14

defined below, there is a separate set of equations for the

compensation components.

In Equations 17, L is the per-channel filter inductance

divided by the number of active channels; C is the sum total

of all output capacitors; ESR is the equivalent-series

the peak-to-peak sawtooth signal amplitude as described in

Figure 5 and Electrical Specifications.

Once selected, the compensation values in Equations 17

assure a stable converter with reasonable transient perfor-

mance. In most cases, transient performance can be

oscilloscope until no further improvement is noted. Normally,

Equations 17 unless some performance issue is noted.

noise away from the PWM comparator (see Figure 5). Keep

frequency capacitor of between 22pF and 150pF in case any

jitter problem is noted.

COMPENSATION WITHOUT LOAD-LINE REGULATION

The non load-line regulated converter is accurately modeled

as a voltage-mode regulator with two poles at the L-C

resonant frequency and a zero at the ESR frequency. A type

III controller, as shown in Figure 12, provides the necessary

compensation.

compensated system. Choose a frequency high enough to

assure adequate transient performance but not higher than

1/3 of the switching frequency. The type-III compensator has

added noise rejection or to assure adequate attenuation at

the error-amplifier high-order pole and zero frequencies. A

problems with too much phase shift below the system

bandwidth.

In the solutions to the compensation equations, there is a

single degree of freedom. For the solutions presented in

compensation components are then selected according to

Equations 18.

1

2

π LC

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

f

0

>

R

C

R

FB

2

πf

0.75V

IN

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

=

C

C

0.75V

IN

2

πV

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

=

Case 1:

1

2

π LC

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

f

0

1

2

πC ESR

()

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

<

≤

R

C

R

FB

V

0

2

LC

0.75 V

IN

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

=

C

C

0.75 V

IN

2

π

0

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

=

Case 2:

(EQ. 17)

f

0

1

2

πC ESR

()

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

>

R

C

R

FB

2

π f

0.75 V

()

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

=

C

C

0.75V

() C

2

πV

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

=

Case 3:

FIGURE 12. COMPENSATION CIRCUIT FOR ISL6219A BASED

CONVERTER WITHOUT LOAD-LINE

REGULATION.

COMP

FB

VSEN

-

+

R

1

R

FB

CESR

()

LC

C ESR

()

–

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

=

C

1

LC

C ESR

()

–

R

FB

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

=

C

2

0.75 V

IN

2

π

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

=

(EQ. 18)

R

C

V



0.75 V

IN

2

π f

HF

LC 1

–





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

=

C

C

0.75V

IN

2

πV

f

1

2

πf

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

–









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

=

ISL6219A