18V, High Efficiency, DC-DC Step-Up Converter AS1345 – 17
Power OK Operation
If desired the POK functionality can be used. In this case a
resistor R1 (~100k) has to be applied between the POK pin and
VIN, because the POK output is an open drain type. If the POK
functionality is not used the pin should be unconnected.
During shut-down the POK pin is high impedance to save
current. Therefore it shows VIN if connected to VIN with a
resistor or is floating otherwise. During start-up the POK goes
to LOW. During normal operation it is usually HIGH but it goes
to LOW if for some reason VOUT drops below 90% of the
nominal output voltage.
Thermal Shutdown
To prevent the AS1345 from short-term misuse and overload
conditions the chip includes a thermal overload protection. To
block the normal operation mode all switches will be turned
off. The device is in thermal shutdown when the junction
temperature exceeds 150°C typ. To resume the normal
operation the temperature has to drop below 140°C typ. A good
thermal path should be provided to dissipate the heat
generated within the package, especially at higher output
power. To dissipate as much heat as possible from the package
into a copper plane with as much area as possible, it’s
recommended to use multiple vias in the printed circuit board.
It is recommended to solder the Exposed Pad to the GND plane.
Continuing operation in thermal overload conditions may
damage the device, and therefore, is considered a bad practice.
Inductor Selection
For best efficiency, choose an inductor with high frequency core
material, such as ferrite, to reduce core losses. The inductor
should have low DCR (DC resistance) to reduce the I²R losses,
and must be able to handle the peak inductor current without
saturating. A 10µH to 22µH inductor with greater than 500mA
current rating and less than 500mΩ DCR is recommended.
When smaller peak currents are selected, the inductor current
specification can be reduced accordingly.
