6

LTC3490

3490fa

OPERATIO

The LTC3490 is a high efficiency, constant current source

for 1W high intensity white LEDs. These high intensity

LEDs require a fixed current of 350mA with a voltage

compliance of 2.8V to 4V.

The LTC3490 operates with 1 or 2 NiMH or alkaline cells.

It functions as a boost converter with a current sense re-

sistor providing the control feedback. If the battery voltage

is greater than the required LED compliance, it will cycle

off periodically to maintain the correct average current. It

features a low voltage start-up circuit that will start with an

input voltage of only 1V. Once the drive voltage exceeds

2.3V, the circuit operates from the drive voltage.

All of the loop compensation is internal; only the main filter

capacitor is needed for stable operation.

Dimming Function

During normal operation with the CTRL/SHDN pin con-

350mA. The drive current can be reduced by changing the

voltage on the CTRL/SHDN pin.

to the battery voltage. The LED drive current is given by the

formula:

I

V

V

mA

LED

CTRL

IN

=

⎛

⎝⎜

⎞

⎠⎟

500

0 2

•– .

at 350mA.

Open-Circuit Protection

Since this is a boost converter attempting to drive a cur-

rent into the load, an open or high impedance load will cause

the regulator loop to increase the output voltage in an ef-

fort to achieve regulation. To protect the device, maximum

output voltage is limited to 4.7V under all conditions.

Undervoltage Sense and Protection

The undervoltage lockout prevents excessive inductor

peak current and protects the batteries from deep dis-

charging which can damage them. The low-battery indica-

tor allows the end user to be made aware that the batteries

are nearing the end of their useful life.