Datasheet LT1374 (Analog Devices) - 9

LT1374
U U W U APPLICATIONS INFORMATION More Than Just Voltage Feedback
foldback operation. Again, it is nearly transparent to the user under normal load conditions. The only loads that may The feedback pin is used for more than just output voltage be affected are current source loads which maintain full sensing. It also reduces switching frequency and current load current with output voltage less than 50% of final value. limit when output voltage is very low (see the Frequency In these rare situations the feedback pin can be clamped Foldback graph in Typical Performance Characteristics). above 1.5V with an external diode to defeat foldback cur- This is done to control power dissipation in both the IC and rent limit. Caution: clamping the feedback pin means that in the external diode and inductor during short-circuit frequency shifting will also be defeated, so a combination conditions. A shorted output requires the switching regu- of high input voltage and dead shorted output may cause lator to operate at very low duty cycles, and the average the LT1374 to lose control of current limit. current through the diode and inductor is equal to the short-circuit current limit of the switch (typically 6A for the The internal circuitry which forces reduced switching LT1374, folding back to less than 3A). Minimum switch on frequency also causes current to flow out of the feedback time limitations would prevent the switcher from attaining pin when output voltage is low. The equivalent circuitry is a sufficiently low duty cycle if switching frequency were shown in Figure 2. Q1 is completely off during normal maintained at 500kHz, so frequency is reduced by about operation. If the FB pin falls below 1V, Q1 begins to 5:1 when the feedback pin voltage drops below 1V (see conduct current and reduces frequency at the rate of Frequency Foldback graph). This does not affect operation approximately 5kHz/µA. To ensure adequate frequency with normal load conditions; one simply sees a gear shift foldback (under worst-case short-circuit conditions), the in switching frequency during start-up as the output external divider Thevinin resistance must be low enough voltage rises. to pull 150µA out of the FB pin with 0.6V on the pin (RDIV In addition to lower switching frequency, the LT1374 also ≤ 4k). The net result is that reductions in frequency and current limit are affected by output voltage divider imped- operates at lower switch current limit when the feedback ance. Although divider impedance is not critical, caution pin voltage drops below 1.7V. Q2 in Figure 2 performs this should be used if resistors are increased beyond the function by clamping the VC pin to a voltage less than its suggested values and short-circuit conditions will occur normal 2.1V upper clamp level. This foldback current limit with high input voltage. High frequency pickup will greatly reduces power dissipation in the IC, diode and increase and the protection accorded by frequency and inductor during short-circuit conditions. External synchro- current foldback will decrease. nization is also disabled to prevent interference with LT1374 VSW TO FREQUENCY SHIFTING OUTPUT 5V 1.6V Q1 ERROR AMPLIFIER + 2.4V R1 R3 R4 1k 1k FB + – R5 5k Q2 R2 5k TO SYNC CIRCUIT VC GND 1374 F02
Figure 2. Frequency and Current Limit Foldback
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