LM2750
Application Information
Manufacturer
TDK
AVX
Murata
Taiyo-Yuden
Vishay-Vitramon
(Continued)
Contact Information
www.component.tdk.com
www.avx.com
www.murata.com
www.t-yuden.com
www.vishay.com
output to ensure stability. For output currents between 60mA
and 120mA, a minimum output capacitance of 2.0µF is
required.
A minimum voltage rating of 10V is recommended for the
output capacitor. This is to account for DC bias properties of
ceramic capacitors. Capacitance of ceramic capacitors re-
duces with increased DC bias. This degradation can be quite
significant (
>
50%) when the DC bias approaches the volt-
age rating of the capacitor.
POWER EFFICIENCY AND POWER DISSIPATION
Efficiency of the LM2750 mirrors that of an unregulated
switched capacitor converter followed by a linear regulator.
The simplified power model of the LM2750, in
will
be used to discuss power efficiency and power dissipation.
In calculating power efficiency, output power (P
OUT
) is easily
determined as the product of the output current and the 5.0V
output voltage. Like output current, input voltage is an
application-dependent variable. The input current can be
calculated using the principles of linear regulation and
switched capacitor 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 qui-
escent 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 effi-
ciency:
INPUT CAPACITOR
The input capacitor (C
IN
) is used as a reservoir of charge,
helping to quickly transfer charge to the flying capacitor
during the charge phase (φ1) of operation. The input capaci-
tor helps to keep the input voltage from drooping at the start
of the charge phase, when the flying capacitor is first con-
nected to the input, and helps to filter noise on the input pin
that could adversely affect sensitive internal analog circuitry
biased off the input line. As mentioned above, an X7R/X5R
ceramic capacitor is recommended for use. For applications
where the maximum load current required is between 60mA
and 120mA, a minimum input capacitance of 2.0µF is re-
quired. For applications where the maximum load current is
60mA or less, 1.0µF of input capacitance is sufficient. Failure
to provide enough capacitance on the LM2750 input can
result in poor part performance, often consisting of output
voltage droop, excessive output voltage ripple and/or exces-
sive input voltage ripple.
A minimum voltage rating of 10V is recommended for the
input capacitor. This is to account for DC bias properties of
ceramic capacitors. Capacitance of ceramic capacitors re-
duces with increased DC bias. This degradation can be quite
significant (
>
50%) when the DC bias approaches the volt-
age rating of the capacitor.
FLYING CAPACITOR
The flying capacitor (C
FLY
) transfers charge from the input to
the output, providing the voltage boost of the doubler. A
polarized capacitor (tantalum, aluminum electrolytic, etc.)
must not be used here, as the capacitor will be reverse-
biased upon start-up of the LM2750. The size of the flying
capacitor and its ESR affect output current capability when
the input voltage of the LM2750 is low, most notable for input
voltages below 3.0V. These issues were discussed previ-
ously in the
Output Current Capability
section. For most
applications, a 1µF X7R/X5R ceramic capacitor is recom-
mended for the flying capacitor.
OUTPUT CAPACITOR
The output capacitor of the LM2750 plays an important part
in determining the characteristics of the output signal of the
LM2750, many of which have already been discussed. The
ESR of the output capacitor affects charge pump output
resistance, which plays a role in determining output current
capability. Both output capacitance and ESR affect output
voltage ripple. For these reasons, a low-ESR X7R/X5R ce-
ramic capacitor is the capacitor of choice for the LM2750
output.
In addition to these issues previously discussed, the output
capacitor of the LM2750 also affects control-loop stability of
the part. Instability typically results in the switching fre-
quency effectively reducing by a factor of two, giving exces-
sive output voltage droop and/or increased voltage ripple on
the output and the input. With output currents of 60mA or
less, a minimum capacitance of 1.0µF is required at the
11
Comparisons of LM2750 efficiency measurements to calcu-
lations using the above equation have shown the equation to
be a quite accurate approximation of actual efficiency. Be-
cause 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 circum-
stances. 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 sub-
tracting output power from input power:
P
D
= P
IN
- P
OUT
= [V
IN
x (2·I
OUT
+ I
Q
)] - [V
OUT
x 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 dissipa-
tion 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 LLP pack-
age enable this power to be dissipated from the LM2750 with
little or no derating, even when the circuit is placed in el-
evated ambient temperatures.
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