11

determined by the ESR (Equivalent Series Resistance) and

voltage rating requirements as well as actual capacitance

requirements. The output voltage ripple is due to the

inductor ripple current and the ESR of the output capacitors

as defined by:

where,

High frequency decoupling capacitors should be placed as

close to the power pins of the load as physically possible. Be

careful not to add inductance in the circuit board wiring that

could cancel the usefulness of these low inductance

components. Consult with the manufacturer of the load

circuitry for specific decoupling requirements.

Use only specialized low-ESR capacitors intended for

switching-regulator applications, at 300kHz, for the bulk

capacitors. In most cases, multiple electrolytic capacitors of

small case size perform better than a single large case

capacitor.

The stability requirement on the selection of the output

30kHz. This range is set by an internal, single compensation

zero at 6kHz. The ESR zero can be a factor of five on either

side of the internal zero and still contribute to increased

phase margin of the control loop. Therefore:

In conclusion, the output capacitors must meet three criteria: By

varying the values of the soft-start capacitors, it is possible to

provide sequencing of the main outputs at start-up.

1. They must have sufficient bulk capacitance to sustain the

output voltage during a load transient while the output

inductor current is slewing to the value of the load

transient

2. The ESR must be sufficiently low to meet the desired

output voltage ripple due to the output inductor current,

and

3. The ESR zero should be placed, in a rather large range,

to provide additional phase margin.

3.3V ALWAYS and 5V ALWAYS Output Capacitors

The output capacitors for the linear regulators insure stability

and provide dynamic load current. The 3.3V ALWAYS and

the 5V ALWAYS linear regulators should have, as a

minimum, 10

µF capacitors on their outputs.

3.3V Main and 5V Main PWM Output Inductor

Selection

The PWM converters require output inductors. The output

inductor is selected to meet the output voltage ripple

requirements. The inductor value determines the converter’s

ripple current and the ripple voltage is a function of the ripple

current and output capacitor(s) ESR. The ripple voltage

expression is given in the capacitor selection section and the

ripple current is approximated by the following equation:

Input Capacitor Selection

The important parameters for the bulk input capacitor(s) are

the voltage rating and the RMS current rating. For reliable

operation, select bulk input capacitors with voltage and

current ratings above the maximum input voltage and largest

RMS current required by the circuit. The capacitor voltage

rating should be at least 1.25 times greater than the

maximum input voltage and 1.5 times is a conservative

guideline.

The AC RMS input current varies with load as shown in

Figure 9. Depending on the specifics of the input power and

it’s impedance, most (or all) of this current is supplied by the

input capacitor(s). Figure 9 also shows the advantage of

having the PWM converters operating out of phase. If the

converters were operating in phase, the combined RMS

current would be the algebraic sum, which is a much larger

value as shown. The combined out-of-phase current is the

square root of the sum of the square of the individual

reflected currents and is significantly less than the combined

in-phase current.

Use a mix of input bypass capacitors to control the voltage

ripple across the MOSFETs. Use ceramic capacitors for the

high frequency decoupling and bulk capacitors to supply the

RMS current. Small ceramic capacitors can be placed very

close to the upper MOSFET to suppress the voltage induced

in the parasitic circuit impedances.

For board designs that allow through-hole components, the

Sanyo OS-CON® series offer low ESR and good

temperature performance.

For surface mount designs, solid tantalum capacitors can be

used, but caution must be exercised with regard to the

capacitor surge current rating. These capacitors must be

capable of handling the surge-current at power-up. The TPS

series available from AVX is surge current tested.

V

RIPPLE

∆IL ESR

×

=

C

OUT

1

2

π ESR f

Z

×

×

×

-------------------------------------------

=

∆IL

V

IN

V

OUT

–

F

S

L

×

--------------------------------

V

OUT

V

IN

----------------

×

=

FIGURE 9. INPUT RMS CURRENT vs LOAD

12345

3.3V AND 5V LOAD CURRENT

5

4.5

4

3.5

3

2.5

2

1.5

1

0.5

0

0

IN PHASE

OUT OF PHASE

3.3V

5V

IPM6220

OS-CON® is a registered trademark of Sanyo Electric Company, Ltd. (Japan)