Preliminary Datasheet

LP6343

LP6343 –00 Version 1.0 Datasheet

Feb.-2010

www.lowpowersemi.com

Page 9 of 12

The input capacitor RMS ripple current varies with the

input and output voltage and will always be less than or

equal to half of the total DC load current.

To minimize stray inductance, the capacitor should be

placed as closely as possible to the IC. This keeps the high

frequency content of the input current localized,

minimizing EMI and input voltage ripple. The proper

placement of the input capacitor (C1) can be seen in the

evaluation board layout in Figures 2 and 3.

A laboratory test set-up typically consists of two long wires

running from the bench power supply to the evaluation

board input voltage pins. The inductance of these wires,

along with the low-ESR ceramic input capacitor, can create

a high Q network that may affect converter performance.

This problem often becomes apparent in the form of

excessive ringing in the output voltage during load

transients. Errors in the loop phase and gain measurements

can also result. Since the inductance of a short PCB trace

feeding the input voltage is significantly lower than the

power leads from the bench power supply, most

applications do not exhibit this problem. In applications

where the input power source lead inductance cannot be

reduced to a level that does not affect the converter

performance, a high ESR tantalum or aluminum electrolytic

should be placed in parallel with the low ESR, ESL bypass

ceramic. This dampens the high Q network and stabilizes

the system.

Output Capacitor Selection

The function of output capacitance is to store energy to

attempt to maintain a constant voltage. The energy is stored

in the capacitor’s electric field due to the voltage applied.

The value of output capacitance is generally selected to

limit output voltage ripple to the level required by the

specification. Since the ripple current in the output inductor

is usually determined by L, VOUT and VIN, the series

impedance of the capacitor primarily determines the out-put

voltage ripple. The three elements of the capacitor that

contribute to its impedance (and output voltage ripple) are

equivalent series resistance (ESR), equivalent series

inductance (ESL), and capacitance (C). The output voltage

droop due to a load transient is dominated by the

capacitance of the ceramic output capacitor. During a step

increase in load current, the ceramic output capacitor alone

supplies the load current until the loop responds. Within

three switching cycles, the loop responds and the inductor

current increases to match the load current demand. The

relationship of the output voltage droop during the three

switching cycles to the output capacitance can be estimated

by:

In many practical designs, to get the required ESR, a

capacitor with much more capacitance than is needed must

be selected. For both continuous or discontinuous inductor

current mode operation, the ESR of the COUT needed to

limit the ripple to ∆VO, V peak-to-peak is:

Ripple current flowing through a capacitor’s ESR causes

power dissipation in the capacitor. This power dissipation

causes a temperature increase internal to the capacitor.

Excessive temperature can seriously shorten the expected

life of a capacitor. Capacitors have ripple current ratings

that are dependent on ambient temperature and should not

be exceeded. The output capacitor ripple cur-rent is the

inductor current, IL, minus the output current, IO. The

RMS value of the ripple current flowing in the output

capacitance (continuous inductor current mode operation) is

given by:

ESL can be a problem by causing ringing in the low

megahertz region but can be controlled by choosing low

ESL capacitors, limiting lead length (PCB and capacitor),

and replacing one large device with several smaller ones

connected in parallel. In conclusion, in order to meet the

requirement of out-put voltage ripple small and regulation

loop stability, ceramic capacitors with X5R or X7R

dielectrics are recommended due to their low ESR and high

ripple current ratings.

The output ripple VOUT is

determined by:

A 22μF ceramic capacitor can satisfy most applications.