AP1159ADSXX Datasheet, PDF (16/19 Pages) Asahi Kasei Microsystems – 14V Input / 100mA Low voltage Output LDO Regulator

11.10 Operating Region and Power Dissipation
[AP1159ADS]
Figure 6. PCB Material: Glass epoxy (t=1.0mm)
Please do derating with 4.0mW/C at Pd=400mW and 25C or more. Thermal resistance (ja) is=250C/W.
The package loss is limited at the temperature that the internal temperature sensor works (about 150C).
Therefore, the package loss is assumed to be an internal limitation. There is no heat radiation characteristic of
the package unit assumed because of the small size. The device being mounted on the PCB carries heat away.
This value changes by the material and the copper pattern etc. of the PCB. The losses are approximately
400mW. Enduring these losses becomes possible in a lot of applications operating at 25C.
The overheating protection circuit operates when there are a lot of losses with the regulator (When outside
temperature is high or heat radiation is bad). The output current cannot be pulled enough and the output
voltage will drop when the protection circuit operates. When the junction temperature reaches 150C, the IC is
shut down. However, operation begins at once when the IC stops operation and the temperature of the chip
decreases.
How to determine the thermal resistance when mounted on PCB
The thermal resistance when mounted is expressed as follows:
Tj=jaPd+Ta
Tj of IC is set around 150C. Pd is the value when the thermal sensor is activated.
If the ambient temperature is 25C, then:
150=jaPd+25
ja=125/Pd (C /mW)
Noise bypass capacitor
The noise and the ripple rejection characteristics depend on the capacitance on the Np terminal.
The ripple rejection characteristic of the low frequency region improves by increasing the capacitance of Cnp.
A standard value is Cnp=0.001F. Increase Cnp in a design with important output noise and ripple rejection
requirements. The IC will not be damaged if the capacitor value is increased.
The on/off switching speed changes depending on the Np terminal capacitance. The switching speed slows
when the capacitance is large.
Rev.0.1
- 16 -
2014/09