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| 型號(hào)： | LM2795 |
| 文件頁(yè)數(shù)： | 12/14頁(yè) |
| 文件大小： | 373K |
| 代理商： | LM2795 |
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Application Information
(Continued)
power additional circuitry in an application. Note that the
P
OUT
voltage is not regulated, and will thus be equal to 1.5
times the input voltage. It is possible to regulate the output
voltage of the LM2794/5 by using a Linear Dropout Regula-
tor (LDO) such as the LP3985-series LDOs.
PARALLEL Dx OUTPUTS FOR INCREASED CURRENT
DRIVE
Outputs D1 -D4 may be connected together in any combi-
nation to drive higher currents through fewer LEDs. For
example, outputs D1 and D2 may be connected together to
drive a single LED. Similarly, D3 and D4 may be connected
together to drive a second LED. With this configuration, two
parallel current sources of equal value provide current to
each LED, so R
and V
should be chosen so that the
current through each of the outputs is programmed to 50% of
the desired LED current. For example, if 30mA is the desired
drive current for the LEDs, R
and V
should be se-
lected so that the current through each of the outputs is
15mA. Other combinations of parallel outputs may be imple-
mented in similar fashions (connecting all four outputs to
drive a single LED, for example). Connecting outputs in
parallel does not affect internal operation of the LM2794/95
and has no impact on the Electrical Characteristics and limits
previously presented. The available Dx output current, maxi-
mum Dx voltage, and all other specifications provided in the
Electrical Characteristics table apply to parallel output con-
figurations, just as they do to the standard 4-LED application
circuit.
THERMAL PROTECTION
The LM2794/5 has internal thermal protection circuitry to
disable the charge pump if the junction temperature exceeds
150C. This feature will protect the device from damage due
to excessive power dissipation. The device will recover and
operate normally when the junction temperature falls below
the maximum operating junction temperature of 125C. It is
important to have good thermal conduction with a proper
layout to reduce thermal resistance.
POWER EFFICIENCY
The efficiency of the LM2794/5 is calculated by dividing the
output power by the input power. This is shown in the follow-
ing equation:
Efficiency = ( V
D1
I
D1
+ V
D2
I
D2
+ V
D2
I
D3
+ V
D4
I
D4
)
/ (V
IN
I
SUPPLY
)
Where V
is the corresponding diode voltage and I
DX
is the
corresponding diode currrent.
An approximation of the efficiency for the LM2794/95 is
given as:
Efficiency = (V
4I
)/ 3/2V
IN
4I
D(AVG)
= V
D(AVG)
/ (3/2 V
IN
)
where V
D(AVG)
is the average diode and I
D(AVG)
It is clear that the efficiency will depend on the supply voltage
in the above equation.As such, the lower the supply voltage,
the higher the efficiency.
POWER DISSIPATION
The maximum allowable power dissipation that this package
is capable of handling can be determined as follows:
P
DMax
= (T
JMax
- T
A
) /
θ
JA
where T
is the maximum junction temperature, T
is the
ambient temperature, and
θ
is the junction-to-ambient
thermal resistance of the specified package.
The actual power dissipation of the device can be calculated
using this equation:
P
Dissipation
= (3/2 V
IN
-V
DIODE
)
I
LOAD
As an example, if V
in the target application is 4.2V, V
= 3.0V and worse case current consumption is 60mA (15mA
for each diode).
P
Dissipation
= ((1.5
4.2) - 3.0)
0.06 = 198mW
Power dissipation must be less than that allowed by the
package. Please refer to the Absolute Maximum Rating of
the LM2794/5.
MICRO SMD MOUNTING
The LM2794/5 is a 14-bump micro SMD with a bump size of
300 micron. The micro SMD package requires specific
mounting techniques which are detailed in National Semi-
conductor Application Note (AN -1112). NSMD (non-solder
mask defined) layout pattern is recommended over the SMD
(solder 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
faciliate placement of the micro SMD device. Micro SMD is a
wafer level chip size package which means the dimensions
of the package is equal to the die size. As such, the micro
SMD package are lacks the plastic encapsulation character-
istic of the larger devices and ; it is sensitive to direct
exposure to sun light and light sources such as infrared light
and halogen light. These wavelenghts may cause unpredit-
abled operation.
L
www.national.com
12
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