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LT1123 Datasheet(PDF) 7 Page - Linear Technology |
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LT1123 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 16 page  7 LT1123 1123fb The following assumptions were made in calculating the data for the curves. Resistors are 5% tolerance and the values shown on the curve are nominal. For 20mA drive current assume: VBE = 0.95V at IC = 1A VDRIVE = 1.75V For 50mA drive current assume: VBE = 1.2V at IC = 2A VDRIVE = 1.9V For 120mA drive current assume: VBE = 1.4V at IC = 4A VDRIVE = 2.1V The RD Selection Chart lists the recommended values for RD for the most useful range of input voltage and output current. The chart includes a number for power dissipa- tion for the LT1123 and RD. Current Limit For regulator circuits using the LT1123, current limiting is achieved by limiting the base drive to the external PNP pass transistor. This means that the actual system current limit will be a function of both the current limit of the LT1123 and the Beta of the external PNP. Beta-based current limit schemes are normally not practical because of uncertainties in the Beta of the pass transistor. Here the drive characteristics of the LT1123 combined with the Beta characteristics of the MJE1123 can provide reliable Beta-based current limiting. This is shown in Figure 5 where the current limit of 30 randomly selected transis- tors is plotted. The spread of current limit is reasonably well controlled. 5.5V RD 120 Ω 43 Ω –– Power (LT1123) 0.05W 0.14W –– Power (RD) 0.12W 0.32W –– 6.0V RD 150 Ω 51 Ω 20 Ω Power (LT1123) 0.05W 0.15W 0.37W Power (RD) 0.13W 0.35W 0.76W 7.0V RD 180 Ω 75 Ω 27 Ω Power (LT1123) 0.06W 0.14W 0.38W Power (RD) 0.16W 0.36W 0.89W 8.0V RD 240 Ω 91 Ω 36 Ω Power (LT1123) 0.06W 0.15W 0.38W Power (RD) 0.17W 0.42W 0.97W 9.0V RD 270 Ω 110 Ω 43 Ω Power (LT1123) 0.20W 0.16W 0.41W Power (RD) 0.07W 0.47W 1.11W 10.0V RD 330 Ω 130 Ω 51 Ω Power (LT1123) 0.22W 0.17W 0.43W Power (RD) 0.07W 0.52W 1.25W INPUT VOLTAGE 0A to 4A 0.75V OUTPUT CURRENT: DROPOUT VOLTAGE: 0A to 1A 0.3V 0A to 2A 0.4V RD Selection Chart Note that in some conditions RD may be replaced with a short. This is possible in circuits where an overload is unlikely and the input voltage and drive requirements are low. See the section on Thermal Considerations for more information. Figure 6. MJE1123 IC vs IB Figure 5. Short-Circuit Current for 30 Random Devices OUTPUT CURRENT (A) 4.00 3 4 5 4.75 5.25 6.00 LT1123 F05 2 1 0 4.25 4.50 5.00 5.50 5.75 6 7 8 9 10 11 12 13 14 15 IB (A) 0 0 1 3 4 5 0.10 9 LT1123 F06 2 0.05 0.15 6 7 8 APPLICATIO S I FOR ATIO |
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