Datasheet

LM2750
SNVS180L –APRIL 2002–REVISED MAY 2013 www.ti.com
The table below lists some leading ceramic capacitor manufacturers.
Manufacturer Contact Information
TDK www.component.tdk.com
AVX www.avx.com
Murata www.murata.com
Taiyo-Yuden www.t-yuden.com
Vishay-Vitramon www.vishay.com
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 capacitor helps to keep the input voltage from
drooping at the start of the charge phase, when the flying capacitor is first connected 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 required.
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 excessive 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 reduces with increased DC bias. This
degradation can be quite significant (>50%) when the DC bias approaches the voltage 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 previously in the OUTPUT CURRENT CAPABILITY section. For most applications, a 1µF
X7R/X5R ceramic capacitor is recommended 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 ceramic 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 frequency effectively reducing by a factor of two,
giving excessive 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.
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 reduces with increased DC bias. This
degradation can be quite significant (>50%) when the DC bias approaches the voltage 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 Figure 15, 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
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