Datasheet

LM2757
SNVS536E –OCTOBER 2007–REVISED MAY 2013 www.ti.com
The voltage rating of the output capacitor should be 10V or more. For example, a 10V 0603 1.0µF is acceptable
for use with the LM2757, as long as the capacitance does not fall below a minimum of 0.5µF in the intended
application. All other capacitors should have a voltage rating at or above the maximum input voltage of the
application. The capacitors should be selected such that the capacitance on the input does not fall below 0.7µF,
and the capacitance of the flying capacitors does not fall below 0.2µF.
The table below lists some leading ceramic capacitor manufacturers.
Manufacturer Contact Information
AVX www.avx.com
Murata www.murata.com
Taiyo-Yuden www.t-yuden.com
TDK www.component.tdk.com
Vishay-Vitramon www.vishay.com
Output Capacitor and Output Voltage Ripple
The output capacitor in the LM2757 circuit (C
OUT
) directly impacts the magnitude of output voltage ripple. Other
prominent factors also affecting output voltage ripple include input voltage, output current and flying capacitance.
Due to the complexity of the regulation topology, providing equations or models to approximate the magnitude of
the ripple can not be easily accomplished. But one important generalization can be made: increasing
(decreasing) the output capacitance will result in a proportional decrease (increase) in output voltage ripple.
In typical high-current applications, a 1.0µF low-ESR ceramic output capacitor is recommended. Different output
capacitance values can be used to reduce ripple, shrink the solution size, and/or cut the cost of the solution. But
changing the output capacitor may also require changing the flying capacitor and/or input capacitor to maintain
good overall circuit performance. Performance of the LM2757 with different capacitor setups in discussed in the
section Recommended Capacitance.
High ESR in the output capacitor increases output voltage ripple. If a ceramic capacitor is used at the output, this
is usually not a concern because the ESR of a ceramic capacitor is typically very low and has only a minimal
impact on ripple magnitudes. If a different capacitor type with higher ESR is used (tantalum, for example), the
ESR could result in high ripple. To eliminate this effect, the net output ESR can be significantly reduced by
placing a low-ESR ceramic capacitor in parallel with the primary output capacitor. The low ESR of the ceramic
capacitor will be in parallel with the higher ESR, resulting in a low net ESR based on the principles of parallel
resistance reduction.
Input Capacitor and Input Voltage Ripple
The input capacitor (C
IN
) is a reservoir of charge that aids a quick transfer of charge from the supply to the flying
capacitors during the charge phase of operation. The input capacitor helps to keep the input voltage from
drooping at the start of the charge phase when the flying capacitors are connected to the input. It also filters
noise on the input pin, keeping this noise out of sensitive internal analog circuitry that is biased off the input line.
Much like the relationship between the output capacitance and output voltage ripple, input capacitance has a
dominant and first-order effect on input ripple magnitude. Increasing (decreasing) the input capacitance will result
in a proportional decrease (increase) in input voltage ripple. Input voltage, output current, and flying capacitance
also will affect input ripple levels to some degree.
In typical high-current applications, a 1.0µF low-ESR ceramic capacitor is recommended on the input. Different
input capacitance values can be used to reduce ripple, shrink the solution size, and/or cut the cost of the
solution. But changing the input capacitor may also require changing the flying capacitor and/or output capacitor
to maintain good overall circuit performance. Performance of the LM2757 with different capacitor setups is
discussed below in Recommended Capacitance.
Flying Capacitors
The flying capacitors (C
1
, C
2
) transfer charge from the input to the output. Flying capacitance can impact both
output current capability and ripple magnitudes. If flying capacitance is too small, the LM2757 may not be able to
regulate the output voltage when load currents are high. On the other hand, if the flying capacitance is too large,
the flying capacitor might overwhelm the input and output capacitors, resulting in increased input and output
ripple.
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