6

LT1937

1937f

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.

(1)

Table 4. R1 Resistor Value Selection

R1 (

Ω)

5

19.1

10

9.53

12

7.87

15

6.34

20

4.75

Open-Circuit Protection

In the cases of output open circuit, when the LEDs are

disconnected from the circuit or the LEDs fail, the feed-

back voltage will be zero. The LT1937 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.

APPLICATIO S I FOR ATIO

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 LT1937

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 LT1937 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. The switching waveforms of the SHDN pin

PWM control are shown in Figures 6a and 6b.

Figure 5. LED Driver with Open-Circuit Protection

200

µs/DIV

1937 F06a

FB

100mV/DIV

SHDN

2V/DIV

(6a) 1kHz

20

µs/DIV

1937 F06b

FB

100mV/DIV

SHDN

2V/DIV

(6b) 10kHz

Figure 6. PWM Dimming Control Using the SHDN Pin

PWM

LT1937

SHDN

SW

LT1937

L

22

µH

0.22

µF

1

µF

1937 F05

SHDN

R1

6.34

Ω

R2

1k

FB

D

GND