SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
APPLICATIONS INFORMATION
LED current control
The LED current is controlled by the feedback resistor. The feedback reference is 95mV. The LED current is 95
mV/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.
(1) R1 = 95 mV / I
LED
Table 1. R1 resistor value selection
ILED (mA)
5
10
12
15
20
R1 (Ω)
19.1
9.53
7.87
6.34
4.75
SM8237
SM8237
Figure 2. LED driver with open-circuit protection
Setting the output voltage for DC to DC Converter
Set the output voltage by selecting the resistive voltage divider ratio. Using 51 KΩ for the High-side resistor R
1
of the voltage divider. Determine the Low-side resistor R
2
according to the following formula:
R
2
=
R
1
· V
FB
Vout - V
FB
Where Vout is the output voltage for R
1
= 51 KΩ and V
FB
= 0.095 V, then R
2
(KΩ) = 4.845 / Vout -0.095
Please see the typical application on page 1.
Open-circuit protection
In the cases of output circuit, when the LEDs are disconnected from the circuit or the LEDs fail, the feedback
voltage will be zero. The SM8237 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 2). The zener voltage should be larger than the maximum forward voltage
of the LED string. The current rating zener should be larger than 0.1mA.
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 the SHDN pin, the SM8237 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 SM8237 and corresponds to zero full 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.
Page 9
V.1.3 Aug 28,2007
SAMHOP [ SAMHOP MICROELECTRONICS CORP. ]
