LM3557 Datasheet, PDF (7/11 Pages) National Semiconductor (TI) – Step-Up Converter for White LED Applications

Application Information (Continued)
FIGURE 3. Programmable Output Voltage
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WHITE LED CURRENT SETTING
For backlighting applications, the white LED current is pro-
grammed by the careful choice of the R2 resistor.
Backlight:
V_En≥ 0.9V
adjust the OVP threshold of a given application. Instead of
having the Ovp pin connected to the output voltage, it can be
adjusted through a resistor divider circuit to only experience
a fraction of the output voltage magnitude. The resistor
divider circuit bias current should be at least 100 times
greater than the Ovp pin bias current. Using Figure 3, the
following equation can be used to adjust the output voltage:
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I_LED: White LED Current.
V_Fb-Sw2: Feedback Voltage.
R2:
Resistor.
The feedback voltage is with respect to the voltage at the
Sw2 pin, not ground. For example, if the voltage on the Sw2
pin were 0.1V then the voltage at the Fb pin would be 0.61V
(typical).
ADJUSTING OVER-VOLTAGE PROTECTION
If the over-voltage protection (OVP) threshold is too low for a
particular application, a resistor divider circuit can be used to
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V_OVP: OVP Voltage Threshold.
V_OUT: Maximum Output Voltage (<35V).
R3: Resistor.
R4: Resistor.
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