Datasheet LT1185 (Analog Devices) - 7

LT1185
U U W U APPLICATIO S I FOR ATIO
Large output capacitors (electrolytic or solid tantalum)
Example:
A commercial version of the LT1185 in the will not cause the LT1185 to oscillate, but they will cause TO-220 package is to be used with a maximum ambient a damped “ringing” at light load currents where the ESR temperature of 60°C. Output voltage is 5V at 2A. Input of the capacitor is several orders of magnitude lower than voltage can vary from 6V to 10V. Assume an interface the load resistance. This ringing only occurs as a result of resistance of 1°C/W. transient load or line conditions and normally causes no First solve for control area, where the maximum junction problems because of its low amplitude (≤ 25mV). temperature is 125°C for the TO-220 package, and θ
Heat Sinking
JC = 1°C/W: The LT1185 will normally be used with a heat sink. The size P = (10V – 5V) (2A) + 2A (10V) = 10.5W of the heat sink is determined by load current, input and 40 output voltage, ambient temperature, and the thermal θ 125°C – 60°C HS = – 1°C/W – 1°C/W = 4.2°C/W resistance of the regulator, junction-to-case (θ 10.5W JC). The LT1185 has two separate values for θJC: one for the power Next, solve for power transistor limitation, with transistor section, and a second, lower value for the TJMAX = 150°C, θJC = 3°C/W: control section. The reason for two values is that the power transistor is capable of operating at higher continu- ous temperature than the control circuitry. At low power θ 150 – 60 HS = – 3 – 1 = 4.6°C/W levels, the two areas are at nearly the same temperature, 10.5 and maximum temperature is limited by the control area. The lowest number must be used, so heat sink resistance At high power levels, the power transistor will be at a must be less than 4.2°C/W. significantly higher temperature than the control area and its maximum operating temperature will be the Some heat sink data sheets show graphs of heat sink limiting factor. temperature rise vs power dissipation instead of listing a value for thermal resistance. The formula for θ To calculate heat sink requirements, you must solve a HS can be rearranged to solve for maximum heat sink temperature thermal resistance formula twice, one for the power rise: transistor and one for the control area. The lowest value obtained for heat sink thermal resistance must be used. In ∆THS = TJMAX – TAMAX – P(θJC + θCHS) these equations, two values for maximum junction tem- perature and junction-to-case thermal resistance are used, Using numbers from the previous example: as given in Electrical Specifications. (T ∆THS = 125°C – 60 – 10.5(1 + 1) = 44°C control θ JMAX – TAMAX) HS = – θJC – θCHS. section P θHS = Maximum heat sink thermal resistance ∆THS = 150°C – 60 – 10.5(3 + 1) = 48°C power θ transistor JC = LT1185 junction-to-case thermal resistance θCHS = Case-to-heat sink (interface) thermal resistance, including any insulating washers The smallest rise must be used, so heat sink temperature T rise must be less than 44°C at a power level of 10.5W. JMAX = LT1185 maximum operating junction temperature For board level applications, where heat sink size may be TAMAX = Maximum ambient temperature in critical, one is often tempted to use a heat sink which customers application barely meets the requirements. This is permissible if P = Device dissipaton correct assumptions were made concerning maximum I = (V OUT IN – VOUT) (IOUT) + (VIN) ambient temperature and power levels. One complicating 40 1185ff 7