LM3557 Datasheet, PDF (8/19 Pages) National Semiconductor (TI) – Step-Up Converter for White LED Applications

LM3557
SNVS338B – NOVEMBER 2004 – REVISED FEBRUARY 2013
APPLICATION INFORMATION
L
2.2 PH
D
Vout
R3
R4
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V_SUPPLY
Cin
4.7 PF
V_IN
Sw1 Ovp
LM3557 Fb
En
NC
Gnd Sw2
Cout
1 PF
R2
Figure 12. Programmable Output Voltage
WHITE LED CURRENT SETTING
For backlighting applications, the white LED current is programmed by the careful choice of the R2 resistor.
Backlight:
V_En≥ 0.9V
(1)
V_Fb-Sw2
I_LED = _R2
where
• I_LED white LED current
• V_Fb-Sw2 is the feedback voltage
• R2 is the resistor
(2)
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 LED CURRENT USING PWM SIGNAL
The LED current can be controlled using a PWM signal on the EN pin with frequencies in the range of 100Hz
(greater than visible frequency spectrum) to 1kHz. For controlling LED currents down to the µA levels, it is best
to use a PWM signal frequency between 200-500Hz. The LM3557 LED current can be controlled with PWM
signal frequencies above 1kHz but the controllable current decreases with higher frequency.
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 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 12, the following equation can be used to adjust the output voltage:
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