LT3492 LINER [Linear Technology], LT3492 Datasheet - Page 11

• Manufacturer Part Number: LT3492
• Description: Triple Output LED Driver with 3000:1 PWM Dimming
• Manufacturer: LINER [Linear Technology]

this capability. Some margin to data sheet limits is included,
along with provision for 200mA inductor ripple current.
For boost mode converters:
For buck mode converters:
For SEPIC and buck-boost mode converters:
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
a higher output current at nominal V
The LED current of each channel is programmed by con-
necting an external sense resistor R
the LED load, and setting the voltage regulation threshold
across that sense resistor using CTRL input. If the CTRL
voltage, V
If V
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.
APPLICATIONS INFORMATION
I
I
I
I
I
LED(MAX)
LED
CTRL
LED
OUT MAX
OUT MAX
(
(
=
=
is higher than 1V, the LED current is:
10 • R
R
100mV
CTRL
SENSE
IN
V
)
)
≅ 0.4A
V
REF
≅ 0 4
≅
(as shown in the Block Diagram) to provide
CTRL
, is less than 1V, the LED current is:
0 4
SENSE
.
.
A
A
45k
470Ω
PTC
(
V
V
OUT MAX
V
OUT MAX
IN MIN
2V
Figure 2
(
(
(
V
)
IN MIN
3492 F02
)
50k
(
)
+
V
)
IN MIN
IN
SENSE
CTRL1-3
(
levels.
)
)
in series with
Thermal Considerations
The LT3492 is rated to a maximum input voltage of 30V
for continuous operation, and 40V for nonrepetitive one
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
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
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
thermal contact between the exposed pad and the ground
plane of the board. Also, in boost confi guration, the
Schottky rectifi er and the capacitor between GND and the
cathode of the Schottky are in the high frequency switching
path where current fl 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
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-
tensive routing of high impedance traces such as OVP and
V
the GND under the IC rather than a GND where switching
currents are fl owing. Finally, the bypass capacitor on the
V
possible to the V
C
IN
. Make sure these sensitive signals are star coupled to
supply to the LT3492 should be placed as close as
IN
terminal of the device.
LT3492
11
IN
from
3492f