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LT3590

3590f

3590 F08

VREG

CTRL

VIN

LED

OUT

GND

CTRL

GND

CTRL

GND

VREG

VIN

(a) SC70 Package

(b) 2mm × 2mm DFN Package

Figure 8. Recommended Component Placement

APPLICATIONS INFORMATION

The dimming range can be further extended by changing

the amplitude of the PWM signal. The height of the PWM

signal sets the commanded sense voltage across the sense

resistor through the CTRL pin. In this manner both analog

dimming and direct PWM dimming extend the dimming

range for a given application. The color of the LEDs no

longer remains constant because the forward current of

the LED changes with the height of the CTRL signal. For

the ten LED application described above, the LEDs can be

dimmed ﬁrst, modulating the duty cycle of the PWM signal.

Once the minimum duty cycle is reached, the height of the

PWM signal can be decreased below 1.5V down to 150mV.

The use of both techniques together allows the average

LED current for the ten LED application to be varied from

50mA down to less than 50μA.

Internal Voltage Regulator

The LT3590 has a 3.3V onboard voltage regulator capable

of sourcing up to 1mA of current for use by an external

device. This feature may be used to power-up a controller

from the LT3590. The 3.3V is available even during shut-

down. It is required to place a 0.1μF capacitor from VREG

to ground. The regulator current is limited to 1.5mA.

Board Layout Considerations

As with all switching regulators, careful attention must be

paid to the PCB board layout and component placement.

To prevent electromagnetic interference (EMI) problems,

proper layout of high frequency switching paths is essen-

tial. Minimize the length and area of all traces connected

to the switching node pin (SW). Keep the sense voltage

pins (VIN and LED) away from the switching node. Place

the output capacitor, C2, next to the VIN pin. Always use

a ground plane under the switching regulator to minimize

interplane coupling. Recommended component placement

is shown in Figure 8.

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