Datasheet LT3592 (Analog Devices) - 10
| Fabricante | Analog Devices |
| Descripción | 500mA Wide Input Voltage Range Step-Down LED Driver with 10:1 Dimming |
| Páginas / Página | 24 / 10 — APPLICATIONS INFORMATION Oscillator. Table 1. RT Values for Selector … |
| Formato / tamaño de archivo | PDF / 323 Kb |
| Idioma del documento | Inglés |
APPLICATIONS INFORMATION Oscillator. Table 1. RT Values for Selector Oscillator Frequencies. fOSC. FB Resistor Network
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LT3592
APPLICATIONS INFORMATION Oscillator
The BRIGHT mode current is given by: The frequency of operation is programmed by an external IBRIGHT = 200mV/RSENSE resistor from RT to ground. Table 1 shows RT values for The DIM mode current is 10% of the BRIGHT mode value. commonly used oscillator frequencies, and refer to the Typi- The maximum allowed DC value of the BRIGHT mode cur- cal Performance Characteristics curve for other values. rent is 500mA. When the recommended component values are used in a 900kHz 2 LED application, the transient from
Table 1. RT Values for Selector Oscillator Frequencies
switching between BRIGHT and DIM currents will be less
fOSC RT
than 50μs in duration. 400kHz 357k 900kHz 140k The sense resistor used should exhibit a low TC to keep 2.2MHz 48.7k the LED current from drifting as the operating temperature changes.
FB Resistor Network
The BRIGHT pin can tolerate voltages as high as 36V and The output voltage limit is programmed with a resistor can be safely tied to VIN even in high voltage applications, divider between the output and the VFB pin. This is the but it also has a low threshold voltage (~0.7V) that allows voltage that the output will be clamped to in case the LED it to interface to logic level control signals. goes open circuit. Choose the resistors according to
Input Voltage Range
R1 = R2([VOUT/1.21V] – 1) The maximum allowed input voltage for the LT3592 is Be sure to choose VOUT such that it does not interfere with 36V. The minimum input voltage is determined by either the operation of the current control loop; it should be set the LT3592’s minimum operating voltage of 3.6V or by at least 10% above the maximum expected LED voltage its maximum duty cycle. The duty cycle is the fraction of for the selected BRIGHT output current. R2 should be 20k time that the internal switch is on and is determined by or less to avoid bias current errors. An optional phase- the input and output voltages: lead capacitor of 22pF between VOUT and VFB reduces light-load ripple. V DC = OUT + VD V – V IN SW + VD
Output Current Selection
where V The output current levels are programmed by the value of D is the forward voltage drop of the catch diode (~0.4V) and V the external current sense resistor between CAP and OUT. SW is the voltage drop of the internal switch
Table 2. Inductor Vendor Information SUPPLIER PHONE FAX WEBSITE
Panasonic (800) 344-2112 www.panasonic.com/industrial/components/components.html Vishay (402) 563-6866 (402) 563-6296 www.vishay.com/resistors Coilcraft (847) 639-6400 (847) 639-1469 www.coilcraft.com CoEv Magnetics (800) 227-7040 (650) 361-2508 www.circuitprotection.com/magnetics.asp Murata (814) 237-1431 (814) 238-0490 www.murata.com (800) 831-9172 Sumida USA: (847) 956-0666 USA: (847) 956-0702 www.sumida.com Japan: 81(3) 3607-5111 Japan: 81(3) 3607-5144 TDK (847) 803-6100 (847) 803-6296 www.component.tdk.com TOKO (847) 297-0070 (847) 699-7864 www.tokoam.com 3592fc 10
APPLICATIONS INFORMATION Oscillator
The BRIGHT mode current is given by: The frequency of operation is programmed by an external IBRIGHT = 200mV/RSENSE resistor from RT to ground. Table 1 shows RT values for The DIM mode current is 10% of the BRIGHT mode value. commonly used oscillator frequencies, and refer to the Typi- The maximum allowed DC value of the BRIGHT mode cur- cal Performance Characteristics curve for other values. rent is 500mA. When the recommended component values are used in a 900kHz 2 LED application, the transient from
Table 1. RT Values for Selector Oscillator Frequencies
switching between BRIGHT and DIM currents will be less
fOSC RT
than 50μs in duration. 400kHz 357k 900kHz 140k The sense resistor used should exhibit a low TC to keep 2.2MHz 48.7k the LED current from drifting as the operating temperature changes.
FB Resistor Network
The BRIGHT pin can tolerate voltages as high as 36V and The output voltage limit is programmed with a resistor can be safely tied to VIN even in high voltage applications, divider between the output and the VFB pin. This is the but it also has a low threshold voltage (~0.7V) that allows voltage that the output will be clamped to in case the LED it to interface to logic level control signals. goes open circuit. Choose the resistors according to
Input Voltage Range
R1 = R2([VOUT/1.21V] – 1) The maximum allowed input voltage for the LT3592 is Be sure to choose VOUT such that it does not interfere with 36V. The minimum input voltage is determined by either the operation of the current control loop; it should be set the LT3592’s minimum operating voltage of 3.6V or by at least 10% above the maximum expected LED voltage its maximum duty cycle. The duty cycle is the fraction of for the selected BRIGHT output current. R2 should be 20k time that the internal switch is on and is determined by or less to avoid bias current errors. An optional phase- the input and output voltages: lead capacitor of 22pF between VOUT and VFB reduces light-load ripple. V DC = OUT + VD V – V IN SW + VD
Output Current Selection
where V The output current levels are programmed by the value of D is the forward voltage drop of the catch diode (~0.4V) and V the external current sense resistor between CAP and OUT. SW is the voltage drop of the internal switch
Table 2. Inductor Vendor Information SUPPLIER PHONE FAX WEBSITE
Panasonic (800) 344-2112 www.panasonic.com/industrial/components/components.html Vishay (402) 563-6866 (402) 563-6296 www.vishay.com/resistors Coilcraft (847) 639-6400 (847) 639-1469 www.coilcraft.com CoEv Magnetics (800) 227-7040 (650) 361-2508 www.circuitprotection.com/magnetics.asp Murata (814) 237-1431 (814) 238-0490 www.murata.com (800) 831-9172 Sumida USA: (847) 956-0666 USA: (847) 956-0702 www.sumida.com Japan: 81(3) 3607-5111 Japan: 81(3) 3607-5144 TDK (847) 803-6100 (847) 803-6296 www.component.tdk.com TOKO (847) 297-0070 (847) 699-7864 www.tokoam.com 3592fc 10