Datasheet

V
OUT
= 5.0VV
IN
LM2750
Switched-
Capacitor
Doubler
Ideal Linear
Regulator
(I
Q
= 0)
V ' # 2 × V
IN
I ' = I
OUT
I
IN
= (2 × I
OUT
) + I
Q
I
OUT
I
Q
Power Model
)II2(V
IV
P
P
E
QOUTIN
OUTOUT
IN
OUT
˜u
u
LM2750
www.ti.com SNVS180L –APRIL 2002–REVISED MAY 2013
conversion. In an ideal linear regulator, the current into the circuit is equal to the current out of the circuit. The
principles of power conservation mandate the ideal input current of a voltage doubler must be twice the output
current. Adding a correction factor for operating quiescent current (I
Q
, 5mA typ.) gives an approximation for total
input current which, when combined with the other input and output parameter(s), yields the following equation
for efficiency:
Comparisons of LM2750 efficiency measurements to calculations using the above equation have shown the
equation to be a quite accurate approximation of actual efficiency. Because efficiency is inversely proportional to
input voltage, it is highest when the input voltage is low. In fact, for an input voltage of 2.9V, efficiency of the
LM2750 is greater than 80% (I
OUT
≥ 40mA) and peak efficiency is 85% (I
OUT
= 120mA). The average efficiency
for an input voltage range spanning the Li-Ion range (2.9V-to-4.2V) is 70% (I
OUT
= 120mA). At higher input
voltages, efficiency drops dramatically. In Li-Ion-powered applications, this is typically not a major concern, as
the circuit will be powered off a charger in these circumstances. Low efficiency equates to high power dissipation,
however, which could become an issue worthy of attention.
LM2750 power dissipation (P
D
) is calculated simply by subtracting output power from input power:
P
D
= P
IN
- P
OUT
= [V
IN
× (2·I
OUT
+ I
Q
)] - [V
OUT
× I
OUT
]
Power dissipation increases with increased input voltage and output current, up to 772mW at the ends of the
operating ratings (V
IN
= 5.6V, I
OUT
= 120mA). Internal power dissipation self-heats the device. Dissipating this
amount power/heat so the LM2750 does not overheat is a demanding thermal requirement for a small surface-
mount package. When soldered to a PCB with layout conducive to power dissipation, the excellent thermal
properties of the WSON package enable this power to be dissipated from the LM2750 with little or no derating,
even when the circuit is placed in elevated ambient temperatures.
Figure 15. LM2750 Model for Power Efficiency and Power Dissipation Calculations
LAYOUT RECOMMENDATIONS
A good board layout of the LM2750 circuit is required to achieve optimal assembly, electrical, and thermal
dissipation performance. Figure 16 is an example of a board layout implementing recommended techniques. For
more information related to layout for the WSON/SON package, see TI's AN-1187 Application Report
(SNOA401). Below are some general guidelines for board layout:
• Place capacitors as close to the as possible to the LM2750, and on the same side of the board. V
IN
and V
OUT
connections are most critical: run short traces from the LM2750 pads directly to these capacitor pads.
• Connect the ground pins of the LM2750 and the capacitors to a good ground plane. The ground plane is
essential for both electrical and thermal disspation performance.
• For optimal thermal performance, make the ground plane(s) as large as possible. Connect the die-attach pad
(DAP) of the LM2750 to the ground plane(s) with wide traces and/or multiple vias. Top-layer ground planes
are most effective in increasing the thermal dissipation capability of the WSON package. Large internal
ground planes are also very effective in keeping the die temperature of the LM2750 within operating ratings.
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