Datasheet LT3430, LT3430-1 (Analog Devices) - 8

LT3430/LT3430-1
APPLICATIONS INFORMATION FEEDBACK PIN FUNCTIONS
average current through the diode and inductor is equal to the short-circuit current limit of the switch (typically The feedback (FB) pin on the LT3430/LT3430-1 is used to 4A for the LT3430/LT3430-1, folding back to less than set output voltage and provide several overload protection 2A). Minimum switch on time limitations would prevent features. The fi rst part of this section deals with selecting the switcher from attaining a suffi ciently low duty cycle if resistors to set output voltage and the second part talks switching frequency were maintained at 200kHz (100kHz about foldback frequency and current limiting created by LT3430-1), so frequency is reduced by about 5:1 (3:1 the FB pin. Please read both parts before committing to LT3430-1) when the feedback pin voltage drops below a fi nal design. 0.8V (see Frequency Foldback graph). This does not affect The suggested value for the LT3430 output divider resistor operation with normal load conditions; one simply sees (see Figure 2) from FB to ground (R2) is 5k or less, and a a gear shift in switching frequency during start-up as the formula for R1 is shown below. For the LT3430-1, choose output voltage rises. the resistors so that the Thevinin resistance of the divider In addition to lower switching frequency, the LT3430/ at the feedback pin is 7.5kΩ. The output voltage error LT3430-1 also operate at lower switch current limit when caused by ignoring the input bias current on the FB pin the feedback pin voltage drops below 0.6V. Q2 in Figure 2 is less than 0.25% with R2 = 5k. A table of standard 1% performs this 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 Please read the following if divider resistors are increased current limit greatly reduces power dissipation in the IC, above the suggested values. diode and inductor during short-circuit conditions. External R2 V ( −1 2 . 2) synchronization is also disabled to prevent interference R OUT 1= with foldback operation. Again, it is nearly transparent to 1.22 the user under normal load conditions. The only loads that
Table 1. *LT3430, **LT3430-1
may be affected are current source loads which maintain
OUTPUT R1 % ERROR AT OUTPUT
full load current with output voltage less than 50% of
VOLTAGE R2 (NEAREST 1%) DUE TO DISCREET 1%
fi nal value. In these rare situations the feedback pin can
(V) (k
Ω
) (k
Ω
) RESISTOR STEPS
be clamped above 0.6V with an external diode to defeat 3* 4.99 7.32 +0.32 foldback current limit. Caution: clamping the feedback 3.3* 4.99 8.45 –0.43 pin means that frequency shifting will also be defeated, 5* 4.99 15.4 –0.30 so a combination of high input voltage and dead shorted 12* 4.12 46.4 –0.27 output may cause the LT3430/LT3430-1 to lose control 3** 12.7 18.7 +0.54 of current limit. 3.3** 12.1 20.5 –0.40 The internal circuitry which forces reduced switching 5** 10 30.9 –0.20 frequency also causes current to fl ow out of the feedback 12** 8.25 73.2 +0.37 pin when output voltage is low. The equivalent circuitry is shown in Figure 2. Q1 is completely off during normal
More Than Just Voltage Feedback
operation. If the FB pin falls below 0.8V, Q1 begins to The feedback pin is used for more than just output voltage conduct current and LT3430 reduces frequency at the rate sensing. It also reduces switching frequency and current of approximately 1.4kHz/µA. To ensure adequate frequency limit when output voltage is very low (see the Frequency foldback (under worst-case short-circuit conditions), the Foldback graph in Typical Performance Characteristics). external divider Thevinin resistance (RTHEV) must be low This is done to control power dissipation in both the IC enough to pull 115µA out of the FB pin with 0.44V on and in the external diode and inductor during short-cir- the pin (RTHEV ≤ 3.8k)(LT3430-1 RTHEV ≈ 7.5k). The net cuit conditions. A shorted output requires the switching result is that reductions in frequency and current limit regulator to operate at very low duty cycles, and the are affected by output voltage divider impedance. Cau- 34301fa 8