SP6655 datasheet(12/16 Pages) SIPEX

12 / 16 page

Date: 7/12/04

SP6655 High Efficiency 400mA Synchronous Buck Regulator

For the 10

µF Ceramic Output Capacitor with

0.003

ΩESR,anda10µHinductoryielding200mA

inductor current ripple ILR, the VOUT ripple would

be 0.6mVpp. Since 0.6mV is a very small signal

level, the actual value would probably be as large

as 10mV due to noise and layout issues, but this

illustrates that the SP6655 output ripple can be

very low indeed. To improve stability, a small

ceramic capacitor, CF = 22pF should be paralleled

with the feedback voltage divider RF, as shown on

the typical application schematic on page 1. An-

other function of the output capacitance is to hold

up the output voltage during the load transients and

prevent excessive overshoot and undershoot. The

typical performance characteristics curves show

very good load step transient response for the

SP6655 with the recommended output capaci-

tance of 10

µF ceramic.

The input capacitor will reduce the peak current

drawn from the battery, improve efficiency and

significantly reduce high frequency noises in-

duced by a switching power supply. The typical

input capacitor for the SP6655 is 10

µF ceramic.

These capacitors will provide good high frequency

bypassing and their low ESR will reduce resistive

losses for higher efficiency. An RC filter is recom-

mended for the VIN pin 2 to effectively reduce the

noise for the ICs analog supply rail which powers

sensitive circuits. This time constant needs to be at

least 5 times greater than the switching period,

which is calculated as 1/FLP during the CCM

mode. The typical application schematic uses the

values of RVIN = 10Ω and CVIN = 1µF to meet these

requirements.

APPLICATION INFORMATION

inductor current ripple of almost 200mA would

produce very stable regulation and fast load

transient response at the expense of slightly

reduced efficiency.

Other inductor parameters are important: the in-

ductor current rating and the DC resistance. When

the current through the inductor reaches the level

ISAT, the inductance drops to 70% of the

nominal value. This non-linear change can cause

stability problems or excessive fluctuation in in-

ductor current ripple. To avoid this, the inductor

should be selected with saturation current at least

equal to the maximum output current of the con-

verter plus half the inductor current ripple. To

provide the best performance in dynamic condi-

tions such as start-up and load transients, inductors

should be chosen with saturation current close to

the SP6655 inductor current limit of 0.625A.

DC resistance, another important inductor charac-

teristic, directly affects the efficiency of the con-

verter, so inductors with minimum DC resistance

should be chosen for high efficiency designs.

Recommended inductors with low DC resistance

are listed in Table 2. Preferred inductors for on

board power supplies with the SP6655 are mag-

netically shielded types to minimize radiated mag-

netic field emissions.

Capacitor Selection

The SP6655 has been designed to work uith very

low ESR output capacitors (listed in Table 2

Component Selection) which for the typical appli-

cation circuit are 10

µF ceramic capacitors. These

capacitors combine small size, low ESR and good

value. To regulate the output with low ESR capaci-

tors of 0.01

Ω or less, an internal ramp voltage

VRAMP has been added to the FB signal to reliably

trip the loop comparator (as described in the Op-

erations section).

Output ripple for a buck regulator is determined

mostly by output capacitor ESR, which for the

SP6655 with a constant inductor current ripple can

be expressed as:

VOUT (ripple) = ILR * RESR

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