LM3570 Datasheet, PDF (1/13 Pages) National Semiconductor (TI) – Low Noise White LED Driver System

July 2004
LM3570
Low Noise White LED Driver System
General Description
The LM3570 device provides a complete LED driver solution
for wireless handsets and other portable devices using a
display and keypad. With three constant current sources, up
to three white LEDs can be used for display lighting with
excellent current matching (0.3% typ.). The regulated 4.35V
output voltage is perfect for driving auxiliary keypad LEDs in
voltage mode.
The LM3570 utilizes a high efficiency 3/2 CMOS charge-
pump with a pre-regulation loop that minimizes conducted
noise on the input. It accepts an input voltage range from
2.7V to 5.5V and maintains a constant current determined by
the current through an external R_SET resistor.
The device supplies up to 80mA of total load current to
accommodate any combination of up to three white LEDs,
and additional current from V_OUT. The switching frequency is
set at 500kHz. (typ.) to keep the conducted noise spectrum
away from sensitive frequencies within portable RF devices.
By applying a pulse width modulated (PWM) signal to the
PWM pin, the user has the ability to independently control
the brightness of the regulated current source outputs with-
out shutting down the regulated output voltage.
Features
n 2.7V to 5.5V Input Voltage
n Regulated Output Voltage (V_OUT = 4.35V)
n Regulated I_Dx with ±0.3% matching between constant
current outputs
n High Efficiency 3/2 Boost function
n Drives one, two, or three white LEDs with no bias
resistor losses
n Drives auxiliary keypad LEDS in voltage mode
n Up to 80mA total output current
n Active-High Enable
n Active-High PWM Control Pin for independent control of
current sources
n Very small solution size
n 1µA(max) shutdown current
n 500kHz switching frequency (typ.)
n Linear regulation generates predictable noise spectrum
n LLP-14 package: 4.0mm X 3.0mm X 0.8mm
Applications
n Portable devices using white or blue LEDs with display
and backlight or frontlight
n 1-Cell LiIon battery-operated equipment including PDAs,
hand-held PCs and cellular phones
Typical Application Circuit
© 2004 National Semiconductor Corporation DS200850
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