Datasheet LT1573 (Analog Devices) - 7

LT1573
U U W U APPLICATIO S I FOR ATIO
The LT1573 is designed to be used in conjunction with an on the regulator circuit under overload conditions. The external PNP transistor. The overall specifications of a resistor RD is chosen based on the operating requirements regulator circuit using the LT1573 and an external PNP will of the circuit, primarily the dropout voltage and the output be heavily dependent on the specifications of the external current. The dropout voltage of an LT1573-based regula- PNP. While there are a wide variety of PNP transistors tor circuit is determined by the VCE saturation voltage of available that can be used with the LT1573, the specifica- the discrete external PNP transistor when it is driven with tions given in a typical transistor data sheet are of little use a base current equal to the available drive current of the in determining overall circuit performance. In the follow- LT1573. ing discussion the critical requirements of the PNP tran- sistors are noted. Design equations are given and
External PNP Transistor Selection Criteria
examples are shown using a readily available discrete PNP The selection of an appropriate external PNP transistor transistor. This device is inexpensive, available from mul- depends on the regulator application specifications. The tiple sources and can be used for a wide range of applica- critical PNP transistor selection criteria include: tions. For applications using other PNP transistors, the 1. The maximum output current of the PNP transistor regulator specifications can be derived by the same method. 2. The dropout voltage at the maximum output current
Basic Regulator Circuit
3. The gain-bandwidth product fT of the transistor The basic regulator circuit is shown in Figure 2. The The PNP transistor must be able to supply the specified adjustable output LT1573 senses the regulator output maximum regulator output current to be qualified for the voltage from its feedback pin via the output voltage regulator application. The VCE saturation voltage of the divider, R1 and R2, and drives the base of the external PNP transistor at the maximum output current determines the transistor to maintain the regulator output at the desired dropout voltage of the circuit. The dropout voltage deter- value. For fixed output versions of the LT1573, the regu- mines the minimum regulator input voltage for a certain lator output voltage is sensed from the feedback pin via an specified output voltage. The gain-bandwidth product fT internal voltage divider. The resistor R of the transistor determines how fast the voltage regulator D is required for the overcurrent latch-off function. R can follow an output load change without losing voltage D is also used to limit the drive current available to the external PNP transistor and regulation. to limit the power dissipation in the LT1573. Limiting the The D45H11 from Motorola and the KSE45H11TU from drive current to the external PNP transistor will limit the Samsung can be used in all LT1573 regulator circuits with output current of the regulator which minimizes the stress current ratings up to 5A. The D45H11 can supply 5A of CC RC FB COMP LATCH VOUT LT1573 + CTIME SHDN VIN R R B D GND DRIVE QOUT VIN VOUT R1 + + LOAD CIN COUT1 COUT2 R2 GND 1573 F02
Figure 2. Basic Regulator Circuit
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