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PDF LM2794 Data sheet ( Hoja de datos )

Número de pieza LM2794
Descripción Current Regulated Switched Capacitor LED Supply with Analog and PWM Brightness Control
Fabricantes National Semiconductor 
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May 2003
LM2794/LM2795
Current Regulated Switched Capacitor LED Supply with
Analog and PWM Brightness Control
General Description
The LM2794/95 is a fractional CMOS charge-pump that
provides four regulated current sources. It accepts an input
voltage range from 2.7V to 5.5V and maintains a constant
current determined by an external sense resistor.
The LM2794/5 delivers up to 80mA of load current to accom-
modate four White LEDs. The switching frequency is
325kHz. (min.) to keep the conducted noise spectrum away
from sensitive frequencies within portable RF devices.
Brightness can be controlled by both linear and PWM tech-
niques. A voltage between 0V and 3.0V may be applied to
the BRGT pin to linearly vary the LED current. Alternatively,
a PWM signal can be applied to the SD pin to vary the
perceived brightness of the LED. The SD pin reduces the
operating current to 2.3µA (typ.) The LM2794 is shut down
when the SD pin is low, and the LM2795 is shut down when
the SD pin is high.
The LM2794/95 is available in a micro SMD-14 CSP pack-
age.
Features
n Regulated current sources with ±0.5% matching
between any two outputs
n High efficiency 3/2 boost function
n Drives one, two, three or four white LEDs
n 2.7V to 5.5V Input Voltage
n Up to 80mA output current
n Analog brightness control
n Active-low or high shutdown input (’94/95)
n Very small solution size and no inductor
n 2.3µA (typ.) shutdown current
n 325kHz switching frequency (min.)
n Constant Frequency generates predictable noise
spectrum
n Standard Micro SMD-14 package: 2.08mm X 2.403mm
X 0.845mm High
n Thin Micro SMD-14 package: 2.08mm X 2.403mm X
0.600mm High
Applications
n White LED Display Backlights
n White LED Keypad Backlights
n 1-Cell Li-Ion battery-operated equipment including
PDAs, hand-held PCs, cellular phones
Basic Application Circuit
© 2003 National Semiconductor Corporation DS200285
20028503
www.national.com

1 page




LM2794 pdf
Typical Performance Characteristics Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1µF,
VIN = 3.6V, BRGT pin = 0V, RSET = 124.
IDIODE vs VIN
IDIODE vs BRGT
IDIODE vs VIN
BRGT = 3V
20028512
IDIODE vs RSET
20028509
IDIODE vs RSET
VBRGT = 0V
20028507
IDIODE vs VDIODE
20028508
20028541
5
20028524
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LM2794 arduino
Application Information (Continued)
Applying a voltage on BRGT between 0 to 3 volts will linearly
vary the LED current. Voltages above 3V do not increase the
LED current any further. The voltage on the VBRGT pin is fed
into an internal resistor network with a ratio of 0.385. The
resulting voltage is then summed with a measured offset
voltage of 0.188V, which comes from the reference voltage
being fed through a resistor network (See Functional Block
Diagram). The brightness control circuitry then uses the
summed voltage to control the voltage across RSET. An
equation for approximating the LED current is:
20028540
ILED CURRENT SELECTION PROCEDURES
The following procedures illustrate how to set and adjust
output current levels. For constant brightness or analog
brightness control, go to “Brightness control using BRGT”.
Otherwise refer to “Brightness control using PWM”.
Brightness Control Using PWM
1. Set the BRGT pin to 0V.
2. Determine the maximum desired ILED current. Use the
ILED equation to calculate RSET by setting BRGT to 0V or
use Table 3 to select a value for RSET when BRGT
equals 0V.
3. Brightness control can be implemented by pulsing a
signal at the SD pin. LED brightness is proportional to
the duty cycle (D) of the PWM signal. For linear bright-
ness control over the full duty cycle adjustment range,
the PWM frequency (f) should be limited to accommo-
date the turn-on time (TON = 100µs) of the device.
D x (1/f) > TON
fMAX = DMIN ÷ TON
If the PWM frequency is much less than 100Hz, flicker
may be seen in the LEDs. For the LM2794, zero duty
cycle will turn off the LEDs and a 50% duty cycle will
result in an average ILED being half of the programmed
LED current. For example, if RSET is set to program
15mA, a 50% duty cycle will result in an average ILED of
7.5mA. For the LM2795 however, 100% duty cycle will
turn off the LEDs and a 50% duty cycle will result in an
average ILED being half the programmed LED current.
Brightness Control Using BRGT
1. Choose the maximum ILED desired and determine the
max voltage to be applied to the BRGT pin. For constant
brightness, set BRGT to a fixed voltage between 0V to
3V.
2. Use Table 3 to determine the value of RSET required or
use the ILED equation above to calculate RSET.
3. Use Table 4 as a reference for the dimming profile of the
LEDs, when BRGT ranges from 0V to 3V.
TABLE 3. RSET Values
LED Current
BRGT
5mA
10mA 15mA 20mA
0.0V
374
187
12493.1
0.5V
768
383
255
191
1.0V 1.15K576
383
287
1.5V 1.54K768
511
383
2.0V 1.91K953
624
475
2.5V 2.32K1.15K768
576
3.0V 2.67K1.33K909
665
RSET values are rounded off to the nearest 1% standard values.
BRGT
0.0V
0.5V
1.0V
1.5V
2.0V
2.5V
3.0V
TABLE 4. LED Current
2.67K
0.7mA
1.4mA
2.1mA
2.9mA
3.6mA
4.3mA
5.0mA
RSET Values
1.33K909
1.4mA 2.1mA
2.9mA 4.2mA
4.3mA 6.3mA
5.8mA 8.4mA
7.2mA 10.5mA
8.7mA 12.7mA
10.1mA 14.8mA
665
2.8mA
5.7mA
8.6mA
11.5mA
14.4mA
17.3mA
20.2mA
CHARGE PUMP OUTPUT (POUT)
The LM2794/5 charge pump is an unregulated switched
capacitor converter with a gain of 1.5. The voltage at the
output of the pump (the POUT pin) is nominally 1.5 x VIN. This
rail can be used to deliver additional current to other circuitry.
Figure 2 shows how to connect additional LEDs to POUT. A
ballast resistor sets the current through each LED, and LED
current matching is dependent on the LED forward voltage
matching. Because of this, LEDs driven by POUT are recom-
mended for functions where brightness matching is not criti-
cal, such as keypad backlighting.
Since POUT is unregulated, driving LEDs directly off POUT is
usually practical only with a fixed input voltage. If the input
voltage is not fixed (Li-Ion battery, for example), using a
linear regulator between the POUT pin and the LEDs is
recommended. National Semiconductor’s LP3985-4.5V low-
dropout linear regulator is a good choice for such an appli-
cation.
The voltage at POUT is dependent on the input voltage
supplied to the LM2794/5, the total LM2794/5 output current,
and the output resistance (ROUT) of the LM2794/5 charge
pump. Output resistance is a model of the switching losses
of the charge pump. Resistances of the internal charge
pump switches (MOS transistors) are a primary component
of the LM2794/5 output resistance. Typical LM2794/5 output
resistance is 3.0. For worst-case design calculations, using
an output resistance of 3.5is recommended. (Worst-case
recommendation accounts for parameter shifts from part-to-
part variation and applies over the full operating temperature
range).
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