LM2759SDEV National Semiconductor, LM2759SDEV Datasheet - Page 12

• Manufacturer Part Number: LM2759SDEV
• Description: BOARD EVAL LM2759
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM2759SDEV

• Current - Output / Channel: 1A
• Outputs And Type: 1, Non-Isolated
• Features: Camera Flash and Torch Modes, I²C Interface, Programmable Output Current
• Voltage - Input: 3 ~ 5.5V
• Utilized Ic / Part: LM2759
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Output: -
• Lead Free Status / Rohs Status: Supplier Unconfirmed

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POWER EFFICIENCY
Efficiency of LED drivers is commonly taken to be the ratio of
power consumed by the LED (P
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 LM2759 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 (100mA - 1A). 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 LM2759
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 120°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, 2x charge pump, P
power consumed by the LED, T
and θ
12 pin LLP package. V
V
grammed LED current.
4.7 µF for C
2.2 µF for C1, C2, C
LED
J
) can be approximated with the equations below. P
C1608X5R0J475
C1608X5R0J225
is the nominal LED forward voltage, and I
JA
JMK107BJ475
JMK107BJ225
MFG Part No.
is the junction-to-ambient thermal resistance for the
Q
) can be neglected, and the resulting efficiency will
Q
= (Gain × V
OUT
will result in a slightly higher efficiency predic-
T
P
J
IN
P
E = V
= T
IN
DISSIPATION
= V
). With a 1x, 1.5x, 2x charge pump, the
P
IN
A
E = (P
IN
LED
IN
IN
P
+ (P
IN
LED
DISSIPATION
x I
IN
× (Gain × I
is the input voltage to the LM2759,
= V
× I
IN
= V
÷ (V
DISSIPATION
LED
) be evaluated rather than power
LED
LED
IN
= P
LED
A
Ceramic X5R
Ceramic X5R
Ceramic X5R
Ceramic X5R
IN
) − (V
÷ P
× I
is the ambient temperature,
× I
IN
) to the power drawn at the
× Gain)
) and junction temperature
IN
IN
Type
LED
LED
- P
)
LED
× θ
LED
+ I
× I
JA
Q
)
LED
)
Suggested Capacitors and Suppliers
LED
)
is the pro-
LED
IN
is the
is the
Taiyo-Yuden
Taiyo-Yuden
Q
MFG
TDK
TDK
12
The junction temperature rating takes precedence over the
ambient temperature rating. The LM2759 may be operated
outside the ambient temperature rating, so long as the junc-
tion temperature of the device does not exceed the maximum
operating rating of 105°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 105°C.
MAXIMUM OUTPUT CURRENT
The maximum LED current that can be used for a particular
application depends on the rated forward voltage of the LED
used, the input voltage range of the application, and the Gain
mode of the LM2759’s charge pump. The following equation
can be used to approximate the relationship between the
maximum LED current, the LED forward voltage, the mini-
mum input voltage, and the charge pump gain:
(V
V
in regulation can be approximated by (I
K
the charge pump according to its gain mode. When using the
equation above, keep in mind that the (V
the equation can not be greater than the nominal output reg-
ulation voltage for a particular gain. In other words, when
making calculations for an application where the term (V
V
next higher gain level must be used for the calculation.
Example: V
a R
V
V
The maximum power dissipation in the LM2759 must also be
taken into account when selecting the conditions for an ap-
plication, such that the junction temperature of the device
never exceeds its rated maximum. The input voltage range,
operating temperature range, and/or current level of the ap-
plication may have to be adjusted to keep the LM2759 within
normal operating ratings.
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 LLP package and board pads. Poor solder joints
can result in erratic or degraded performance.
HR
HR
HR
IN_MIN
IN_MIN
IN_MIN
OUT
) is higher than a particular gain’s regulation voltage, the
or the voltage required across the current sink to remain
is 0.8 mV/mA (typ). R
of 2.25Ω (typ.)
> [(4V + 0.8V) + (1A x 2.25Ω) ] ÷ 2
> 3.53V (typ.)
x Gain) > (V
Voltage Rating
F
= 4V @ 1A, Charge Pump in the Gain of 2x with
6.3V
6.3V
6.3V
6.3V
F
+ V
OUT_GAIN
HR
) + (I
Case Size
LED
is the output impedance of
x R
0603 (1608)
0603 (1608)
0603 (1608)
0603 (1608)
OUT_GAIN
LED
F
+ V
Inch (mm)
x K
HR
)
) portion of
HR
), where
F
+