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LT1507I Datasheet(PDF) 8 Page - Linear Technology |
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LT1507I Datasheet(HTML) 8 Page - Linear Technology |
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8 / 20 page  8 LT1507 APPLICATIONS INFORMATION committing to a final design. The fixed 3.3V LT1507-3.3 has internal divider resistors and the FB pin is renamed SENSE, connected directly to the output. The suggested value for the output divider resistor from FB to ground (R2) is 5k or less and the 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. Please read below if R2 is increased above the suggested value. R1 R2(V – 2.42) 2.42 OUT = 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 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 regulator 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 2A of the LT1507, folding back to less than 1A). 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 LT1507 also operates at lower switch current limit when the feedback pin voltage drops below 1.5V. This foldback current limit greatly reduces power dissipation in the IC, diode and inductor during short-circuit conditions. Again, it is nearly transparent to the user under normal load conditions. The only loads which 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 current 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 LT1507 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. If the FB pin falls below 1V, current begins to flow out of the pin and reduces frequency at the rate of approximately 5kHz/ µA. To ensure adequate frequency foldback (under worst-case short-circuit con- ditions) the external divider Thevinin resistance must be low enough to pull 150 µA out of the FB pin with 0.6V on the VIN SHDN VSW FB BOOST GND VC LT1507 D2 1N914 C2 0.1 µF L1*** 5 µH D1 1N5818 R2 4.99k R1 5.36k CC 3.3nF + C1** 100 µF 10V TANTALUM OUTPUT 5V DEFAULT (OPEN) = ON + C3* 33 µF 20V TANTALUM VIN * ** *** AVX TPSD337M020R0200 OR SPRAGUE 593 EQUIVALENT. RIPPLE CURRENT RATING ≥ 0.6A AVX TPSD108M010R0100 OR SPRAGUE 593 EQUIVALENT COILTRONICS CTX5-1. SUBSTITUTION UNITS SHOULD BE RATED AT ≥ 1.25A, USING LOW LOSS CORE MATERIAL. LOAD CURRENTS ABOVE 0.85A MAY NEED A 10 µH OR 20µH INDUCTOR LT1507 • F01 Figure 2. Typical Schematic for LT1507 Adjustable Application |
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