ISL78227

FN8808 Rev.6.02

Page 40 of 43

Feb 28, 2025

COMPENSATOR DESIGN

Generally, a simple Type-2 compensator can be used to stabilize

the system. In the actual application, however, an extra phase

margin is provided by a Type-3 compensator.

The transfer function at the error amplifier and its compensation

network can be expressed as Equation 36.

If ROEA>>RCP, CCP1>>CCP2, and ROEA = infinite, the equation

can be simplified as shown in Equation 37:

where:

If Type-3 compensation is needed, the transfer function at the

feedback resistor network is:

where:

The total transfer function with compensation network and gain

stage is expressed as:

Use f = ω/2π to convert the pole and zero expressions to

frequency domain, and from Equations 33, 38, and 39, select

the compensator’s pole and zero locations.

In general, as described earlier, a Type-2 compensation is

enough. Typically, the crossover frequency is set 1/5 to 1/3 of

the ωRHZ frequency. As a general rule, set the compensator

ωp2/2π at the very low end frequency; set ωz2/2π at 1/5 of the

crossover frequency, and set ωp3/2π at the ESR zero or the RHZ

frequency ωRHZ/2π, whichever is lower.

VCC Input Filter

To provide a quiet power rail to the internal analog circuitry, it is

recommended to place an RC filter between PVCC and VCC. A

10Ω resistor between PVCC and VCC and at least 1µF ceramic

capacitor from VCC to GND are recommended.

Current Sense Circuit

To set the current sense resistor, the voltage across the current

sense resistor should be limited to within ±0.3V. In a typical

application, it is recommended to set the voltage across the

current sense resistor in range around 30mV to 100mV for the

typical load current condition.

Configuration to Support Single Phase

Boost

The IC can be configured to support single phase operation either

using Phase 1 or Phase 2. The configurations needed to use

Phase 1 for single phase operation are listed below (to use

Phase 2 for single phase operation, change the corresponding

phase number to the other phase number):

• BOOT2 connects to PVCC

• PH2 connects to GND

• Remove or short RSEN2.

• ISEN2P and ISEN2N connect to VIN through the same RSET

and RBIAS resistor respectively as Phase 1

• Leave UG2 and LG2 open

The extra notes are listed below with upper single phase

configurations:

• If BOOT2 pin connects to GND, both UG2 and UG1 are disabled.

• IMON pin output current signal has only Phase 1’s inductor

current sensed signal. Equation 12, which calculates the IMON

output current, is expressed as Equation 40, for ISEN2P and

ISEN2N connecting to VIN.

• If ISEN2P and ISEN2N connect to GND, the offset current

17×10-6 in Equation 40 becomes to 8.5×10-6.

The Constant Current Loop works on the same principle.

FIGURE 66. TYPE-3 COMPENSATOR

-

+

Gm

VREF

VOUT

RCP

CCP2

FB

Vc

RFB2

RFB1

R1

C1

CCP1

ROEA

He2(s)

He1(s)

COMP

VOUT

He2 s

 

VC

VFB

-----------

gm ZCOMP =



=

=

(EQ. 36)

gm

1 sRCPCCP1

+





ROEA

1 s RCPCCP1 ROEA CCP1 CCP2

+





+





CCP2CCP1RCPROEAS

2

+

+

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

He2 s

 

gm

1 s RCP CCP1





+

s CCP1 1 s RCP CCP2





+









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



1

s

-------

1

s

z2

----------

+

1

s

p2

----------

+

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



=

=

(EQ. 37)

1

gm

CCP1

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

=

z2

1

RCP CCP1



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

=

p2

1

RCP CCP2



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

=

He1 s

 

RFB1

RFB1 RFB2

+

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

1

s

z1

----------

+

1

s

p1

----------

+

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



=

(EQ. 38)

z1

1

C1 RFB2 R1

+







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

=

p1

1

C1

RFB2 RFB1



RFB2 R1



RFB1 R1



+

+

RFB2 RFB1

+

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



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

=

Gopen s

 

Gvcvo s

 

He1 s

 

He2 s

 





=

(EQ. 39)

IMON

IL1 R



SEN1

RSET1

-------------------------------- 0.125



17 10



6

–

+

=

(EQ. 40)