Datasheet LT3431 (Analog Devices) - 8

LT3431
U U W U APPLICATIO S I FOR ATIO FEEDBACK PIN FUNCTIONS
short-circuit current limit of the switch (typically 4A for the LT3431, folding back to less than 2A). Minimum switch on The feedback (FB) pin on the LT3431 is used to set output time limitations would prevent the switcher from attaining voltage and provide several overload protection features. a sufficiently low duty cycle if switching frequency were The first part of this section deals with selecting resistors maintained at 500kHz, so frequency is reduced by about to set output voltage and the second part talks about 5:1 when the feedback pin voltage drops below 0.8V (see foldback frequency and current limiting created by the FB Frequency Foldback graph). This does not affect operation pin. Please read both parts before committing to a final with normal load conditions; one simply sees a gear shift design. in switching frequency during start-up as the output The suggested value for the output divider resistor (see voltage rises. Figure 2) from FB to ground (R2) is 5k or less, and a In addition to lower switching frequency, the LT3431 also formula for R1 is shown below. The output voltage error operates at lower switch current limit when the feedback caused by ignoring the input bias current on the FB pin is pin voltage drops below 0.6V. Q2 in Figure 2 performs this less than 0.25% with R2 = 5k. A table of standard 1% function by clamping the V values is shown in Table 1 for common output voltages. C pin to a voltage less than its normal 2.1V upper clamp level. This foldback current limit Please read the following if divider resistors are increased greatly reduces power dissipation in the IC, diode and in- above the suggested values. ductor during short-circuit conditions. External synchro- nization is also disabled to prevent interference with fold- R2 V ( . ) OUT − 1 22 R1= back operation. Again, it is nearly transparent to the user 1.22 under normal load conditions. The only loads that may be affected are current source loads which maintain full load
Table 1
current with output voltage less than 50% of final value. In
OUTPUT R1 % ERROR AT OUTPUT
these rare situations the feedback pin can be clamped above
VOLTAGE R2 (NEAREST 1%) DUE TO DISCREET 1% (V) (k
Ω
) (k
Ω
) RESISTOR STEPS
0.6V with an external diode to defeat foldback current limit. 3 4.99 7.32 + 0.32 Caution: clamping the feedback pin means that frequency 3.3 4.99 8.45 – 0.43 shifting will also be defeated, so a combination of high in- 5 4.99 15.4 – 0.30 put voltage and dead shorted output may cause the LT3431 to lose control of current limit. 6 4.75 18.7 + 0.40 8 4.47 24.9 + 0.20 The internal circuitry which forces reduced switching 10 4.32 30.9 – 0.54 frequency also causes current to flow out of the feedback 12 4.12 36.5 + 0.24 pin when output voltage is low. The equivalent circuitry is 15 4.12 46.4 – 0.27 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
More Than Just Voltage Feedback
approximately 3.5kHz/µA. To ensure adequate frequency The feedback pin is used for more than just output voltage foldback (under worst-case short-circuit conditions), the sensing. It also reduces switching frequency and current external divider Thevinin resistance must be low enough limit when output voltage is very low (see the Frequency to pull 115µA out of the FB pin with 0.44V on the pin (RDIV Foldback graph in Typical Performance Characteristics). ≤ 3.8k). The net result is that reductions in frequency and This is done to control power dissipation in both the IC and current limit are affected by output voltage divider imped- in the external diode and inductor during short-circuit ance. Although divider impedance is not critical, caution conditions. A shorted output requires the switching regu- should be used if resistors are increased beyond the lator to operate at very low duty cycles, and the average suggested values and short-circuit conditions can possi- current through the diode and inductor is equal to the bly occur with high input voltage. High frequency pickup sn3431 3431fs 8