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LT3492
APPLICATIONS INFORMATION
this capability. Some margin to data sheet limits is included,
along with provision for 200mA inductor ripple current.
For boost mode converters:
Thermal Considerations
The LT3492 is rated to a maximum input voltage of 30V
for continuous operation, and 40V for nonrepetitive one
I OUT(MAX) ? 0.4A
V IN(MIN)
V OUT(MAX)
second transients. Careful attention must be paid to the
internal power dissipation of the LT3492 at higher input
voltages and higher switching frequencies/output voltage
I OUT(MAX) ? 0.4A
For buck mode converters:
I LED(MAX) ? 0.4A
For SEPIC and buck-boost mode converters:
V IN(MIN)
(V OUT(MAX) + V IN(MIN) )
If some level of analog dimming is acceptable at minimum
supply levels, then the CTRL pin can be used with a resistor
divider to V IN (as shown in the Block Diagram) to provide
a higher output current at nominal V IN levels.
The LED current of each channel is programmed by con-
necting an external sense resistor R SENSE in series with
the LED load, and setting the voltage regulation threshold
across that sense resistor using CTRL input. If the CTRL
voltage, V CTRL , is less than 1V, the LED current is:
to ensure that a junction temperature of 125°C is not
exceeded. This is especially important when operating
at high ambient temperatures. Consider driving V IN from
5V or higher to ensure the fastest switching edges, and
minimize one source of switching loss. The exposed
pad on the bottom of the package must be soldered to
a ground plane. This ground should then be connected
to an internal copper ground plane with thermal vias
placed directly under the package to spread out the heat
dissipated by the LT3492.
Board Layout
The high speed operation of the LT3492 demands careful
attention to board layout and component placement. The
exposed pad of the package is the only GND terminal of
the IC and is important for thermal management of the
IC. Therefore, it is crucial to achieve a good electrical and
I LED =
V CTRL
10 ? R SENSE
thermal contact between the exposed pad and the ground
plane of the board. Also, in boost con?guration, the
Schottky recti?er and the capacitor between GND and the
I LED =
If V CTRL is higher than 1V, the LED current is:
100mV
R SENSE
The CTRL pins should not be left open. The CTRL pin
can also be used in conjunction with a PTC thermistor to
provide overtemperature protection for the LED load as
shown in Figure 2.
cathode of the Schottky are in the high frequency switching
path where current ?ow is discontinuous. These elements
should be placed so as to minimize the path between SW
and the GND of the IC. To reduce electromagnetic interfer-
ence (EMI), it is important to minimize the area of the SW
node. Use the GND plane under SW to minimize interplane
coupling to sensitive signals. To obtain good current
regulation accuracy and eliminate sources of channel to
channel coupling, the ISP and ISN inputs of each channel
V REF
45k
2V
50k
of the LT3492 should be run as separate lines back to the
terminals of the sense resistor. Any resistance in series
with ISP and ISN inputs should be minimized. Avoid ex-
470Ω
PTC
3492 F02
Figure 2
CTRL1-3
tensive routing of high impedance traces such as OVP and
V C . Make sure these sensitive signals are star coupled to
the GND under the IC rather than a GND where switching
currents are ?owing. Finally, the bypass capacitor on the
V IN supply to the LT3492 should be placed as close as
possible to the V IN terminal of the device.
3492fa
12
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