Datasheet LT3474, LT3474-1 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | Step-Down 1A LED Driver |
| Pages / Page | 20 / 9 — APPLICATIONS INFORMATION. Open Circuit Protection. Undervoltage Lockout. … |
| File Format / Size | PDF / 226 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Open Circuit Protection. Undervoltage Lockout. Figure 3. Undervoltage Lockout
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LT3474/LT3474-1
APPLICATIONS INFORMATION
The switch driver operates either from VIN or from the An internal comparator will force the part into shutdown BOOST pin. An external capacitor and internal Schottky when VIN falls below 3.5V. If an adjustable UVLO threshold diode are used to generate a voltage at the BOOST pin is required, the SHDN pin can be used. The threshold that is higher than the input supply. This allows the driver voltage of the SHDN pin comparator is 2.65V. A internal to saturate the internal bipolar NPN power switch for ef- resistor pulls 10.3μA to ground from the SHDN pin at the fi cient operation. UVLO threshold. Choose resistors according to the following formula:
Open Circuit Protection
2.65V The LT3474 has internal open circuit protection. If the LED is R2 = absent or fails open, the LT3474 clamps the voltage on the V – 2.65V TH – 10.3μA LED pin at 14V. The switching regulator then skips cycles R1 to limit the input current. The LT3474-1 has no internal V open circuit protection. With the LT3474-1, be careful not TH = UVLO Threshold to violate the ABSMAX voltage of the BOOST pin; if VIN > Example: Switching should not start until the input is 25V, external open circuit protection circuitry (as shown in above 8V. Figure 2) may be necessary. The output voltage during an VTH = 8V open LED condition is shown in the Typical Performance Characteristics section. R1 = 100k 2.65V
Undervoltage Lockout
R2 = = 61.9k 8V – 2.65V –10.3μA Undervoltage lockout (UVLO) is typically used in situations 100k where the input supply is current limited, or has high source resistance. A switching regulator draws constant power Keep the connections from the resistors to the SHDN pin from the source, so the source current increases as the short and make sure the coupling to the SW and BOOST source voltage drops. This looks like a negative resistance pins is minimized. If high resistance values are used, the load to the source and can cause the source to current limit SHDN pin should be bypassed with a 1nF capacitor to or latch low under low source voltage conditions. UVLO prevent coupling problems from switching nodes. prevents the regulator from operating at source voltages where these problems might occur. LT3474 VIN VIN OUT 2.65V R1 V 10k C SHDN 27V VC 10.3μA C1 R2 GND 100k 3474 F03 3474 F02
Figure 3. Undervoltage Lockout Figure 2. External Overvoltage Protection Circuitry for the LT3474-1.
3474fd 9
APPLICATIONS INFORMATION
The switch driver operates either from VIN or from the An internal comparator will force the part into shutdown BOOST pin. An external capacitor and internal Schottky when VIN falls below 3.5V. If an adjustable UVLO threshold diode are used to generate a voltage at the BOOST pin is required, the SHDN pin can be used. The threshold that is higher than the input supply. This allows the driver voltage of the SHDN pin comparator is 2.65V. A internal to saturate the internal bipolar NPN power switch for ef- resistor pulls 10.3μA to ground from the SHDN pin at the fi cient operation. UVLO threshold. Choose resistors according to the following formula:
Open Circuit Protection
2.65V The LT3474 has internal open circuit protection. If the LED is R2 = absent or fails open, the LT3474 clamps the voltage on the V – 2.65V TH – 10.3μA LED pin at 14V. The switching regulator then skips cycles R1 to limit the input current. The LT3474-1 has no internal V open circuit protection. With the LT3474-1, be careful not TH = UVLO Threshold to violate the ABSMAX voltage of the BOOST pin; if VIN > Example: Switching should not start until the input is 25V, external open circuit protection circuitry (as shown in above 8V. Figure 2) may be necessary. The output voltage during an VTH = 8V open LED condition is shown in the Typical Performance Characteristics section. R1 = 100k 2.65V
Undervoltage Lockout
R2 = = 61.9k 8V – 2.65V –10.3μA Undervoltage lockout (UVLO) is typically used in situations 100k where the input supply is current limited, or has high source resistance. A switching regulator draws constant power Keep the connections from the resistors to the SHDN pin from the source, so the source current increases as the short and make sure the coupling to the SW and BOOST source voltage drops. This looks like a negative resistance pins is minimized. If high resistance values are used, the load to the source and can cause the source to current limit SHDN pin should be bypassed with a 1nF capacitor to or latch low under low source voltage conditions. UVLO prevent coupling problems from switching nodes. prevents the regulator from operating at source voltages where these problems might occur. LT3474 VIN VIN OUT 2.65V R1 V 10k C SHDN 27V VC 10.3μA C1 R2 GND 100k 3474 F03 3474 F02
Figure 3. Undervoltage Lockout Figure 2. External Overvoltage Protection Circuitry for the LT3474-1.
3474fd 9
