Circuit Description

The LM3595 is a parallel white-LED driver with four matched

current outputs. This part is intended to be driven off of an

existing power supply rail or other regulated voltage source

found within the surrounding system. The matched current

regulators each have a 100:1 current ratio between the

OUTx outputs and the I

current through the LEDs without the use of external ballast

resistors. With a total of 100mA of total output current avail-

able, the LM3595 is easily capable of supplying 25mA per

each of the four outputs through the proper selection of the

R

the LM3595 with a PWM signal and/or an analog voltage.

Application Information

ENABLE / SHUTDOWN

When the voltage on the active-high-logic enable pin is low,

the LM3595 will be in shutdown. While disabled, the LM3595

typically draws 0.1µA. When the EN pin is unconnected, the

part automatically goes into shutdown due to an internal

1M

Ω pull-down resistor that is tied between EN and GND.

OUTPUT CURRENT CAPABILITY

The LM3595 is capable of providing up to 25mA of current to

each of the four outputs given an input voltage of 3.0V to

5.5V. The outputs have a typical current matching of 0.5%

between adjacent sources. An external resistor can be used

to set the output current, as approximated with the following

the equation:

R

In order for the output currents to be regulated properly,

sufficient headroom voltage (V

headroom voltage refers to the minimum amount of voltage

that must be present across the current source in order to

ensure the desired current is realizable. To ensure the de-

sired current is obtained, apply the following equations to

find the minimum input voltage required:

V

≥ V

HR

V

the following equation:

V

I

OUTX

is the desired diode current, and k

typically

23mV/mA in the LM3595, is a proportionality constant that

represents the ON-resistance of the internal current mirror

transistors. For worst-case design calculations, using a k

HR

of 30mV/mA is recommended. (Worst-case recommendation

accounts for parameter shifts from part-to-part variation and

applies over the full operating temperature range). Changes

in headroom voltage from one output to the next, possible

with LED forward voltage mismatch, will result in different

output currents and LED brightness mismatch. Thus, oper-

ating the LM3595 with insufficient headroom voltage across

all current sources should be avoided.

TABLE 1. I

Voltage (BR-ON = 0V), k

I

OUT

R

SET

V

HEADROOM

10mA

12.4k

Ω

285mV

15mA

8.25k

Ω

430mV

25mA

4.88k

Ω

715mV

PWM BRIGHTNESS CONTROL

Brightness control can be implemented by pulsing a signal at

the EN pin. When a PWM signal is used to drive the EN pin

of the LM3595, the BR-ON pin should be driven low. The

R

when the analog brightness control feature is disabled. LED

brightness is proportional to the duty cycle (D) of the PWM

signal. For linear brightness control over the full duty cycle

adjustment range, the PWM frequency (f) should be limited

to accommodate the turn-on time (T

vice.

ON

f

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

be seen in the LEDs. For the LM3595, zero duty cycle will

turn off the LEDs and a 50% duty cycle will result in an

average I

example, if R

will result in an average I

programmed LED current. R

exceed the maximum current delivery capability of the de-

vice.

ANALOG BRIGHTNESS CONTROL

The LM3595 allows for analog brightness control using the

BR-ON, and BRGT pin. To enable this function, the BR-ON

pin must be set to a voltage greater or equal to 0.45*V

With analog brightness enabled, the voltage delivered to pin

BRGT can be used to adjust the output currents. Use the

maximum desired current (I

analog voltage to be used in the adjustment (V(BR)

size R

Table 2 shows the current through each LED for the LM3595

with various BRGT and RSET values.

When analog brightness is used, applying 0V to the BRGT

pin does not turn the LEDs off completely. There is a small

amount of leakage current (100µA) that typically causes a

small amount of light to be emitted from the LED. In order to

completely turn off the LEDs, drive the EN pin low to disable

the part.

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