated after the ?rst PWM
edge. The open LED detection is enabled only when the
PWM signal is enabled. There is a delay for OPENLED
?ag generation when the PWM signal is enabled to avoid
generating a spurious ?ag signal. The maximum current
the OPENLED can sink is typically 2mA.
During start-up (see the Operation section), the open LED
detection is disabled. If an LED string is not used and tied
to V OUT , the string will not be in any fault detection.
Loop Compensation
The LT3598 has an internal transconductance error
ampli?er for LED current regulation whose V C output
compensates the control loop. During an open LED
event where all LED strings are open, the V C node also
compensates the control loop. The external inductor,
output capacitor, and the compensation resistor and
capacitor determine the loop stability. The inductor and
output capacitor are chosen based on performance, size
and cost. The compensation resistor and capacitor at V C
are selected to optimize control loop stability. For typical
LED applications, a 15nF compensation capacitor in series
with a 3k resistor at V C is adequate.
Thermal Considerations
The LT3598 provides six channels for LED strings with
internal NPN devices serving as constant-current sources.
When LED strings are regulated, the lowest LED pin voltage
is typically 0.8V. The higher the programmed LED current,
the more power dissipation in the LT3598. For 30mA LED
programming current with a 100% PWM dimming ratio,
at least 144mW is dissipated within the IC due to current
sources. If the forward voltages of the six LED strings are
very dissimilar, there can be signi?cant power dissipation.
Thermal calculations shall include the power dissipation
on current sources in addition to conventional switch DC
loss, switch AC loss and input quiescent loss. For best
ef?ciency, it is recommended that all channels have the
same number of LEDs, and each string has a similar voltage
drop across the LEDs.
Board Layout Considerations
As with all switching regulators, careful attention must be
paid to the PCB board layout and component placement.
To prevent electromagnetic interference (EMI) problems,
proper layout of high frequency switching paths is essential.
Minimize the length and area of all traces connected to the
switching node pin (SW). Always use a ground plane under
the switching regulator to minimize interplane coupling.
Good grounding is essential in LED fault detection.
3598fb
15
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