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LT3435 Datasheet(PDF) 9 Page - Linear Technology |
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LT3435 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  LT3474/LT3474-1 9 3474fd APPLICATIONS INFORMATION The switch driver operates either from VIN or from the BOOST pin. An external capacitor and internal Schottky diode are used to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to saturate the internal bipolar NPN power switch for ef- ficient operation. Open Circuit Protection The LT3474 has internal open circuit protection. If the LED is absent or fails open, the LT3474 clamps the voltage on the LED pin at 14V. The switching regulator then skips cycles to limit the input current. The LT3474-1 has no internal open circuit protection. With the LT3474-1, be careful not to violate the ABSMAX voltage of the BOOST pin; if VIN > 25V, external open circuit protection circuitry (as shown in Figure 2) may be necessary. The output voltage during an open LED condition is shown in the Typical Performance Characteristics section. Undervoltage Lockout Undervoltage lockout (UVLO) is typically used in situations where the input supply is current limited, or has high source resistance. A switching regulator draws constant power from the source, so the source current increases as the source voltage drops. This looks like a negative resistance load to the source and can cause the source to current limit or latch low under low source voltage conditions. UVLO prevents the regulator from operating at source voltages where these problems might occur. An internal comparator will force the part into shutdown when VIN falls below 3.5V. If an adjustable UVLO threshold is required, the SHDN pin can be used. The threshold voltage of the SHDN pin comparator is 2.65V. A internal resistor pulls 10.3μA to ground from the SHDN pin at the UVLO threshold. Choose resistors according to the following formula: R2 = 2.65V V TH – 2.65V R1 – 10.3μA VTH = UVLO Threshold Example: Switching should not start until the input is above 8V. VTH = 8V R1 = 100k R2 = 2.65V 8V – 2.65V 100k – 10.3μA = 61.9k Keep the connections from the resistors to the SHDN pin short and make sure the coupling to the SW and BOOST pins is minimized. If high resistance values are used, the SHDN pin should be bypassed with a 1nF capacitor to prevent coupling problems from switching nodes. GND 10.3μA 2.65V VIN VC LT3474 R1 C1 R2 VIN SHDN 3474 F03 Figure 3. Undervoltage Lockout Figure 2. External Overvoltage Protection Circuitry for the LT3474-1. VC 100k 10k 27V OUT 3474 F02 |
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