| Tj - TA | = P(RθJC + RθSA) | 
                      
                        | Tj | = Junction Temperature (°C) 125°C | 
                      
                        | TA | =Ambient Temperature (°C) | 
                      
                        | P | =Power dissipation (watts)Voltage drop x load current | 
                      
                        | RθJC | =Thermal resistance junction to case °CW | 
                      
                        | RθCS | =Thermal resistance of Heat sink compound (o.2°C/WTYP) | 
                      
                        | RθSA | Thermal resistance of external heat 
                          sink(°C/W) it depend upon length, width, expose aluminum (0.5 to 5) |