APPLICATION HINTS 





Package Power Dissipation


The package power dissipation is the level at which the thermal sensor monitoring the junction temperature is activated. The SMS112 shuts down when the junction temperature exceeds the limit of 150°C. The junction temperature rises as the difference between the input power and output power increases. The mounting pad configuration on the PCB, the board material, as well as the ambient temperature affect the rate of temperature rise. The junction temperature will be low, even if the power dissipation is high, when the mounting of the device has good thermal conductivity. When mounted on the recommended mounting pad the power dissipation for the SOT-23 package is 400mW. For operation above 25°C derate the power dissipation at 3.2 mW/°C. To determine the power dissipation for shutdown when mounted, attach the device on the PCB and increase the input-to-output voltage until the thermal protection circuit is activated. Calculate the power dissipation of the device by subtracting the output voltage from the input voltage and multiply by the output current. The measurements should allow for the ambient temperature of the PCB. The value obtained from PD/(150°C - TA) is the derating factor. The PCB mounting pad should provide maximum thermal conductivity in order to maintain low device temperatures. As a general rule, the lower the temperature, the better the reliability of the device.


The thermal resistance when the device is mounted is equal to:


TJ = θJA x PD + TA


The internal limit for junction temperature is 150°C. If the ambient temperature is 25°C, then:


150°C = θJA x PD + 25°C


θJA = 125°C/ PD


A simple way to determine PD is to calculate Vin x Iin when the output is shorted. As the temperature rises, the input gradually will decrease. The PD value obtained when the thermal equilibrium is reached, is the value that should be used.


The maximum operating current is:


Iout = ΔPD/(VIN(MAX) - VO)





The range of usable currents can be found from the graph in figure 2.


Procedure:


1. Find PD.


2. PD1 is calculated as PD x (0.8 - 0.9).


3. Plot PD1 against 25°C.


4. Connect PD1 to the point corresponding to the 150°C.








External Capacitors


The SMS112 series require input and output decoupling capacitors. The required value of these capacitors depends on the application circuit and other factors. Because high frequency characteristics of electrolytic capacitors depend greatly on the type and even the manufacturer, the value


of capacitance that works well with SMS112 for one brand or type may not necessary be sufficient with an electrolytic of different origin. Sometimes actual bench testing will be the only means to determine the proper capacitor type and value. To obtain stability in all general applications a high quality 4.7mF aluminum electrolytic or a 2.2mF tantalum electrolytic can be used.


A critical characteristic of the electrolytic capacitors is their performance over temperature. The SMS112 is designed to operate to -30°C, but some electrolytics will freeze around -30°C therefore becoming ineffective. In such case the result is oscillation at the regulator output. For all application circuits where cold operation is necessary, the output capacitor must be rated to operate at the minimum temperature.


In order to determine the minimum value of the output capacitor, for an application circuit, the entire circuit including the capacitor should be bench tested at minimum operating temperatures and maximum operating currents. After the minimum capacitance value has been found, the value should be doubled for actual use to cover for production variations both in the regulator and the capacitor. The recommended minimum capacitance for SMS112 is 2.2mF. As a general rule, with higher output voltages the value of the output capacitance decreases, since the internal loop gain is reduced.





Noise Bypass Capacitor


The noise bypass capacitor should be connected as close as possible to pin 3 and ground. The recommended value for this capacitor is 0.01mF. The noise bypass terminal is susceptible to external noise, and oscillation can occur when the bypass capacitor is not used and the solder pad for this pin is too large. 


Because of the high impedance of the noise bypass terminal, care should be taken if the bypass capacitor is not used.