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LT1374HVIR Datasheet(PDF) 9 Page - Linear Technology |
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LT1374HVIR Datasheet(HTML) 9 Page - Linear Technology |
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9 / 32 page  9 LT1374 1374fb 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 LT1374, 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 LT1374 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 APPLICATIONS INFORMATION Figure 2. Frequency and Current Limit Foldback – + 2.4V VSW VC GND TO SYNC CIRCUIT 1374 F02 TO FREQUENCY SHIFTING R3 1k R4 1k R1 R2 5k OUTPUT 5V R5 5k ERROR AMPLIFIER FB 1.6V Q1 LT1374 Q2 + 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 LT1374 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 1V, Q1 begins to conduct current and reduces frequency at the rate of approximately 5kHz/ µA. To ensure adequate frequency foldback (under worst-case short-circuit conditions), the external divider Thevinin resistance must be low enough to pull 150 µA out of the FB pin with 0.6V on the pin (RDIV ≤ 4k). 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 with high input voltage. High frequency pickup will increase and the protection accorded by frequency and current foldback will decrease. |
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