AN2115 datasheet(20/33 Pages) STMICROELECTRONICS

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Application information

AN2115

20/33

Doc ID 11165 Rev 5

6.1.4

Compensation network (R1 C3)

As shown in Section 4.3 the system stability can be studied using the loop transfer function

given by Equation 3. If the output capacitor is of the ceramic type, the zero due to the ESR

will generally be outside of the system bandwidth. Thus, the stability of the system will be

ensured by the cancellation between the pole due to the output capacitor, the equivalent

load and the R1C3 zero. Equation 15 represents a simplified gain loop expression,

applicable around the transition frequency fT:

Equation 15

Assuming that C2 = 10 µF, the transition frequency at 0 dB = 30 kHz (fT is equal to the

system bandwidth), and the output voltage = 1.8 V, the value of R1 can be calculated as:

Equation 16

The nearest standard E12 series value is R1 = 24 kΩ.

The higher the bandwidth, the faster the transient response will be, but the bandwidth (and

so the R1 value) must be lower than fSW/10 to avoid being influenced by the sampling effect

poles as mentioned in Section 4.3. The zero due to the compensation network must be at

the least 5 times before the frequency transition, so the value of C3 will be:

Equation 17

The nearest standard value is C3 = 220 pF

Table 5.

Recommended inductors

Manufacturer

Series

Inductor value (µH)

Saturation current (A)

Coilcraft

DO1607C

6.8 to 15

0.72 to 0.96

DT1608C

6.8 to 15

0.6 to 1

LPO1704

6.8 to 10

0.8 to 0.9

CH4192-A

4.7

1.3

DO1606T

6.8 to 10

1 to 1.1

Panasonic

ELL6RH

6.2 to 22

0.7 to 1.4

ELL6GM

6.8 to 10

0.93 to 1.1

Toko

D62CB

10 to 22

0.71 to 1.07

D62C

10 to 22

0.63 to 0.99

Coiltronics

SD10

3.3 to 6.2

0.92 to 1.31

SD12

3.3 to 6.2

1.08 to 1.42

LOOP s

()

mR1α

-------------------

πf

TC2

mα

-------------------

24k

πf

TR1

-------------------

220pF

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