PT6937 Datasheet, PDF (5/15 Pages) Princeton Technology Corp – White LED Step-Up Converter

White LED Step-Up Converter
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PT6937
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 PT6937 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 33V rating. A zener diode
can be used at the output to limit the voltage on the SW pin (Figure 1). 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.
L1
22µH
D
VIN
CIN
1µF
VIN
SW
COUT
0.22µF
R2
PT6937
1K
/SHDN FB
GND
R1
6.43Ω
Figure1. LED driver with open-circuit protection
DIMMING CONTROL
There are four different types of dimming control circuits:
USING A PWM SIGNAL TO /SHDN PIN
With the PWM signal applied to the /SHDN pin, the PT6937 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 PT6937 and corresponds to zero
LED current; and 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. The switching waveforms of the /SHDN pin PWM control are shown in the Figure 2 as
below.
PT6937 V1.1
-5-
March, 2006