IR3894M_15 Datasheet, PDF(32/45 Page) International Rectifier – Single‐Input Voltage, Synchronous Buck Regulator

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12A Highly Integrated SupIRBuck

SingleâInput V-o3l2ta-g` e, Synchronous Buck Regulator

PDâ97745

IR3894

ZIN

VOUT

R4 R5

R3 C3

Gain (dB)

VREF

E/A

Comp

|H(s)| dB

FZ1

FZ 2

FP2

F Frequency

Figure 27: Type III Compensation network

and its asymptotic gain plot

Again, the transfer function is given by:

Vout

ï½

( s)

ï½

Z IN

By replacing Zin and Zf, according to Fig. 27, the transfer

function can be expressed as:

H (s) ï½

(1 ï« sR3C3 ) ï©ï«1 ï« sC 4 ï¨ R4 ï« R5 ï©ï¹ï»

sR5 (C2

ï«

C3 )

ï©ïª1

ï«

sR3

ï¦

ï§

ï¨

ï«

ï¶ï¹

ï·ïº

ï¸ï»

ï«

sR4C4

)

ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ( 2 7 )

The compensation network has three poles and two

zeros and they are expressed as follows:

FP1 ï½ 0ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï (28)

FP2

ï½

2ï°

* R4

*C4 ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï (29)

FP3

ï½

2ï°

ï¦

ï§

ï¨

* C3

ï« C3

ï¶

ï·

ï¸

2ï°

* C2

ï ï ï ï ï ï ï ï ï (30)

FZ 1

ï½

2ï°

*C3 ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï (31)

FZ 2

ï½

2ï°

* C4

* (R4

ï«

R5 )

2ï°

* C4

ï ï ï ï ï ï ï ï ï ï ï (32)

Cross over frequency is expressed as:

ï½

*C4

* Vin

Vosc

2ï°

* Lo

*Co

(33)

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. Table 3 shows the

compensation types for relative locations of the crossover

frequency.

TABLE 3: DIFFERENT TYPES OF COMPENSATORS

Compensator

Type

Type II

Type III

FESR vs FO

FLC < FESR < FO < FS/2

FLC < FO < FESR

Typical Output

Capacitor

Electrolytic

SP Cap, Ceramic

The higher the crossover frequency is, the potentially faster

the load transient response will be. However, the crossover

frequency should be low enough to allow attenuation of

switching noise. Typically, the control loop bandwidth or

crossover frequency (Fo) is selected such that:

The DC gain should be large enough to provide high

DCâregulation accuracy. The phase margin should be greater

than 45o for overall stability.

For this design we have:

Vin=12V

Vo=1.2V

Vosc=1.8V (This is a function of Vin, pls. see feed forward

section)

Vref=0.5V

Lo=0.51uH

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 10uf at 1.2 V dc bias and 600 kHz frequency. It

is this value that must be used for all computations related to the

compensation.

32 JANUARY 18, 2013 | DATA SHEET | Rev 3.4

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