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LT3430IFE Datasheet(PDF) 8 Page - Linear Technology |
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LT3430IFE Datasheet(HTML) 8 Page - Linear Technology |
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8 / 28 page  8 LT3430 sn3430 3430is APPLICATIO S I FOR ATIO FEEDBACK PIN FUNCTIONS The feedback (FB) pin on the LT3430 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 second part talks about foldback frequency and current limiting created by the FB pin. Please read both parts before committing to a final design. 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. A table of standard 1% values is shown in Table 1 for common output voltages. Please read the following if divider resistors are increased above the suggested values. R RVOUT 1 21 22 122 = − () . . 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 + 0.32 3.3 4.99 8.45 – 0.43 5 4.99 15.4 – 0.30 6 4.75 18.7 + 0.38 8 4.47 24.9 + 0.20 10 4.32 30.9 – 0.54 12 4.12 36.5 + 0.24 15 4.12 46.4 – 0.27 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 4A for the LT3430, folding back to less than 2A). 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 0.8V (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 LT3430 also operates at lower switch current limit when the feedback pin voltage drops below 0.6V. 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 in- ductor during short-circuit conditions. External synchro- nization is also disabled to prevent interference with fold- back 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 0.6V with an external diode to defeat foldback current limit. Caution: clamping the feedback pin means that frequency shifting will also be defeated, so a combination of high in- put voltage and dead shorted output may cause the LT3430 to lose control of current limit. The internal circuitry which forces reduced switching frequency also causes current to flow out of the feedback pin when output voltage is low. The equivalent circuitry is shown in Figure 2. Q1 is completely off during normal operation. If the FB pin falls below 0.8V, Q1 begins to conduct current and reduces frequency at the rate of approximately 1.4kHz/ µA. To ensure adequate frequency foldback (under worst-case short-circuit conditions), the external divider Thevinin resistance must be low enough to pull 115 µA out of the FB pin with 0.44V on the pin (RDIV ≤ 3.8k). The net result is that reductions in frequency and current limit are affected by output voltage divider imped- ance. Although divider impedance is not critical, caution should be used if resistors are increased beyond the suggested values and short-circuit conditions will occur |
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