LTC3255_15 Datasheet, PDF (12/16 Pages) Linear Technology – Wide VIN Range Fault Protected 50mA Step-Down Charge Pump

LTC3255
APPLICATIONS INFORMATION
Table 1
CERAMIC CAPACITOR MANUFACTURER
AVX
Kemet
Murata
Taiyo Yuden
TDK
WEBSITE
www.avxcorp.com
www.kemet.com
www.murata.com
www.t-yuden.com
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
node. Stray capacitance from FB to C⁺ or C^– can degrade
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
large ground plane under the device can reduce the thermal
resistance of the package and PC board considerably. 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
Step-Down, 1:1 Step-Down, and V_IN Shunt Regulator
Operation sections provide equations for calculating the
power dissipation (P_D) in each mode.
For example, if it is determined that the maximum power
dissipation (P_D) is 1.2W under normal operation, then
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
below 150°C. For ambient temperatures exceeding roughly
127°C, the device will cycle in and out of the thermal
shutdown.
3255f
12
For more information www.linear.com/LTC3255