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LT1506IR-3.3SYNC Datasheet(PDF) 8 Page - Linear Technology |
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LT1506IR-3.3SYNC Datasheet(HTML) 8 Page - Linear Technology |
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8 / 24 page  8 LT1506 APPLICATIONS INFORMATION FEEDBACK PIN FUNCTIONS The feedback (FB) pin on the LT1506 is used to set output voltage and provide several overload protection features. The first part of this section deals with selecting resistors to set output voltage and the remaining part talks about foldback frequency and current limiting created by the FB pin. Please read both parts before committing to a final design. The fixed 3.3V LT1506-3.3 has internal divider resistors and the FB pin is renamed SENSE, connected directly to the output. The suggested value for the output divider resistor (see Figure 2) from FB to ground (R2) is 5k or less, and a formula for R1 is shown below. The output voltage error caused by ignoring the input bias current on the FB pin is less than 0.25% with R2 = 5k. Please read the following if divider resistors are increased above the suggested values. R RVOUT 1 22 42 242 = − () . . 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 in the external diode and inductor during short-circuit conditions. A shorted output requires the switching regu- lator 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 6A for the LT1506, folding back to less than 3A). Minimum switch on time limitations would prevent the switcher from attaining a sufficiently low duty cycle if switching frequency were maintained at 500kHz, so frequency is reduced by about 5:1 when the feedback pin voltage drops below 1V (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 LT1506 also operates at lower switch current limit when the feedback pin voltage drops below 1.7V. 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 1.5V with an external diode to defeat foldback cur- rent limit. Caution: clamping 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 LT1506 to lose control of current limit. Figure 2. Frequency and Current Limit Foldback – + 2.4V VSW VC GND TO SYNC CIRCUIT 1506 F02 TO FREQUENCY SHIFTING R3 1k R4 1k R1 R2 5k OUTPUT 5V R5 5k ERROR AMPLIFIER FB 1.6V Q1 LT1506 Q2 + |
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