LM2750LD-5.0 National Semiconductor, LM2750LD-5.0 Datasheet - Page 9

• Manufacturer Part Number: LM2750LD-5.0
• Description: Low Noise/ 5.0V Regulated Switched Capacitor Voltage Converter
• Manufacturer: National Semiconductor

Application Information
A low-ESR ceramic capacitor is recommended on the output
to keep output voltage ripple low. Placing multiple capacitors
in parallel can reduce ripple significantly, both by increasing
capacitance and reducing ESR. When capacitors are in
parallel, ESR is in parallel as well. The effective net ESR is
determined according to the properties of parallel resistance.
Two identical capacitors in parallel have twice the capaci-
tance and half the ESR as compared to a single capacitor of
the same make. On a similar note, if a large-value, high-ESR
capacitor (tantalum, for example) is to be used as the pri-
mary output capacitor, the net output ESR can be signifi-
cantly reduced by placing a low-ESR ceramic capacitor in
parallel with this primary output capacitor.
CAPACITORS
The LM2750 requires 3 external capacitors for proper opera-
tion. Surface-mount multi-layer ceramic capacitors are rec-
ommended. These capacitors are small, inexpensive and
have very low equivalent series resistance (≤10mΩ typ.).
Tantalum capacitors, OS-CON capacitors, and aluminum
electrolytic capacitors generally are not recommended for
use with the LM2750 due to their high ESR, as compared to
ceramic capacitors.
For most applications, ceramic capacitors with X7R or X5R
temperature characteristic are preferred for use with the
LM2750. These capacitors have tight capacitance tolerance
(as good as
and typically have little voltage coefficient. Capacitors with
Y5V and/or Z5U temperature characteristic are generally not
recommended. These types of capacitors typically have
wide capacitance tolerance (+80%, -20%), vary significantly
over temperature (Y5V: +22%, -82% over -30
range; Z5U: +22%, -56% over +10
have poor voltage coefficients. Under some conditions, a
nominal 1µF Y5V or Z5U capacitor could have a capacitance
of only 0.1µF. Such detrimental deviation is likely to cause
these Y5V and Z5U of capacitors to fail to meet the minimum
capacitance requirements of the LM2750.
The table below lists some leading ceramic capacitor manu-
facturers.
INPUT CAPACITORS
The input capacitor (C
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
±
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TDK
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±
10%), hold their value over temperature (X7R:
o
C to 125
IN
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) is used as a reservoir of charge,
C; X5R:
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15% over -55
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C to +85
(Continued)
o
C range), and
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C to +85
C to 85
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C
9
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.
FLYING CAPACITOR
The flying capacitor (C
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
output to ensure stability. For output currents between 60mA
and 120mA, a minimum output capacitance of 2.0µF is
required.
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 Figure 2, will
be used to discuss power efficiency and power dissipation.
In calculating power efficiency, output power (P
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
input current which, when combined with the other input and
output parameter(s), yields the following equation for effi-
ciency:
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-
Q
, 5mA typ.) gives an approximation for total
FLY
) transfers charge from the input to
OUT
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) is easily