LM2794TL National Semiconductor, LM2794TL Datasheet - Page 10
LM2794TL
Manufacturer Part Number
LM2794TL
Description
IC LED DRVR WHT BCKLT 14MICROSMD
Manufacturer
National Semiconductor
Datasheet
1.LM2794TLX.pdf
(16 pages)
Specifications of LM2794TL
Constant Current
Yes
Topology
PWM, Switched Capacitor (Charge Pump), Step-Up (Boost)
Number Of Outputs
4
Internal Driver
Yes
Type - Primary
Backlight
Type - Secondary
White LED
Frequency
325kHz ~ 675kHz
Voltage - Supply
2.7 V ~ 5.5 V
Voltage - Output
3.8V
Mounting Type
Surface Mount
Package / Case
14-UFBGA
Operating Temperature
-30°C ~ 85°C
Current - Output / Channel
20mA
Lead Free Status / RoHS Status
Contains lead / RoHS non-compliant
Other names
*LM2794TL
LM2794TLCT
LM2794TLCT
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Company
Part Number
Manufacturer
Quantity
Price
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Part Number:
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Manufacturer:
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Quantity:
10 000
Part Number:
LM2794TLX
Manufacturer:
NS/国半
Quantity:
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Company:
Part Number:
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Manufacturer:
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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
20028540
BRGT
. An