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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)： | 11/16頁 |
| 文件大?。?/td> | 487K |
| 代理商： | LM2794 |
Application Information
(Continued)
I
LED
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 I
current. Use the
I
equation to calculate R
by setting BRGT to 0V or
use
Table 3
to select a value for R
SET
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 (T
ON
= 100μs) of the device.
D x (1/f)
>
T
ON
f
MAX
= D
MIN
÷ T
ON
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 I
being half of the programmed
LED current. For example, if R
is set to program
15mA, a 50% duty cycle will result in an average I
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 I
LED
being half the programmed LED current.
Brightness Control Using BRGT
1.
Choose the maximum I
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 R
required or
use the I
LED
equation above to calculate R
SET
.
3.
Use
Table 4
as a reference for the dimming profile of the
LEDs, when BRGT ranges from 0V to 3V.
TABLE 3. R
SET
Values
LED Current
10mA
187
383
576
768
953
BRGT
0.0V
0.5V
1.0V
1.5V
2.0V
5mA
374
768
1.15K
1.54K
1.91K
15mA
124
255
383
511
624
20mA
93.1
191
287
383
475
LED Current
10mA
1.15K
1.33K
BRGT
2.5V
3.0V
R
SET
values are rounded off to the nearest 1% standard values.
5mA
2.32K
2.67K
15mA
768
909
20mA
576
665
TABLE 4. LED Current
R
SET
Values
1.33K
1.4mA
2.9mA
4.3mA
5.8mA
7.2mA
8.7mA
10.1mA
BRGT
0.0V
0.5V
1.0V
1.5V
2.0V
2.5V
3.0V
2.67K
0.7mA
1.4mA
2.1mA
2.9mA
3.6mA
4.3mA
5.0mA
909
2.1mA
4.2mA
6.3mA
8.4mA
10.5mA
12.7mA
14.8mA
665
2.8mA
5.7mA
8.6mA
11.5mA
14.4mA
17.3mA
20.2mA
CHARGE PUMP OUTPUT (P
OUT
)
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 P
pin) is nominally 1.5 x V
. This
rail can be used to deliver additional current to other circuitry.
Figure 2
shows how to connect additional LEDs to P
OUT
. 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 P
OUT
are recom-
mended for functions where brightness matching is not criti-
cal, such as keypad backlighting.
Since P
OUT
is unregulated, driving LEDs directly off P
OUT
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 P
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 P
is dependent on the input voltage
supplied to the LM2794/5, the total LM2794/5 output current,
and the output resistance (R
) 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 3.5
is recommended. (Worst-case
recommendation accounts for parameter shifts from part-to-
part variation and applies over the full operating temperature
range).
L
www.national.com
11
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