2.

Brightness control can be implemented by pulsing a

signal at the EN pin. LED brightness is proportional to

the duty cycle (D) of the PWM signal. For linear bright-

ness control over the full duty cycle adjustment range,

the PWM frequency (f) should be limited to accommo-

date the turn-on time (T

ON

f

If the PWM frequency is much less than 100Hz, flicker

may be seen in the LEDs. For the LM3590, zero duty

cycle will turn off the LEDs and a 50% duty cycle will

result in an average I

LED current. For example, if R

15mA, a 50% duty cycle will result in an average I

7.5mA.

POWER DISSIPATION

The power dissipation (P

ture (T

is the product of the input current and input voltage, P

the power consumed by the LEDs, T

perature, and

θ

tance for the SOT23-5 package. V

the LM3590, V

LEDs connected to the I

LED current.

P

=(V

T

θ

The junction temperature rating takes precedence over the

ambient temperature rating. The LM3590 may be operated

outside the ambient temperature rating, so long as the junc-

tion temperature of the device does not exceed the maxi-

mum operating rating of 110˚C. The maximum ambient tem-

perature rating must be derated in applications where high

power dissipation and/or poor thermal resistance causes the

junction temperature to exceed 110˚C.

Application Circuits

Figure 2 shows how to program the LED current to four

different DC levels using two digital logic signals. The pro-

grammed LED current is a function of the equivalent resis-

tance on the I

on SET1 and SET2. Example values for R1, R2, and RSET

an the resulting 4 current levels are shown below.

TABLE 3. Digital LED Current Programming

EN

SET1

SET2

R

ISET

Example R

ISET

Example I

OUT

0

X

X

Shutdown

Shutdown

Shutdown

11

1

R

31.6k

Ωi15.kΩi31.6kΩ

16mA

11

0

R

31.6k

Ωi15.kΩ

12mA

10

1

R

31.6k

Ωi31.6kΩ

8mA

10

0

R

SET

31.6k

Ω

4mA

20081313

FIGURE 2. Example: R

Ω,R

Ω,R

Ω

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