Datasheet LT3475, LT3475-1 (Analog Devices) - 8

LT3475/LT3475-1
APPLICATIONS INFORMATION Open Circuit Protection
OUT The LT3475 has internal open-circuit protection. If the LED 10k is absent or is open circuit, the LT3475 clamps the voltage 22V V on the LED pin at 14V. The switching regulator then oper- C ates at a very low frequency to limit the input current. The 100k LT3475-1 has no internal open circuit protection. With the LT3475-1, be careful not to violate the ABSMAX voltage of th BOOST pin; if V 3475 F01 IN > 25V, external open circuit protection circuitry (as shown in Figure 1) may be necessary.The
Figure 1. External Overvoltage Protection Circuitry for the LT3475-1
output voltage during an open LED condition is shown in the Typical Performance Characteristics section. LT3475 VIN VIN
Undervoltage Lockout
2.6V R1 VC SHDN Undervoltage lockout (UVLO) is typically used in situations where the input supply is current limited, or has high source 9μA C1 R2 GND resistance. A switching regulator draws constant power 3475 F02 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
Figure 2. Undervoltage Lockout
or latch low under low source voltage conditions. UVLO Keep the connections from the resistors to the SHDN pin prevents the regulator from operating at source voltages short and make sure the coupling to the SW and BOOST where these problems might occur. pins is minimized. If high resistance values are used, the An internal comparator will force the part into shut- SHDN pin should be bypassed with a 1nF capacitor to down when VIN falls below 3.7V. If an adjustable UVLO prevent coupling problems from switching nodes. threshold is required, the SHDN pin can be used. The threshold voltage of the SHDN pin comparator is 2.6V. An
Setting the Switching Frequency
internal resistor pulls 9μA to ground from the SHDN pin The LT3475 uses a constant frequency architecture that at the UVLO threshold. can be programmed over a 200kHz to 2MHz range with a Choose resistors according to the following formula: single external timing resistor from the RT pin to ground. A graph for selecting the value of R 2.6V T for a given operating R2 = frequency is shown in the Typical Applications section. VTH – 2.6V – 9μA R1
Table 1. Switching Frequencies SWITCHING FREQUENCY (MHz) RT (kΩ)
VTH = UVLO Threshold 2 4.32 Example: Switching should not start until the input is 1.5 6.81 above 8V. 1.2 9.09 V 1 11.8 TH = 8V 0.8 16.9 R1=100k 0.6 24.3 2.6V R2 = = 57.6k 0.4 40.2 8V – 2.6V – 9μA 0.3 57.6 100k 0.2 100 3475fb 8