ISL854102DEMO2Z datasheet(17/21 Pages) RENESAS

17 / 21 page

ISL854102

FN8870 Rev.1.00

Page 17 of 21

Mar 15, 2019

Figure 37 shows the type II compensator and its transfer function

is expressed as shown in Equation 10:

where:

Compensator design goal:

•High DC gain

• Choose loop bandwidth fc less than 100kHz

• Gain margin: >10dB

• Phase margin: >40°

The compensator design procedure is as follows:

The loop gain at crossover frequency of fc has a unity gain.

Therefore, the compensator resistance R6 is determined by

Equation 11.

where GM is the transconductance, gm, of the voltage error

amplifier in each phase. Compensator capacitor C6 is then given

by Equation 12.

Put one compensator pole at zero frequency to achieve high DC

gain, and put another compensator pole at either ESR zero

frequency or half switching frequency, whichever is lower in

Equation 12. An optional zero can boost the phase margin.



CZ2

is a zero due to R2 and C3.

Put compensator zero 2 to 5 times fc.

Example: VIN = 12V, VO = 5V, IO = 1.2A, fSW = 500kHz,

R2 = 90.9kΩ, Co = 22µF/5mΩ, L = 39µH, fc = 50kHz, then

compensator resistance R6:

It is acceptable to use 124kΩ as the closest standard value for

R6.

It is also acceptable to use the closest standard values for C6 and

C7. There is approximately 3pF parasitic capacitance from VCOMP

to GND; Therefore, C7 is optional. Use C6 = 1500pF and

C7 =OPEN.

Use C3 = 68pF. Note that C3 may increase the loop bandwidth

from previous estimated value. Figure 38 shows the simulated

voltage loop gain. It is shown that it has a 75kHz loop bandwidth

with a 61° phase margin and 6dB gain margin. It may be more

desirable to achieve an increased gain margin, which can be

accomplished by lowering R6 by 20% to 30%. In practice,

ceramic capacitors have significant derating on voltage and

temperature, depending on the type. See the ceramic capacitor

datasheet for more details.

VREF

VFB

VCOMP

FIGURE 37. TYPE II COMPENSATOR

Av S



vˆCOMP

vˆFB

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

GM R3



C6 C7

 R





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



cz1

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



 1



cz2

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







cp1

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



 1



cp2

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





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

(EQ. 10)



cz1

R6C6

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



cz2

R2C3

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



cp1



C6 C7

R6C6C7

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



cp2

R2 R3

C3R2R3

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



f

cVoCoRt

GM VFB



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

22.75

10

fcVoCo



(EQ. 11)

RoCo

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

VoCo

IoR6

--------------- C7 max

RcCo

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

f

SWR6

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

)

(EQ. 12)

f

cR2

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

(EQ. 13)

22.75

10

50kHz 5V 22

F



125.12k



(EQ. 14)

5V 22

F

1A 124k





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

0.88nF

(EQ. 15)

C7 max

 22F



124k



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

 500kHz 124k



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

)

0.88pF 5.1pF

)

(EQ. 16)

 50kHz 90.9k



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

70pF

(EQ. 17)

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