Application Information

CIRCUIT DESCRIPTION

The LM3590 is a constant current series White-LED Driver,

providing up to 20mA from an input voltage between 7.5V to

12.6V. To set the LED drive current, the LM3590 uses a

resistor connected to the I

This reference current is then multiplied and mirrored to the

constant current output, I

controlled by applying a PWM (Pulse Width Modulation)

signal to the Enable pin (EN). (see PWM BRIGHTNESS

CONTROL PROCEDURES section).

ENABLE MODE

The Enable pin (EN) disables the part and reduces the

quiescent current to 0.1µA (typ.). The LM3590 has an active-

high enable pin (LOW = shut down, HIGH = operating). The

LM3590 EN pin can be driven with a low-voltage CMOS logic

signal (1.5V logic, 1.8V logic, etc). There is an internal

500k

Ω pull-down between the EN and GND pins of the

LM3590.

CAPACITOR SELECTION

Although not required for normal operation, a capacitor can

be added to the voltage input of the LM3590 to reduce line

noise. A surface-mount

multi-layer

ceramic

capacitor

(MLCC) is recommended. MLCCs are small, inexpensive

and have very low equivalent series resistance (ESR,

≤15mΩ typ.). MLCCs with a X5R or X7R temperature char-

acteristic are preferred for use with the LM3590. Table 1.

Ceramic Capacitor Manufacturers lists suggested capacitor

suppliers for the typical application circuit.

TABLE 1. Ceramic Capacitor Manufacturers

Manufacturer

Contact

TDK

www.component.tdk.com

Murata

www.murata.com

Taiyo Yuden

www.t-yuden.com

LED SELECTION

The LM3590 is designed to drive up to 3 LEDs with the

combined forward voltages of the LEDs being no greater

than 11.5V, when using a 12V input supply. The typical and

maximum diode forward voltage depends highly on the

manufacturer and their technology. Table 2. White LED Se-

lection lists two suggested manufacturers. LED Forward cur-

rent matching is guaranteed by design, due to the series

LED configuration of the LM3590.

TABLE 2. White LED Selection

Manufacturer

Contact

Osram

www.osram-os.com

Nichia

www.nichia.com

LED HEADROOM VOLTAGE (V

A single current source is connected internally between V

IN

and I

V

rent source requires a sufficient amount of headroom voltage

to be present across it in order to regulate properly. Minimum

required headroom voltage is proportional to the current

flowing through the current source, as dictated by the equa-

tion:

V

The parameter k

proportionality constant that represents the ON-resistance of

the internal current mirror transistors. For worst-case design

calculations, using a k

(Worst-case recommendation accounts for parameter shifts

from part-to-part variation and applies over the full operating

temperature range). Figure 1 shows how output current of

the LM3590 varies with respect to headroom voltage.

On the flat part of the graph, the current is regulated properly

as there is sufficient headroom voltage for regulation. On the

sloping part of the graph the headroom voltage is too small,

the current source is squeezed, and the current drive capa-

bility is limited. Thus, operating the LM3590 with insufficient

headroom voltage across the current source should be

avoided.

I

An external resistor, R

output current. The internal current mirror sets the series

LED output current with a 100:1 ratio to the current through

R

by the series LED drive topology. The following equation

approximates the LED current:

I

PWM BRIGHTNESS CONTROL PROCEDURES

The brightness of the LEDs can be linearly varied from zero

up to the maximum programmed current level by applying a

Pulse-Width-Modulated signal to the EN pin of the LM3590.

The following procedures illustrate how to program the LED

drive current and adjust the output current level using a

PWM signal.

1.

Determine the maximum desired I

I

20081312

FIGURE 1. I

V

V

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