LT3757 datasheet(19/22 Pages) AD

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LT3935

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Planning for Thermal Shutdown

TheLT3935automaticallystopsswitchingwhentheinternal

temperatureistoohigh.Thetemperaturelimitisguaranteed

to be higher than the operational temperature of the part.

During thermal shutdown, all switching is terminated, SS

is forced low, and the LEDs are off.

The exposed pad on the bottom of the package must be

soldered to a ground plane. Vias placed directly under the

package are necessary to dissipate heat.

Following these guidelines, the official four-layer demo

board DC2987A reduces the thermal resistance, θJA to

27°C/W. With a compromised board design, θJA could

be 40°C/W or higher.

Designing the Printed Circuit Board

Note that large switched currents flow through the local

input capacitors and the VIN and GND pins. The loops

traveled by these currents should be made as small as

possible by keeping the capacitors as close as possible

to these pins. These capacitors, as well as the inductor,

should be placed on the same side of the board as the

LT3935 and connected on the same layer. Other large,

bulk input capacitors can be safely placed farther from

the chip and on the other side of the board.

Create a Kelvin ground network by keeping the ground

connection for all of the other components separate.

It should only join the ground for the input and output

capacitors and the return path for the LED current at the

exposed pad.

There are a few other aspects of the board design that

improve performance. An unbroken ground plane on

the second layer dissipates heat, but also reduces noise.

Likewise minimizing the area of the SW and BST nodes

reduces noise. The traces for FB and VC should be kept

short to lessen the susceptibility to noise of these high

impedance nodes. Matched kelvin connections from the

external current sense resistor RS to the ISP and ISN pins

are essential for current regulation accuracy. The 2.2μF

INTVCC and VREF capacitors as well as the 100nF BST

capacitor should be placed as closely as possible to their

respective pins. A capacitor for the CTRL pin should be

placed near the pin for analog dimming. The PWM pin can

be tied to VREF or INTVCC. Please refer to the demo board

layout of the LT3935 for an example of how to implement

these recommendations.

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