LM2794EVAL National Semiconductor, LM2794EVAL Datasheet - Page 9

• Manufacturer Part Number: LM2794EVAL
• Description: BOARD EVALUATION LM2794
• Manufacturer: National Semiconductor

Specifications of LM2794EVAL

• Current - Output / Channel: 80mA
• Outputs And Type: 4, Non-Isolated
• Voltage - Output: 4 V
• Features: Charge Pump
• Voltage - Input: 2.7 ~ 5.5V
• Utilized Ic / Part: LM2794
• Lead Free Status / RoHS Status: Not applicable / Not applicable

Functional Block Diagram
Application Information
CIRCUIT DESCRIPTION
The LM2794/5 is a 1.5x/1x CMOS charge pump with four
matched constant current outputs, each capable of driving
up to 20mA through White LEDs. This device operates over
the extended Li-Ion battery range from 2.7V to 5.5V. The
LM2794/5 has four regulated current sources connected to
the device’s 1.5x charge pump output (P
ages below 4.7V (typ.), the charge-pump provides the
needed voltage to drive high forward voltage drop White
LEDs. It does this by stepping up the P
the input voltage. The charge pump operates in Pass Mode,
providing a voltage on P
the input voltage is at or above 4.7V (typ.). The device can
drive up to 80mA through any combination of LEDs con-
nected to the constant current outputs D
To set the LED drive current, the device uses a resistor
connected to the I
SET
pin to set a reference current. This
OUT
equal to the input voltage, when
OUT
1
OUT
-D
voltage 1.5 times
4
). At input volt-
.
9
reference current is then multiplied and mirrored to each
constant current output. The LED brightness can then be
controlled by analog and/or digital methods. Applying an
analog voltage in the range of 0V to 3.0V to the Brightness
pin (BRGT) adjusts the dimming profile of the LEDs. The
digital technique uses a PWM (Pulse Width Modulation)
signal applied to the Shutdown pin (SD). (see I
BRGT PINS section).
SOFT START
Soft start is implemented internally by ramping the reference
voltage more slowly than the applied voltage. During soft
start, the current through the LED outputs will ramp up in
proportion to the rate that the reference voltage is being
ramped up.
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