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LT3598 Datasheet(PDF) 11 Page - Linear Technology |
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LT3598 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LT3598 11 3598fb Overvoltage Protection The LT3598 uses the FB pin to provide regulated overvoltage protection when all LED strings are open. 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 capacitor 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 resistors 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: VV R R OUT MAX () . =+ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 1 230 1 2 1 APPLICATIONS INFORMATION Figure 2. Overvoltage Protection Voltage Feedback Connections The output voltage should be set 15% higher than the normal LED string operating voltage. Under normal operation, LED1 to LED6 pin voltages are monitored and provide feedback information to the converter for output voltage regulation given the programmed LED current. The maximum output regulation loop is activated only when all LEDs are open. Programming Maximum LED Current Maximum LED current is programmed by placing a resistor between the ISET pin and ground (RISET). The ISET pin resistor can be selected from 10k to 100k. The LED current is programmed according to the following equation: I V R LED ISET ≈ 294 See Table 4 and Figure 3 for resistor values and corr- esponding 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) 3mA 97.6 10mA 29.4 20mA 14.7 30mA 9.76 LED Current Dimming Two different types of dimming control can be used with the LT3598. The LED current can be set by modulating the CTRL pin or the PWM pin. For some applications, a variable DC voltage that adjusts the LED current is the preferred method of brightness control. The CTRL pin voltage can be modulated to set the dimming of the LED string (see Figures 4 and 5). As the voltage on the CTRL pin increases from 0V to 1.0V, the LED current increases from 0 to the programmed LED current level. As the CTRL pin voltage increases beyond 1V, it has no effect on the LED current. Figure 3. RISET Value Selection for LED Current 3598 F02 LT3598 VO_SW FB VOUT R2 R1 RISET (k) 0 20 25 15 10 40 80 20 60 100 5 0 30 3598 F03 |
Similar Part No. - LT3598_15 |
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Similar Description - LT3598_15 |
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