LTC3255

12

3255f

APPLICATIONS INFORMATION

Table 1

CERAMIC CAPACITOR MANUFACTURER WEBSITE

AVX

www.avxcorp.com

Kemet

www.kemet.com

Murata

www.murata.com

Taiyo Yuden

www.t-yuden.com

TDK

www.tdk.com

Layout Considerations

Due to the high switching frequency and transient cur-

rents produced by the LTC3255, careful board layout is

necessary for optimal performance. A true ground plane

and short connections to all capacitors will optimize

performance, reduce noise and ensure proper regulation

over all conditions.

When using the LTC3255 with an external resistor divider

it is important to minimize any stray capacitance to the FB

performance significantly and should be minimized and/

or shielded if necessary.

Thermal Management

The on chip power dissipation in the LTC3255 will cause

the junction to ambient temperature to rise at a rate of

40°C/W or more in the MSE package, or 43°C/W or more

in the DD package. To reduce the maximum junction

temperature, a good thermal connection to the PC board

is recommended. Connecting the die paddle (Pin 11) to a

largegroundplaneunderthedevicecanreducethethermal

resistanceofthepackageandPCboardconsiderably.Poor

board layout and failure to connect the die paddle (Pin

11) to a large ground plane can result in thermal junction

to ambient impedance well in excess of 40°C/W (MSE

package) or in excess of 43°C/W (DD package). Thermal

junction to ambient impedance is specified per JEDEC

standard JESD 51-5.

Because of the wide input operating range it is possible to

exceed the specified operating junction temperature and

even reach thermal shutdown. It is the responsibility of

the user of the LTC3255 to calculate worst-case operat-

ing conditions (temperature and power) to make sure

the LTC3255’s specified operating junction temperature

is not exceeded for extended periods of time. The 2:1

Operation sections provide equations for calculating the

For example, if it is determined that the maximum power

the maximum junction to ambient temperature rise in the

MSE package will be:

Junction to Ambient = 1.2W • 40°C/W = 48°C

Thus, the ambient temperature under this condition can

not exceed 102°C if the junction temperature is to remain

below150°C.Forambienttemperaturesexceedingroughly

127°C, the device will cycle in and out of the thermal

shutdown.