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LT1576IS8-5 Datasheet(PDF) 9 Page - Linear Technology |
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LT1576IS8-5 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 28 page  9 LT1576/LT1576-5 APPLICATIONS INFORMATION Figure 2. Frequency and Current Limit Foldback – + 1.21V VSW VC GND TO SYNC CIRCUIT 1576 F02 TO FREQUENCY SHIFTING R3 1k R4 1k R1 R2 5k OUTPUT 5V R5 5k ERROR AMPLIFIER FB 1.4V Q1 LT1576 Q2 + Table 1 OUTPUT R1 % ERROR AT OUTPUT VOLTAGE R2 (NEAREST 1%) DUE TO DISCREET 1% (V) (k Ω)(kΩ) RESISTOR STEPS 3 4.99 7.32 – 0.50 3.3 4.99 8.66 + 0.30 5 4.99 15.8 + 0.83 6 4.99 19.6 – 0.62 8 4.99 28.0 – 0.01 10 4.99 36.5 + 0.61 12 4.99 44.2 – 0.60 15 4.99 56.2 – 1.08 More Than Just Voltage Feedback The feedback pin is used for more than just output voltage sensing. It also reduces switching frequency and current limit when output voltage is very low (see the Frequency Foldback graph in Typical Performance Characteristics). This is done to control power dissipation in both the IC and the external diode and inductor during short-circuit con- ditions. A shorted output requires the switching regulator to operate at very low duty cycles, and the average current through the diode and inductor is equal to the short-circuit current limit of the switch (typically 2A for the LT1576, folding back to less than 0.77A). Minimum switch on time limitations would prevent the switcher from attaining a sufficiently low duty cycle if switching frequency were maintained at 200kHz, so frequency is reduced by about 5:1 when the feedback pin voltage drops below 0.7V (see Frequency Foldback graph). This does not affect operation with normal load conditions; one simply sees a gear shift in switching frequency during start-up as the output voltage rises. In addition to lower switching frequency, the LT1576 also operates at lower switch current limit when the feedback pin voltage drops below 0.7V. Q2 in Figure 2 performs this function by clamping the VC pin to a voltage less than its normal 2.1V upper clamp level. This foldback current limit greatly reduces power dissipation in the IC, diode and inductor during short-circuit conditions. External synchro- nization is also disabled to prevent interference with foldback operation. Again, it is nearly transparent to the user under normal load conditions. The only loads that may be affected are current source loads which maintain full load current with output voltage less than 50% of final value. In these rare situations the feedback pin can be clamped above 0.7V to defeat foldback current limit. Caution: clamp- ing the feedback pin means that frequency shifting will also be defeated, so a combination of high input voltage and dead shorted output may cause the LT1576 to lose control of current limit. |
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