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参数资料
| 型号： | LM2794BL/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHED CAPACITOR REGULATOR, 675 kHz SWITCHING FREQ-MAX, PBGA14 |
| 封装： | LEAD FREE, CSP-14 |
| 文件页数： | 3/16页 |
| 文件大小： | 487K |
| 代理商： | LM2794BL/NOPB |
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
LED current. Use the
I
LED equation to calculate RSET 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 = 100s) of the device.
D x (1/f) > T
ON
f
MAX =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 I
LED being half of the programmed
LED current. For example, if R
SET is set to program
15mA, a 50% duty cycle will result in an average I
LED 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
LED 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
SET required or
use the I
LED 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. R
SET Values
LED Current
BRGT
5mA
10mA
15mA
20mA
0.0V
374
187
124
93.1
0.5V
768
383
255
191
1.0V
1.15K
576
383
287
1.5V
1.54K
768
511
383
2.0V
1.91K
953
624
475
LED Current
BRGT
5mA
10mA
15mA
20mA
2.5V
2.32K
1.15K
768
576
3.0V
2.67K
1.33K
909
665
RSET values are rounded off to the nearest 1% standard values.
TABLE 4. LED Current
R
SET Values
BRGT
2.67K
1.33K
909
665
0.0V
0.7mA
1.4mA
2.1mA
2.8mA
0.5V
1.4mA
2.9mA
4.2mA
5.7mA
1.0V
2.1mA
4.3mA
6.3mA
8.6mA
1.5V
2.9mA
5.8mA
8.4mA
11.5mA
2.0V
3.6mA
7.2mA
10.5mA
14.4mA
2.5V
4.3mA
8.7mA
12.7mA
17.3mA
3.0V
5.0mA
10.1mA
14.8mA
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
OUT 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 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 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 P
OUT 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
OUT is dependent on the input voltage
supplied to the LM2794/5, the total LM2794/5 output current,
and the output resistance (R
OUT) 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).
LM2794/LM2795
www.national.com
11
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