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LT3599 Datasheet(PDF) 11 Page - Linear Technology |
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LT3599 Datasheet(HTML) 11 Page - Linear Technology |
11 / 26 page LT3599 3599fd The companies below offer Schottky diodes with high voltage and current ratings. Standard silicon diodes (PN junction diodes) should not be used. Table 3. Suggested Diodes PART MAX CURRENT (A) MAX REVERSE VOLTAGE (V) MANUFACTURER B250A DFLS240 B240A B350A B340A 2 2 2 3 3 50 40 40 50 40 Diodes, Inc. www.diodes.com HSM150G HSM150J HSM350G 1 1 3 50 50 50 Microsemi www.microsemi.com Overvoltage Protection The LT3599 uses the FB pin to provide overvoltage protec- tion. A resistor divider is connected between the VO_SW pin and ground (Figure 2). There is an internal PMOS switch between VOUT and VO_SW, which is controlled by the PWM signal. The PMOS switch addition prevents the feedback resistor divider from draining the output capaci- tor during PWM off-period, allowing for a higher dimming range without falsely tripping the OPENLED flag. It also reduces the system current in shutdown. This PMOS has about 1k resistance, so select FB resistor values taking this resistance into account. To set the maximum output voltage, select the values of R1 and R2 (see Figure 2) according to the following equation: VOUT(MAX) =1.223V 1+ R2 R1 The output voltage should be set 10% higher than the normal LED string operating voltage. Under normal operation, LED1 to LED4 pin voltages are monitored and provide feedback information to the converter for output voltage regulation given the programmed LED current. The output voltage regulation loop is activated only when all LEDs are open. Programming Maximum LED Current Maximum LED current can be programmed by placing a resistor between the ISET pin and ground (RISET). The ISET pin resistor can be selected from 11k to 44.2k. The LED current can be programmed according to the following equation: ILED ≈ 1330 RISET Amps ( ) (CTRL>1V) See Table 4 and Figure 3 for resistor values and corre- sponding programmed LED current. LED current can also be adjusted by programming the CTRL pin voltage. Table 4. RISET Value Selection for LED Current LED CURRENT (mA) RESISTOR ON ISET PIN (k) 30mA 44.2 50mA 26.7 99mA 13.3 120mA 11 applicaTions inForMaTion Figure 2. Overvoltage Protection Voltage Feedback Connections Figure 3. RISET Value Selection for LED Current 0 20 40 60 80 120 100 140 160 RISET (k) 80 60 40 20 0 120 100 3599 F03 3599 F02 LT3599 VO_SW FB VOUT R2 R1 |
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