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參數(shù)資料
| 型號： | LM2793LDX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Low Noise White LED Constant Current Supply with Dual Function Brightness Control |
| 中文描述： | SWITCHED CAPACITOR REGULATOR, 675 kHz SWITCHING FREQ-MAX, DSO10 |
| 封裝： | LLP-10 |
| 文件頁數(shù)： | 8/10頁 |
| 文件大小： | 211K |
| 代理商： | LM2793LDX |
Application Information
(Continued)
TABLE 2. White LED Selection
Manufacturer
Osram
Nichia
Contact
www.osram-os.com
www.nichia.com
PARALLEL DX OUTPUTS FOR INCREASED CURRENT
DRIVE
Outputs D
1
and D
2
may be connected together to drive a
single LED. In such a configuration, two parallel current
sources of equal value drive the single LED. The voltage on
SD-BRGT should be chosen so that the current through
each of the outputs is programmed to 50% of the total
desired LED current. For example, if 30mA is the desired
drive current for the single LED, SD-BRGT should be se-
lected so that the current through each of the outputs is
15mA. Connecting the outputs in parallel does not affect
internal operation of the LM2793 and has no impact on the
Electrical Characteristics and limits previously presented.
The available diode output current, maximum diode voltage,
and all other specifications provided in the Electrical Char-
acteristics table apply to this parallel output configuration,
just as they do to the standard 2-LED application circuit.
P
OUT
P
uses pre-regulation to loosely regulate the output of
the LM2793 1.5x charge pump. Pre-regulation uses the
voltage present at P
to limit the gate drive of the 1.5x
switched capacitor charge pump. Pre-regulation helps to
reduce input current noise and large input current spikes
normally associated with switched capacitor charge pumps.
At voltages below 3.3V (typ.), the LM2793 acts as an open
loop charge pump. When the device is in open loop, the
voltage at P
OUT
is 1.5 times the input voltage. At input
voltages higher than 3.3V (typ.) P
OUT
is loosely regulated to
4.9V (typ.).
POWER EFFICIENCY
Figure 2
shows the efficiency of the LM2793.
Efficiency (E) of the LM2793 is defined here as the ratio of
the power consumed by LEDs (P
LED
) to the power drawn
from the input source (P
IN
). In the equations below, I
Q
is the
quiescent current of the LM2793, I
LED
is the current flowing
through one LED, and V
is the forward voltage at that
LED current. In the input power calculation, the 1.5 multiplier
reflects the 3/2 switched capacitor gain of the LM2793.
P
LED
= N x V
LED
x I
LED
P
IN
= V
IN
x I
IN
P
IN
= V
IN
x (1.5 x N x I
LED
+ I
Q
)
E = (P
LED
÷ P
IN
)
It is also worth noting that 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 recommended that power consumed by the
circuit (V
IN
x I
IN
) be evaluated rather than power efficiency.
Power consumption of the LM2793 Typical Application Cir-
cuit is shown in
Figure 3
.
THERMAL PROTECTION
When the junction temperature exceeds 150C, the LM2793
internal thermal protection circuitry disables the part. This
feature protects the device from damage due to excessive
power dissipation. The device will recover and operate nor-
mally when the junction temperature falls below 125C. It is
important to have good thermal conduction with a proper
layout to reduce thermal resistance.
POWER DISSIPATION
When operating within specified operating ratings, the peak
power dissipation (P
DISSIPATION
) of the LM2793 occurs at an
input voltage of 5.5V. Assuming a typical junction-to-ambient
thermal resistance (
θ
JA) for the LLP-10 package of 55C/W,
a LED forward voltage (V
) of 3.6V, and a total load (I
LOAD
)
of 32mA for two White LEDs connected to D
1
and D
power dissipation and junction temperature (T
) are calcu-
lated below for a part operating at the maximum rated am-
bient temperature (T
) of 85C. In the equations below, V
is
the input voltage to the LM2793, P
IN
is the power generated
by the 1.5x charge pump, and P
LED
is the power consumed
by the LEDs.
P
= P
- P
LED
= (1.5V
V
) x I
= ((1.5 x 5.5V) - 3.6V) x 0.032A
= 149mW
20063612
FIGURE 2. Efficiency vs. V
2 LEDs, V
LED
= 3.6V, I
LED
= 16mA
20063613
FIGURE 3. I
LED
current vs. P
2 LEDs, 2.5
≤
V
LED
≤
3.9V, I
LED
= 16mA
L
www.national.com
8
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