SP6654EU datasheet(12/17 Pages) SIPEX

12 / 17 page

Date: 2/1/05

SP6654 High Efficiency 800mA Synchronous Buck Regulator

For the 22

µF POSCAP with 0.04Ω ESR, and a

µH inductor yielding 200mA inductor current

ripple ILR, the VOUT ripple would be 8mVpp.

Since 8mV is a very small signal level, the actual

value would probably be larger due to noise and

layout issues, but this illustrates that the SP6654

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. Another function of the

output capacitance is to hold up the output voltage

during the load transients and prevent excessive

overshoot and undershoot. The typical perfor-

mance characteristics curves show very good load

step transient response for the SP6654 with the

recommended output capacitance of 22

µF ce-

ramic.

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 SP6654 is 22

µF ceramic,

POSCAP or Aluminum Polymer. These capaci-

tors will provide good high frequency bypassing

and their low ESR will reduce resistive losses for

higher efficiency. An RC filter is recommended

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 require-

ments.

APPLICATION INFORMATION

current ripple of almost 300mA 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 SP6654 inductor current limit of 1.25A.

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 SP6654 are mag-

netically shielded types to minimize radiated mag-

netic field emissions.

Capacitor Selection

The SP6654 has been designed to work with very

low ESR output capacitors (listed in Table 2

Component Selection) which for the typical appli-

cation circuit are 22

µF ceramic, POSCAP or Alu-

minum Polymer. These capacitors combine small

size, low ESR and good value. To regulate the

output with low ESR capacitors 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 Operations section).

Output ripple for a buck regulator is determined

mostly by output capacitor ESR, which for the

SP6654 with a constant inductor current ripple can

be expressed as:

VOUT (ripple) = ILR * RESR

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