LM2757

www.ti.com

SNVS536D – OCTOBER 2007 – REVISED AUGUST 2009

For most applications, ceramic capacitors with an X7R or X5R temperature characteristic are preferred for use

with the LM2757. These capacitors have tight capacitance tolerance (as good as ±10%) and hold their value over

temperature (X7R: ±15% over -55ºC to 125ºC; X5R: ±15% over -55ºC to 85ºC).

Capacitors with a Y5V or Z5U temperature characteristic are generally not recommended for use with the

LM2757. These types of capacitors typically have wide capacitance tolerance (+80%, -20%) and vary

significantly over temperature (Y5V: +22%, -82% over -30ºC to +85ºC range; Z5U: +22%, -56% over +10ºC to

+85ºC range). Under some conditions, a 1µF-rated Y5V or Z5U capacitor could have a capacitance as low as

0.1µF. Such detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum

capacitance requirements of the LM2757.

Net capacitance of a ceramic capacitor decreases with increased DC bias. This degradation can result in lower

capacitance than expected on the input and/or output, resulting in higher ripple voltages and currents. Using

capacitors at DC bias voltages significantly below the capacitor voltage rating will usually minimize DC bias

effects. Consult capacitor manufacturers for information on capacitor DC bias characteristics.

Capacitance characteristics can vary quite dramatically with different application conditions, capacitor types, and

capacitor manufacturers. It is strongly recommended that the LM2757 circuit be thoroughly evaluated early in the

design-in process with the mass-production capacitors of choice. This will help ensure that any such variability in

capacitance does not negatively impact circuit performance.

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

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 Capacitor Configurations.

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.

Copyright © 2007–2009, Texas Instruments Incorporated

Submit Documentation Feedback

11

Product Folder Links: LM2757