ISL85014 datasheet(18/22 Pages) RENESAS

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ISL85014

FN8925 Rev.3.00

Page 18 of 21

Nov 11, 2021

The simplified transfer function is derived in Equation 15:

where:

Note that Co is the actual capacitance seen by the regulator,

which may include ceramic high frequency decoupling and bulk

output capacitors. Ceramic capacitors may have to be derated by

approximately 40% depending on dielectric, voltage stress, and

temperature.

Usually, a Type II compensation network is used to compensate

the peak current mode control converter. Figure 40 shows a

typical Type II compensation network and its transfer function is

expressed in Equation 17. The frequency response is shown in

Figure 41.

where:

Design example: VIN = 12V, VO = 1.8V, IO = 14A, fSW = 600kHz,

R1 = 200kΩ, R2 = 100kΩ, Co = 4x100µF/3mΩ 6.3V ceramic

(actually ~200µF), L = 0.68µH,

Select fc = 60kHz. The gain of the Gp(s)xAv(s) should have a unity

gain at crossover frequency. Thus, R3 can be derived as:

Select 800kΩ for R3. Place the zero fz1 around the pole fp to

achieve -20db/dec roll off.

where Rc is the ESR of the output capacitor.

Select 30pF for C2. Zero fz2 is a phase boost zero to increase the

phase margin. Place it between fc and 1/2 switching frequency.

In this case, the ESR zero is calculated to determine if a

feed-forward capacitor is required to introduce phase boost.

Since the ESR zero falls between fc and 1/2 switching frequency,

an additional zero using C1 may not be required. The calculated

values for R1, R2, R3, and C2 match with the 1.8V output

application in the recommended design with internal

compensation shown in Table 1 on page 2. Do not select

resistance higher than 370kΩ for R1 in real applications, since it

will result in a very small C1, which might be overwhelmed by

parasitic capacitances.

In practice, it is recommended to select lower resistance for

R1/R2 and R3 in the external compensation applications.

Usually, 10 times lower compared with the internal

compensation is a good start.

FIGURE 38. POWER TRAIN SMALL SIGNAL MODEL

FIGURE 39. POWER TRAIN SMALL FREQUENCY RESPONSE

FIGURE 40. TYPE II COMPENSATION NETWORK

1/Rt

Vcomp

Gdc

Gp S



vˆo

vˆcomp

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

Gdc



------



-------

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

(EQ. 15)

Gdc

-------



;

fz

RcxCo

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



;

fp

Ro Rc

xC

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

(EQ. 16)

VOUT

Vcomp

VFB

VREF

FIGURE 41. TYPE II COMPENSATION FREQUENCY RESPONSE

Av S



vˆcomp

vˆo

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



cz1

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



 1



cz2

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





SC2R1

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

(EQ. 17)



cz1

f

R3C2

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



cz2

f



R1C1

--------------- fpc



R

1C2

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

R3/R1

fZ2

fZ1

fpc

f

cCoRtR1

829k



(EQ. 18)

Ro Rc

xC

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

33pF

(EQ. 19)

fzesr

R

cCo

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

265kHz

(EQ. 20)

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