Detailed Block Diagram

20085003

Circuit Description

The LM3570 is a low noise white LED driver system. The LM3570 system revolves around a highly efficient regulated three

halves CMOS charge pump producing a output voltage (V

the output voltage is achieved through the use of a pre-regulation loop that produces a stable output voltage while also minimizing

conducted noise on the input rail. For voltages between 2.7V and 3.0V, the LM3570 behaves like an open loop 3/2 charge pump

where the output will be 1.5X the input voltage minus the losses associated with the output resistance (R

OUT

≈ 4Ω).

Connected to the regulated output are three internal tightly matched current sources ideal for driving white LEDs. The amount of

current driven through the LEDs is user selectable through the use of one external R

adjacent outputs on the LM3570 is +/-0.3%(typ.) allowing for uniform brightness across the LEDs. The LM3570 is capable of

delivering up to 80mA of total output current. Current may be pulled out of the dedicated current outputs (I

fixed output voltage rail is ideal for driving keypad LEDs in voltage mode through the use of external current limit resistors.

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