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參數(shù)資料
| 型號： | LM2794BLX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Current Regulated Switched Capacitor LED Supply with Analog and PWM Brightness Control |
| 中文描述： | SWITCHED CAPACITOR REGULATOR, 675 kHz SWITCHING FREQ-MAX, PBGA14 |
| 封裝： | CSP-14 |
| 文件頁數(shù)： | 14/16頁 |
| 文件大小： | 487K |
| 代理商： | LM2794BLX |
Application Information
(Continued)
E = (P
LED
÷ P
IN
)
Efficiency, as defined here, is in part dependent on LED
voltage. Variation in LED voltage does not affect power
consumed by the circuit and typically does not relate to the
brightness of the LED. For an advanced analysis, it is rec-
ommended that power consumed by the circuit (V
x I
) be
evaluated rather than power efficiency.
Figure 7
shows the
power consumption of the LM2794/5 Typical Application Cir-
cuit.
POWER DISSIPATION
The power dissipation (P
DISSIPATION
) and junction tempera-
ture (T
J
) can be approximated with the equations below. P
IN
is the power generated by the 1.5x charge pump, P
LED
is the
power consumed by the LEDs, P
POUT
is the power provided
through the P
OUT
pin, T
A
is the ambient temperature, and
θ
JA
is the junction-to-ambient thermal resistance for the micro
SMD-14 package. V
IN
is the input voltage to the LM2794/5,
V
is the LED forward voltage, I
is the programmed LED
current, and I
POUT
is the current drawn through P
OUT
.
P
DISSIPATION
= P
IN
- P
LED
P
POUT
= [1.5xV
IN
x(4I
DX
+ I
POUT
)] (V
DX
x4I
DX
) (1.5xV
IN
xI
POUT
)
T
J
= T
A
+ (P
DISSIPATION
x
θ
JA
)
The junction temperature rating takes precedence over the
ambient temperature rating. The LM2794/5 may be operated
outside the ambient temperature rating, so long as the junc-
tion temperature of the device does not exceed the maxi-
mum operating rating of 100C. The maximum ambient tem-
perature rating must be derated in applications where high
power dissipation and/or poor thermal resistance causes the
junction temperature to exceed 100C.
MICRO SMD MOUNTING
The LM2794/5 is a 14-bump micro SMD with a bump size of
300 micron diameter. The micro SMD package requires
specific mounting techniques detailed in National Semicon-
ductorApplication Note (AN -1112). NSMD (non-solder mask
defined) layout pattern is recommended over the SMD (sol-
der mask defined) since the NSMD requires larger solder
mask openings over the pad size as opposed to the SMD.
This reduces stress on the PCB and prevents possible
cracking at the solder joint. For best results during assembly,
alignment ordinals on the PC board should be used to
facilitate placement of the micro SMD device. Micro SMD is
a wafer level chip size package, which means the dimen-
sions of the package are equal to the die size. As such, the
micro SMD package lacks the plastic encapsulation charac-
teristics of the larger devices and is sensitive to direct expo-
sure to light sources such as infrared, halogen, and sun light.
The wavelengths of these light sources may cause unpre-
dictable operation.
20028537
FIGURE 6. Efficiency vs V
IN
4 LEDs, V
LED
= 3.6V, I
LED
= 15mA
20028538
FIGURE 7. P
vs V
IN
4 LEDs, 2.5
≤
V
DX
≤
3.9V, I
DX
= 15mA
L
www.national.com
14
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