LM2794EVAL National Semiconductor, LM2794EVAL Datasheet - Page 10

• 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

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Application Information
SHUTDOWN MODE
The shutdown pin (SD) disables the part and reduces the
quiescent current to 2.3µA (typ.).
The LM2795 has an active-high shutdown pin (HIGH =
shutdown, LOW = operating). An internal pull-up is con-
nected between SD and V
use of open-drain logic control of the LM2795 shutdown, as
shown in Figure 1. The LM2795 SD pin can also be driven
with a rail-to-rail CMOS logic signal.
The LM2794 has an active-low shutdown pin (LOW = shut-
down, HIGH = operating). The LM2794 SD pin can be driven
with a low-voltage CMOS logic signal (1.5V logic, 1.8V logic,
etc). There is no internal pull-up or pull-down on the SD pin
of the LM2794.
CAPACITOR SELECTION
The LM2794/5 requires 4 external capacitors for proper
operation. Surface-mount multi-layer ceramic capacitors are
recommended. These capacitors are small, inexpensive and
have very low equivalent series resistance (ESR, ≤15mΩ
typ.). Tantalum capacitors, OS-CON capacitors, and alumi-
num electrolytic capacitors are generally not recommended
for use with the LM2794/5 due to their high ESR, as com-
pared to ceramic capacitors.
For most applications, ceramic capacitors with X7R or X5R
temperature characteristic are preferred for use with the
LM2794/5. These capacitors have tight capacitance toler-
ance (as good as
(X7R:
to 85˚C), and typically have little voltage coefficient. Capaci-
tors with Y5V or Z5U temperature characteristic are gener-
ally not recommended for use with the LM2794/5. Capaci-
tors with these temperature characteristics typically have
wide capacitance tolerance (+80%, −20%), vary significantly
over temperature (Y5V: +22%, −82% over −30˚C to +85˚C
range; Z5U: +22%, −56% over +10˚C to +85˚C range), and
have poor voltage coefficients. Under some conditions, a
nominal 1µF Y5V or Z5U capacitor could have a capacitance
of only 0.1µF. Such detrimental deviation is likely to cause
Y5V and Z5U capacitors to fail to meet the minimum capaci-
tance requirements of the LM2794/5. Table 1 lists suggested
capacitor suppliers for the typical application circuit.
FIGURE 1. Open-Drain Logic Shutdown Control
±
15% over −55˚C to 125˚C; X5R:
±
10%), hold their value over temperature
IN
of the LM2795. This allows the
±
(Continued)
20028536
15% over −55˚C
10
LED SELECTION
The LM2794/5 is designed to drive LEDs with a forward
voltage of about 3.0V to 4.0V. The typical and maximum
diode forward voltage depends highly on the manufacturer
and their technology. Table 2 lists two suggested manufac-
turers. Forward current matching is assured over the LED
process variations due to the constant current output of the
LM2794/5.
I
An external resistor, R
the current to be mirrored in each of the LED outputs. The
internal current mirror sets each LED output current with a
10:1 ratio to the current through R
circuitry matches the current through each LED to within
0.5%.
In addition to R
pin to vary the LED current. By adjusting current with the
Brightness pin (BRGT), the brightness of the LEDs can be
smoothly varied.
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 V
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 R
equation for approximating the LED current is:
SET
AND BRGT PINS
Manufacturer
Manufacturer
TABLE 1. Ceramic Capacitor Manufacturers
Taiyo Yuden
Murata
Osram
Nichia
TDK
TABLE 2. White LED Selection
SET
, a voltage may be applied to the V
SET
, is connected to the I
www.component.tdk.com
www.osram-os.com
www.t-yuden.com
www.murata.com
www.nichia.com
SET
Contact
Contact
. The current mirror
BRGT
SET
pin to set
pin is fed
SET
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BRGT
. An