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LTC3490
OPERATIO
I LED = 500 ? ? CTRL – 0 . 2 ? mA
TheLTC3490isahighefficiency,constantcurrentsource
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-
nected to V IN , the LED drive current is controlled at
350mA. The drive current can be reduced by changing the
voltage on the CTRL/SHDN pin.
For V IN ? 0.2 < V CTRL < V IN ? 0.9, the LED current is
to the battery voltage. The LED drive current is given by the
formula:
? V ?
? V IN ?
When V CTRL > V IN ? 0.9, the LED drive current is clamped
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.
proportional to V CTRL /V IN . This allows a simple potenti-
ometer from V IN to control the current without sensitivity
3490fa
6
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