Datasheet LT1573 (Analog Devices) - 10

LT1573
U U W U APPLICATIO S I FOR ATIO Thermal Considerations
2.
Power Dissipation of the Resistor RD:
The worst-case power dissipation in resistor R The thermal characteristics of several components need D needs to be calculated so that the power rating of the resistor can be deter- to be considered; the LT1573, the pass transistor and mined. The worst-case power dissipation in this resis- resistor RD. Power dissipation should be calculated based tor will occur when the drive current is at a maximum. on the worst-case conditions seen by each component The power dissipation can be calculated from the fol- during normal operation. lowing formula: 1.
Power Dissipation of the LT1573:
The worst-case power dissipation in the LT1573 is a function of drive current, supply voltage and the value of R V − V − V ( )2 D. Worst-case IN BE DRIVE P = (4) dissipation for the LT1573 occurs when the drive cur- RD RD rent is equal to approximately one half of its maximum value. The worst-case power dissipation in the LT1573 where, can be calculated by the following formula: VIN = the maximum input voltage 2 VBE = the minimum emitter/base voltage of the PNP V − V ( IN BE) VDRIVE = the voltage at the LT1573 DRIVE pin PD = = V R 4 SAT of the DRIVE pin in the worst case D (2) R Following the previous design example, the power D > minimum R for latch - off function D dissipation of resistor RD is calculated from Eq (4): where, 2 VIN = the maximum input voltage to the circuit 5 5 . − 0 6 . 5 − ( 0 3 . ) 9 V P = 0 8 . W 3 BE = the minimum emitter/base voltage of the PNP RD = 24 pass transistor 3.
Power Dissipation of the PNP Transistor:
The worst- Following the previous design example for selecting case power dissipation in the PNP pass transistor is resistor RD, the power dissipation of LT1573 is calcu- simply equal to: lated from Eq (2): PPNP = (VIN – VOUT)(IOUT) (5) 2 5.5 − ( 0.6 ) 5 where, P 0.2 W D = 5 4(2 ) = 4 VIN = the maximum input voltage IOUT = the maximum output current For some operating conditions RD may be replaced with a short. This is possible in applications where the Following the previous design example, the power operating requirements (input voltage and drive cur- dissipation of PNP transistor is calculated from Eq (5): rent) are at the low end and the output will not be PPNP = (5.5 – 3.3)(5) = 11W shorted. For RD = 0, the following formula may be used to calculate the maximum power dissipation in the The LT1573 series regulators have internal thermal LT1573: limiting designed to protect the device during overload conditions. For continuous normal load conditions, PD = (VIN – VBE)(IDRIVE) (3) the maximum junction temperature rating of 125°C where, must not be exceeded. It is important to give careful V consideration to all sources of thermal resistance from IN = the maximum input voltage V junction to ambient. For surface mount devices, heat BE = the minimum emitter/base voltage of the PNP I sinking is accomplished by using the heat spreading DRIVE = the required maximum drive current sn1573 1573fas 10