LP3913 datasheet(59/70 Pages) TI1

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L * f * Ioutmax * Vin

Irms = Ioutmax

1 -

Vout

Vin

Vout

Vin

where

r =

(Vin - Vout) * Vout

LP3913

www.ti.com

SNVS489I – NOVEMBER 2006 – REVISED SEPTEMBER 2013

The ceramic capacitor’s capacitance can vary with temperature. The capacitor type X7R, which operates over a

temperature range of

−55°C to +125°C, will only vary the capacitance to within ±15%. The capacitor type X5R

has a similar tolerance over a reduced temperature range of

−55°C to +85°C. Many large value ceramic

capacitors, larger than 1

μF are manufactured with Z5U or Y5V temperature characteristics. Their capacitance

can drop by more than 50% as the temperature varies from 25°C to 85°C. Therefore X7R is recommended over

Z5U and Y5V in applications where the ambient temperature will change significantly above or below 25°C.

Tantalum capacitors are less desirable than ceramic for use as output capacitors because they are more

expensive when comparing equivalent capacitance and voltage ratings in the 0.47

μF to 4.7 μF range.

Another important consideration is that tantalum capacitors have higher ESR values than equivalent size

ceramics. This means that while it may be possible to find a tantalum capacitor with an ESR value within the

stable range, it would have to be larger in capacitance (which means bigger and more costly) than a ceramic

capacitor with the same ESR value. It should also be noted that the ESR of a typical tantalum will increase about

2:1 as the temperature goes from +25°C down to

−40°C, so some guard band must be allowed.

Noise Bypass Capacitors for VREFH Pin

Connecting respectively 100 nF and 1 nF grounded bypass capacitors to the VREFH pin significantly reduces

noise on the LDO outputs. VREFH is a high impedance nodes connected to a bandgap reference used for the

LDOs. Any significant loading on this node will cause a change on the regulated output voltages. For this reason,

DC leakage current through these pins must be kept as low as possible for best output voltage accuracy. The

types of capacitors best suited for the noise bypass capacitors are ceramic and film capacitors. High-quality

ceramic capacitors with either NPI or COG dielectric typically have very low leakage. Polypropylene and

polycarbonate film capacitors are available in small surface-mount packages and typically have extremely low

leakage current. Residual solder flux is another potential source of leakage, which mandates thorough cleaning

of the assembled PCBs.

Input Capacitor Selection for BUCK1, BUCK2 and BUCK3

A ceramic input capacitor of 10

μF, 6.3V is sufficient for the magnetic DC/DC converters. Place the input

capacitor as close as possible to the input of the device. A large value may be used for improved input voltage

filtering. The recommended capacitor types are X7R or X5R. Y5V type capacitors should not be used. DC bias

characteristics of ceramic capacitors must be considered when selecting case sizes like 0805 and 0603. The

input filter capacitor supplies current to the PFET switch of the DC/DC converter in the first half of each cycle and

reduces voltage ripple imposed on the input power source. A ceramic capacitor’s low ESR (Equivalent Series

Resistance) provides the best noise filtering of the input voltage spikes due to fast current transients. A capacitor

with sufficient ripple current rating should be selected. The Input current ripple can be calculated as:

The worse case is when VIN = 2VOUT

Output Capacitor Selection for BUCK1, BUCK2 and BUCK3

A 10

μF, 6.3V ceramic capacitor should be used on the output of the BUCK1 and BUCK2 magnetic DC/DC

converters. BUCK3 needs a 22

μF capacitor. The output capacitor needs to be mounted as close as possible to

the output of the device. A large value may be used for improved input voltage filtering. The recommended

capacitor types are X7R or X5R. Y5V type capacitors should not be used. DC bias characteristics of ceramic

capacitors must be considered when selecting case sizes like 0805 and 0603. DC bias characteristics vary from

manufacturer to manufacturer and DC bias curves should be requested from them and analyzed as part of the

capacitor selection process.

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