PCMB065T-R68MS datasheet(29/46 Pages) IRF

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JANURARY 18, 2013 | DATA SHEET | Rev 3.5

IR3898

6A Highly Integrated SupIRBuck

Single-Input Voltage, Synchronous Buck Regulator

PD-97662

can be reduced to the desired maximum ripple current

in the inductor (Δi). The optimum point is usually found

between 20% and 50% ripple of the output current.

For the buck converter, the inductor value for the

desired operating ripple current can be determined

using the following relation:





;

(15)

i F





 

  













Where:

Vin = Maximum input voltage

V0 = Output Voltage

Δi = Inductor Peak-to-Peak Ripple Current

Fs = Switching Frequency

Δt = On time

D = Duty Cycle

If Δi ≈ 30%*Io, then the output inductor is calculated to

be 1.0μH. Select L=1.0μH, SPM6550T-1R0M, from TDK

which provides a compact, low profile inductor suitable

for this application.

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 be described as:

(

)

(

)

( )

()

(16)

o ESR

o ESL

o C

o ESR

o ESL

ESR

ESL

  

 



 





 









Where:

ΔV0 = Output Voltage Ripple

ΔIL = 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

IR3898 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 of TDK

C2012X5R0J226M (22uF/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 IR3898 is a voltage mode controller. The control loop

is a single voltage feedback path including an error amplifier

and a comparator. To achieve fast transient response

and accurate output regulation, a compensation circuit is

necessary. The goal of the compensation network is to close

the control loop at high crossover frequency with phase

margin greater than 45

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:

(17)









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

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