IR3840WM datasheet(22/34 Pages) IRF

22 / 34 page

Rev 12.0

PD-97657

IR3840WMPbF

Tantalum

Ceramic

LC<Fo<FESR

Type III

Electrolytic

Tantalum

FLC<FESR<Fo<Fs/2

Type II

Output

Capacitor

ESR vs Fo

Compensator

Type

Tantalum

Ceramic

LC<Fo<FESR

Type III

Electrolytic

Tantalum

FLC<FESR<Fo<Fs/2

Type II

Output

Capacitor

ESR vs Fo

Compensator

Type

The compensation network has three poles and

two zeros and they are expressed as follows:

Cross over frequency is expressed as:

Fig.15. Type III Compensation network and

its asymptotic gain plot

(30)

..........

)

(

.....(29)

..........

(28)

.....

..........

)

.......(27

..........

......(26)

..........

≅

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

(31)

..........

osc

The

higher

the

crossover

frequency,

the

potentially faster the load transient response.

However, the crossover frequency should be low

enough to allow attenuation of switching noise.

Typically, the control loop bandwidth or crossover

frequency is selected such that

The DC gain should be large enough to provide

high DC-regulation accuracy. The phase margin

should be greater than 45o for overall stability.

For this design we have:

in=12V

o=1.8V

osc=1.8V

ref=0.7V

o=0.6uH

o=6x22uF, ESR=3mOhm each

It must be noted here that the value of the

capacitance used in the compensator design

must be the small signal value. For instance, the

small signal capacitance of the 22uF capacitor

used in this design is 12uF at 1.8 V DC bias and

600 kHz frequency. It is this value that must be

used for all computations related to the

compensation. The small signal value may be

obtained from the manufacturer’s datasheets,

design tools or SPICE models. Alternatively, they

may also be inferred from measuring the power

stage transfer function of the converter and

measuring the double pole frequency F

LC and

using equation (16) to compute the small signal

These result to:

LC=24.2 kHz

ESR=4.4 MHz

s/2=300 kHz

Select crossover frequency F

o=100 kHz

Since F

LC<Fo<Fs/2<FESR, TypeIII is selected to

place the pole and zeros.

(

)

1/10

1/5

≤

VOUT

VREF

R10

FZ1

FZ2

FP2

FP3

E/A

ZIN

Frequency

Gain(dB)

H(s) dB

Comp

Based on the frequency of the zero generated by

the output capacitor and its ESR, relative to

crossover frequency, the compensation type can

be different. The table below shows the

compensation types for relative locations of the

crossover frequency.

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