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參數(shù)資料
| 型號： | LM2794 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Current Regulated Switched Capacitor LED Supply with Analog and PWM Brightness Control |
| 中文描述： | 電流調(diào)節(jié)開關(guān)電容LED電源與模擬和PWM亮度控制 |
| 文件頁數(shù)： | 10/16頁 |
| 文件大?。?/td> | 487K |
| 代理商： | LM2794 |
Application Information
(Continued)
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
of the LM2795. This allows the
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,
≤
15m
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
±
10%), hold their value over temperature
(X7R:
±
15% over 55C to 125C; X5R:
±
15% over 55C
to 85C), 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 30C to +85C
range; Z5U: +22%, 56% over +10C to +85C 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.
TABLE 1. Ceramic Capacitor Manufacturers
Manufacturer
TDK
Murata
Taiyo Yuden
Contact
www.component.tdk.com
www.murata.com
www.t-yuden.com
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.
TABLE 2. White LED Selection
Manufacturer
Osram
Nichia
Contact
www.osram-os.com
www.nichia.com
I
SET
AND BRGT PINS
An external resistor, R
, is connected to the I
pin to set
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
SET
. The current mirror
circuitry matches the current through each LED to within
0.5%.
In addition to R
SET
, a voltage may be applied to the V
BRGT
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
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 R
SET
. An
equation for approximating the LED current is:
20028540
20028536
FIGURE 1. Open-Drain Logic Shutdown Control
L
www.national.com
10
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