PCDC1008-R215EMO datasheet(28/51 Pages) IRF

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IR3447

www.irf.com

July 17, 2013

Output Capacitor Selection

The voltage ripple and transient requirements

determine the output capacitors type and values. The

criterion is normally based on the value of the

Effective Series Resistance (ESR). However the

actual capacitance value and the Equivalent Series

Inductance (ESL) are other contributing components.

These components can be described as:







)

ESL

ESR















ESR









)

(

ESL





















)

(









)

(

(19)

Where:

∆V0 = Output Voltage Ripple

∆IL = Inductor Ripple Current

Since the output capacitor has a major role in the

overall performance of the converter and determines

the result of transient response, selection of the

capacitor is critical. The IR3447 can perform well with

all types of capacitors.

As a rule, the capacitor must have low enough ESR to

meet output ripple and load transient requirements.

The goal for this design is to meet the voltage ripple

requirement in the smallest possible capacitor size.

Therefore it is advisable to select ceramic capacitors

due to their low ESR and ESL and small size. Ten of

TDK

C2012X5R0J476M

(47uF/0805/X5R/6.3V)

capacitors is a good choice.

It is also recommended to use a 0.1µF ceramic

capacitor at the output for high frequency filtering.

Feedback Compensation

The IR3447 is a voltage mode controller. The control

loop is a single voltage feedback path including error

amplifier and error comparator. To achieve fast

transient response and accurate output regulation, a

compensation circuit is necessary. The goal of the

compensation network is to provide a closed-loop

transfer function with the highest 0 dB crossing

frequency and adequate phase margin (greater than

o).

The output LC filter introduces a double pole, -

40dB/decade gain slope above its corner resonant

frequency, and a total phase lag of 180

o. The resonant

frequency of the LC filter is expressed as follows:







(20)

Figure 25 shows gain and phase of the LC filter. Since

we already have 180

o phase shift from the output filter

alone, the system runs the risk of being unstable.

Phase

Frequency

-180

0dB

-40dB/Decade

-90

Gain

Figure 25: Gain and Phase of LC filter

The IR3447 uses a voltage-type error amplifier with

high-gain and high-bandwidth. The output of the

amplifier is available for DC gain control and AC

phase compensation.

The error amplifier can be compensated either in type

II or type III compensation. Local feedback with Type

II compensation is shown in Figure 26.

This method requires that the output capacitor have

enough ESR to satisfy stability requirements. If the

output capacitor’s ESR generates a zero at 5kHz to

50kHz, the zero generates acceptable phase margin

and the Type II compensator can be used.

The ESR zero of the output capacitor is expressed as

follows:

ESR







(21)

PCDC1008-R215EMO Datasheet(PDF) 28 Page - International Rectifier

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