MCR03EZPFX1002 datasheet(21/35 Pages) IRF

21 / 35 page

Rev 4.0

PD-97528

IR3856MPbF

(16)

.....

..........

current

ripple

Inductor

ripple

voltage

Output

⎟

⎠

⎞

⎜

⎝

⎛

−

)

(

)

ESL

(

)

ESR

(

)

(

)

ESL

(

)

ESR

(

ESL

ESR

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 IR3856

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.

Four

the

TDK

C2102X5R0J226M

(22uF,

6.3V,

3mOhm)

capacitors is a good choice.

Feedback Compensation

The IR3856 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 45o).

The output LC filter introduces a double pole,

–40dB/decade gain slope above its corner

resonant frequency, and a total phase lag of 180o

(see figure 13). The resonant frequency of the LC

filter is expressed as follows:

Figure 14 shows gain and phase of the LC filter.

Since we already have 180o phase shift from the

output filter alone, the system runs the risk of

being unstable.

The IR3856 uses a voltage-type error amplifier

with high-gain (110dB) and wide-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 14.

This method requires that the output capacitor

should have enough ESR to satisfy stability

requirements.

general,

for

Type-II

compensation

the

output

capacitor’s

ESR

generates a zero typically at 5kHz to 50kHz

which is essential for an acceptable phase

margin.

The ESR zero of the output capacitor is

expressed as follows:

(17)

..........

∗

Fig. 14. Gain and Phase of LC filter

(18)

.......

..........

ESR

*ESR*C

∗

Output Capacitor Selection

The voltage ripple and transient requirements

determine the output capacitors type and values.

The criteria 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

described as

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