SP6650 datasheet(11/17 Pages) SIPEX

11 / 17 page

Date: 5/25/04

SP6650 High Efficiency 600mA Synchronous Buck Regulator

External Component Selection

Inductor

According to the pulse frequency modulation

(PFM) algorithm, the peak to peak output ripple

current can be calculated as:

ILR ≈

KON

KON = 2.7µs*V is a constant for SP6650 and is

set by the parameters of the internal ON-time

calculation circuitry. For the recommended 22

µH

inductor, typical ripple currents are ILR = 123mA

in discontinuous conduction mode (DCM) op-

eration. During continuous conduction mode,

the speed of the loop comparator determines the

current ripple. It is approximately equal to 200mA

with a 22

µH inductor.

The value of the inductor is chosen based on the

constant KON and acceptable current ripple. Two

additional inductor parameters are important: its

current rating and its DC resistance.

When the current through the inductor reaches

the level of Isat, inductance drops down to 70%

from the nominal. This non-linear change can

cause stability problems or excessive fluctuation

in current ripple. To avoid this, the inductor

should be chosen with saturation current at least

equal to the maximum output current of the

converter plus half of the ripple. To provide the

best converter performance in dynamic condi-

tions such as start-up and load transients, induc-

tors with saturation current close to the chosen

ILIM are recommended.

The second important inductor parameter, DC

resistance, directly defines the efficiency of the

converter, therefore, the inductor should be cho-

sen with the minimum possible DC resistance

for a particular design. Recommended types of

the inductors for different applications are given

in Table 1. Preferred inductors for on board

power supplies with the SP6650 converter are

shielded inductors to minimize radiated mag-

netic fields emissions.

APPLICATION INFORMATION

All components recommended for typical de-

signs like those shown in the applications sche-

matics are given in Table 1.

Input and Output Capacitors

Output capacitor is often selected based on the

requirement on the output ripple voltage. In a

Buck regulator, the output ripple is determined

by ESR (equivalent series resistor) of the output

capacitors and inductor ripple current

VOR = ESR * ILR,

where VOR = peak to peak output ripple voltage.

SP6650’s adaptive on-time scheme provides a

constant inductor ripple that is independent of

input voltages and thus makes it easier to select

the output capacitor. In many power supply

designs, the ripple voltage needs to be less than

3% of the DC output voltage. Using low ESR

tantalum or electrolytic capacitors to reduce the

output ripple.

Due to the nature of the PFM control, certain

output ripple is required for stable operation.

The loop comparator requires minimum of 15mV

ripple on the FB pin to reliably toggle the com-

parator output. That translates to an output ripple

VOR(MIN) =

15mV * VOUT

VREF

where VREF = 1.25V is the internal reference

voltage.

To reduce the output ripple and improve stabil-

ity, a small capacitor can be paralleled with the

feedback voltage divider as shown on page 1.

This capacitor forms a high pass filter with

feedback resistor to increase the ripple voltage

seen by the FB pin. The value of the capacitor

should be in the range of 100pF to 500pF.

Although the 3.3V output can be programmed

simply by connecting the FB pin to the ground,

using this external feedback scheme can signifi-

cantly reduce the output ripple. For output ripple

less than 15mV, for instance when ceramic

capacitors are used, an artificial ramp can be

generated and superimposed onto the output.

SP6650 데이터시트(PDF) 11 Page - Sipex Corporation

SP6650 데이터시트(HTML) 11 Page - Sipex Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

데이터시트 다운로드

링크 URL