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LT1074CT Datasheet(PDF) 11 Page - Linear Technology |
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LT1074CT Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LT1074/LT1076 11 Figure 8. ILIM Pin Circuit Figure 9. Foldback Current Limit DESCRIPTIO S PI ILIM PIN The ILIM pin is used to reduce current limit below the preset value of 6.5A. The equivalent circuit for this pin is shown in Figure 8. Q1 R1 8K TO LIMIT CIRCUIT VIN 320 A µ D1 D3 6V ILIM 4.3V D2 LT1047•PD12 When ILIM is left open, the voltage at Q1 base clamps at 5V through D2. Internal current limit is determined by the current through Q1. If an external resistor is connected between ILIM and ground, the voltage at Q1 base can be reduced for lower current limit. The resistor will have a voltage across it equal to (320 µA)(R), limited to ≈5V when clamped by D2. Resistance required for a given current limit is: RLIM = ILIM(2kΩ) + 1kΩ (LT1074) RLIM = ILIM(5.5kΩ) + 1kΩ (LT1076) As an example, a 3A current limit would require 3A(2k) + 1k = 7k Ω for the LT1074. The accuracy of these formulas is ±25% for 2A ≤ ILIM ≤ 5A (LT1074) and 7A ≤ ILIM ≤ 1.8A (LT1076), so ILIM should be set at least 25% above the peak switch current required. Foldback current limiting can be easily implemented by adding a resistor from the output to the ILIM pin as shown in Figure 9. This allows full desired current limit (with or without RLIM) when the output is regulating, but reduces current limit under short-circuit conditions. A typical value for RFB is 5kΩ, but this may be adjusted up or down to set the amount of foldback. D2 prevents the output voltage from forcing current back into the ILIM pin. To calculate a value for RFB, first calculate RLIM, the RFB: R IR Rk I Rin k FB SC L LSC L = − ()( ) −Ω ()− Ω () 044 05 1 .* .* *Change 0.44 to 0.16, and 0.5 to 0.18 for LT1076. Example: ILIM = 4A, ISC = 1.5A, RLIM = (4)(2k) + 1k = 9k R k kk k FB = − () Ω () − ()− Ω () 15 044 9 05 9 1 15 38 .. .. . Error Amplifier The error amplifier in Figure 10 is a single stage design with added inverters to allow the output to swing above and below the common mode input voltage. One side of the amplifier is tied to a trimmed internal reference voltage of 2.21V. The other input is brought out as the FB (feed- back) pin. This amplifier has a GM (voltage “in” to current “out”) transfer function of ≈5000µmho. Voltage gain is determined by multiplying GM times the total equivalent output loading, consisting of the output resistance of Q4 and Q6 in parallel with the series RC external frequency compensation network. At DC, the external RC is ignored, and with a parallel output impedance for Q4 and Q6 of 400k Ω, voltage gain is ≈2000. At frequencies above a few hertz, voltage gain is determined by the external compen- sation, RC and CC. VOUT ILIM FB RLIM RFB D2 1N4148 LT1074•PD13 LT1074 |
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