Datasheet LT1573 (Analog Devices) - 8

LT1573
U U W U APPLICATIO S I FOR ATIO
output current with dropout voltage as low as 0.35V. The
Current Limit
gain-bandwidth product fT of the D45H11 is typically For regulator circuits using the LT1573, current limiting is 40MHz which enables the regulator, composed of this achieved by limiting the base drive current to the external PNP transistor and the LT1573, to handle the load changes PNP pass transistor. This means that the actual system of several amps in a few hundred nanoseconds with a current limit will be a function of both the current limit of minimum amount of output capacitance. the LT1573 and the Beta of the external PNP. Motorola The following sections describe how specifications can be provides the following Beta information for the D45H11. determined for the basic regulator based on the LT1573 The minimum Beta of the D45H11 is 60 when VCE = 1V and and D45H11 from Motorola. To determine the specifica- IC = 2A. The minimum Beta is 40 when VCE = 1V and IC = tions for regulators formed by the LT1573 and other PNP 4A. For other PNP transistors, the user should first find out transistors, a similar method can be used. the Beta information from the external PNP transistor manufacturer to determine the appropriate LT1573 base
Dropout Voltage
drive current limit. The current limit of the regulator system then can be achieved by selecting the appropriate The dropout voltage of an LT1573-based regulator circuit amount of resistance R is determined by the V D in Figure 2. CE saturation voltage of the discrete external PNP transistor when it is driven with a base
Selecting RD
current equal to the available drive current of the LT1573. The LT1573 is guaranteed to sink 250mA of base current Resistor RD can be used to limit the available drive current (440mA typ). The available drive current of the LT1573 can to the external PNP transistor. In order to select RD, the be reduced by adding a resistor (R user should first choose the value of the drive current that D in Figure 2) in series with the DRIVE pin. Table 1 lists some useful operating will give the required value of output current and dropout points for the D45H11. These points were empirically voltage. For a circuit using the D45H11 as a pass transistor determined using a sampling of devices. this can be done using Table 1. For circuits using transis- tors other than D45H11, the user must characterize the
Table 1. D45H11 Dropout Voltage
transistor to determine the drive current requirements for the specified output current and dropout voltage. In gen-
TYPICAL DRIVE CURRENT OUTPUT CURRENT DROPOUT VOLTAGE
eral, it is recommended that the user choose the lowest
(mA) (A) (V)
value of drive current that will satisfy the output current 20 1 0.20 requirements. This will minimize the stress on circuit 20 2 0.50 components during overload conditions. 40 2 0.25 The formula used to determine the resistor RD is: 40 3 0.50 R 60 3 0.25 D = (VIN – VBE – VDRIVE)/(IDRIVE + IRB) (1) 60 4 0.70 where, 80 4 0.45 VIN = the minimum input voltage to the circuit 100 4 0.35 VBE = the maximum emitter/base voltage of the PNP 100 5 0.70 pass transistor 150 5 0.40 IDRIVE = the minimum PNP base current required 200 5 0.35 IRB = the current through RB = VBE/RB 150 6 0.65 VDRIVE = the DRIVE pin saturation voltage when the DRIVE 200 6 0.45 pin current equals (IDRIVE + IRB) 250 7 0.50 sn1573 1573fas 8