LM1937 HTC Korea TAEJIN Technology Co., LM1937 Datasheet - Page 5

• Manufacturer Part Number: LM1937
• Description: White Led Step-up Converter
• Manufacturer: HTC Korea TAEJIN Technology Co.

WHITE LED STEP-UP CONVERTER
LED Current Control
The LED current is controlled by the feedback resistor
(R1 in Figure 1). The feedback reference is 95mV.
The LED current is 95mV/R1. In order to have
accurate LED current, precision resistors are preferred
(1% is recommended). The formula and table for R1
selection are shown below. R1 = 95mV/ILED
Table 4. R1 Resistor Value Selection
Open-Circuit Protection
In the cases of output open circuit, when the LEDs are
disconnected from the circuit or the LEDs fail, the
feedback voltage will be zero. The LM1937 will then
switch at a high duty cycle resulting in a high output
voltage, which may cause the SW pin voltage to
exceed its maximum 36V rating. A zener diode can be
used at the output to limit the voltage on the SW pin
(Figure 5). The zener voltage should be larger than
the maximum forward voltage of the LED string. The
current rating of the zener should be larger than
0.1mA.
Figure 4. LED Driver with Open-Circuit Protection
− 5 −
Dimming Control
There are four different types of dimming control
circuits:
1. Using a PWM Signal to SHDN Pin
With the PWM signal applied to theSHDN pin, the
LM1937 is turned on or off by the PWM signal. The
LEDs operate at either zero or full current. The
average LED current increases proportionally with the
duty cycle of the PWM signal. A 0% duty cycle will
turn off the LM1937 and corresponds to zero LED
current. A 100% duty cycle corresponds to full current.
The typical frequency range of the PWM signal is
1kHz to 10kHz. The magnitude of the PWM signal
should be higher than the minimum SHDN voltage
high.
2. Using a DC Voltage
For some applications, the preferred method of
brightness control is a variable DC voltage to adjust
the LED current. The dimming control using a DC
voltage is shown in Figure 5. As the DC voltage
increases, the voltage drop on R2 increases and the
voltage drop on R1 decreases. Thus, the LED current
decreases. The selection of R2 and R3 will make the
current from the variable DC source much smaller
than the LED current and much larger than the FB pin
bias current. For VDC range from 0V to 2V, the
selection of resistors in Figure 7 gives dimming control
of LED current from 0mA to 15mA.
3. Using a Filtered PWM Signal
The filtered PWM signal can be considered as an
adjustable DC voltage. It can be used to replace the
variable DC voltage source in dimming control. The
circuit is shown in Figure 6.
Figure 5. Dimming Control Using a DC Voltage
LM1937
HTC