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LT1166CN8 Datasheet(PDF) 7 Page - Linear Technology |
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LT1166CN8 Datasheet(HTML) 7 Page - Linear Technology |
7 / 16 page 7 LT1166 APPLICATIONS INFORMATION how tapping the sense resistors gives twice the limit current or one half the limit current. Foldback current limit can be added to the normal or “square” current limit by including two resistors (30k typical) from the power supplies to the ILIM pins as shown in Figure 5. With square current limit the maximum output current is independent of the voltage across the power devices. Foldback limit simply makes the output current dependent on output voltage. This scheme puts dissipa- tion limits on the output devices. The larger the voltage across the power device, the lower the available output current. This is represented in Figure 6, Output Voltage vs Output Current for the circuit of Figure 5. Driving the Shunt Regulator It is possible to current drive the shunt regulator directly without driving the input transconductance stage. This has the advantage of higher speed and eliminates the need to compensate the gm stage. With Pin 2 floating, the LT1166 can be placed inside a feedback loop and driven through the biasing current sources. The input transconductance stage remains biased but has no effect on circuit operation. The RL in Figure 7 is used to modu- late the op amp supply current with input signal. This op amp functions as a V-to-I with the supply leads acting as current source outputs. The load resistor and the positive input of the op amp are connected to the output of the LT1166 for feedback to set AV = 1V/V. The capacitor CF eliminates output VOS due to mismatch between ITOP and IBOTTOM, and it also forms a pole at DC and a zero at 1/RFCF. The zero frequency is selected to give a –1V/V gain in the op amp before the phase of the MOSFETs degenerate the stability of the loop. 8 7 3 6 5 SENSE+ ILIM + VOUT ILIM – SENSE– LT1166 VTOP VBOTTOM 0.5 Ω 0.5 Ω 1 Ω VOUT V+ V – 4 1 VIN VIN RIN 2 1166 • F04 1 Ω (2)(ILIM) (1/2)(ILIM) 8 7 3 6 5 SENSE+ ILIM + VOUT ILIM – SENSE– LT1166 VTOP VBOTTOM 1 µF 1 µF 1k 1k 100 Ω 10 Ω 10 Ω 30k 30k VOUT 4 1 100 Ω IRFR9024 IRFR024 15V –15V VIN 5.1k 2 1166 • F05 + 330pF 330pF 20mA 20mA mA Figure 5. Unity Gain Buffer Amp with Foldback Current Limit Figure 4. Tapping Current Limit Resistors OUTPUT VOLTAGE (V) –10 200 160 120 80 40 0 –40 –80 –120 –160 –200 6 LT1166 • F06 –6 –8 –4 0 4 8 –2 2 10 SQUARE ILIM + SQUARE ILIM – FOLDBACK ILIM + FOLDBACK ILIM – Figure 6. Output Current vs Output Voltage |
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