lm2758tlx National Semiconductor Corporation, lm2758tlx Datasheet - Page 10

• Manufacturer Part Number: lm2758tlx
• Description: Switched Capacitor Flash Led Driver In Micro Smd
• Manufacturer: National Semiconductor Corporation

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POWER EFFICIENCY
Efficiency of LED drivers is commonly taken to be the ratio of
power consumed by the LEDs (P
the input of the part (P
input current is equal to the charge pump gain times the output
current (total LED current). The efficiency of the LM2758 can
be predicted as follows:
For a simple approximation, the current consumed by internal
circuitry (I
become:
Neglecting I
tion, but this impact will be negligible due to the value of I
being very low compared to the typical Torch and Flash cur-
rent levels (100-500 mA). 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 con-
sumed by the circuit (V
efficiency.
THERMAL PROTECTION
Internal thermal protection circuitry disables the LM2758
when the junction temperature exceeds 150°C (typ.). This
feature protects the device from being damaged by high die
temperatures that might otherwise result from excessive pow-
er dissipation. The device will recover and operate normally
when the junction temperature falls below 125°C (typ.). It is
important that the board layout provide good thermal conduc-
tion to keep the junction temperature within the specified
operating ratings.
POWER DISSIPATION
The power dissipation (P
(T
power generated by the 1x/1.5x charge pump, P
power consumed by the LEDs, T
and θ
12–bump micro SMD package. V
LM2758, V
is the programmed LED current.
The junction temperature rating takes precedence over the
ambient temperature rating. The LM2758 may be operated
outside the ambient temperature rating, so long as the junc-
J
) can be approximated with the equations below. P
JA
is the junction-to-ambient thermal resistance for the
Q
) can be neglected, and the resulting efficiency will
LED
Q
= (Gain × V
P
will result in a slightly higher efficiency predic-
is the nominal LED forward voltage, and I
IN
T
J
= V
P
E = V
= T
P
DISSIPATION
LED
IN
A
E = (P
IN
IN
P
× (Gain × N × I
+ (P
IN
LED
= N × V
DISSIPATION
x I
). With a 1x/1.5x charge pump, the
IN
× I
IN
= V
÷ (V
DISSIPATION
LED
) be evaluated rather than power
LED
IN
= P
LED
A
IN
) − (V
÷ P
× I
IN
is the ambient temperature,
LED
IN
× Gain)
) and junction temperature
IN
is the input voltage to the
× I
IN
- P
) to the power drawn at
)
LED
LED
LED
× θ
LED
× I
+ I
JA
LED
)
Q
)
)
LED
IN
is the
is the
LED
Q
10
tion temperature of the device does not exceed the maximum
operating rating of 125°C. The maximum ambient tempera-
ture rating must be derated in applications where high power
dissipation and/or poor thermal resistance causes the junc-
tion temperature to exceed 125°C.
MICRO SMD PACKAGE ASSEMBLY AND USE
Use of the micro SMD package requires specialized board
layout, precision mounting and careful re-flow techniques as
detailed in National Semiconductor Application Note 1112.
Refer to the section "Surface Mount Technology (SMD) As-
sembly Considerations". For best results in assembly, align-
ment ordinals on the PC board should be used to facilitate
placement of the device. The pad style used with the micro
SMD package must be the NSMD (non-solder mask defined)
typ. This means that the solder-mask opening is larger than
the pad size. This prevents a lip that otherwise forms if the
solder mask and pad overlap, from holding the device off the
surface of the board and interfering with mounting. See Ap-
plication Note 1112 for specific instructions how to do this.
The 12-bump package used for LM2758 has 300 micron sol-
der balls and requires 10.82 mils pads for mounting on the
circuit board. The trace to each pad should enter the pad with
a 90° entry angle to prevent debris from being caught in deep
corners. Initially, the trace to each pad should be 7 mil wide,
for a section approximately 7 mil. long or longer, as a thermal
relief. Then each trace should neck up or down to its optimal
width. The important criteria is symmetry. This ensures the
solder bumps on the LM2758 re-flow evenly and that the de-
vice solders level to the board. In particular, special attention
must be paid to the pads for bumps A1 and A3, because GND
and VIN are typically connected to large copper planes, thus
inadequate thermal relief can result in late or inadequate re-
flow of these bumps.
The micro SMD package is optimized for the smallest possi-
ble size in applications with red or infrared opaque cases.
Because the micro SMD package lacks the plastic encapsu-
lation characteristic of larger devices, it is vulnerable to light.
Backside metallization and/or epoxy coating, along with front
side shading by the printed circuit board, reduce this sensi-
tivity. However, the package has exposed die edges. In par-
ticular, micro SMD devices are sensitive to light, in the red
and infrared range, shining on the package’s exposed die
edges.
BOARD LAYOUT CONSIDERATIONS
PC board layout is an important part of DC-DC converter de-
sign. Poor board layout can disrupt the performance of a DC-
DC converter and surrounding circuitry by contributing to EMI,
ground bounce, and resistive voltage loss in the traces. These
can send erroneous signals to the DC-DC converter IC, re-
sulting in poor regulation or instability. Poor layout can also
result in re-flow problems leading to poor solder joints be-
tween the micro SMD package and board pads. Poor solder
joints can result in erratic or degraded performance.