ISL6561 datasheet(21/27 Pages) RENESAS

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ISL6561

FN9098 Rev 6.00

Page 21 of 27

November 10, 2015

In Equation 22, make sure that

T2 is the desired temperature

rise above the ambient temperature, and

T1 is the measured

temperature rise above the ambient temperature. While a

single adjustment according to Equation 22 is usually

sufficient, it may occasionally be necessary to adjust RISEN

two or more times to achieve optimal thermal balance

between all channels.

Load-Line Regulation Resistor

The load-line regulation resistor is labeled RFB in Figure 7.

Its value depends on the desired full-load droop voltage

(VDROOP in Figure 7). If Equation 21 is used to select each

ISEN resistor, the load-line regulation resistor is as shown

in Equation 23.

If one or more of the ISEN resistors is adjusted for thermal

balance, as in Equation 23, the load-line regulation resistor

should be selected according to Equation 24 where IFL is the

full-load operating current and RISEN(n) is the ISEN resistor

connected to the nth ISEN pin.

Compensation

The two opposing goals of compensating the voltage

regulator are stability and speed. Depending on whether the

regulator employs the optional load-line regulation as

described in Load-Line Regulation, there are two distinct

methods for achieving these goals.

COMPENSATING LOAD-LINE REGULATED

CONVERTER

The load-line regulated converter behaves in a similar

manner to a peak-current mode controller because the two

poles at the output-filter L-C resonant frequency split with

the introduction of current information into the control loop.

The final location of these poles is determined by the system

function, the gain of the current signal, and the value of the

compensation components, RC and CC.

Since the system poles and zero are effected by the values

of the components that are meant to compensate them, the

solution to the system equation becomes fairly complicated.

Fortunately there is a simple approximation that comes very

close to an optimal solution. Treating the system as though it

were a voltage-mode regulator by compensating the L-C

poles and the ESR zero of the voltage-mode approximation

yields a solution that is always stable with very close to ideal

transient performance.

The feedback resistor, RFB, has already been chosen as

outlined in Load-Line Regulation Resistor. Select a target

bandwidth for the compensated system, f0. The target

bandwidth must be large enough to assure adequate

transient performance, but smaller than 1/3 of the per-

channel switching frequency. The values of the

compensation components depend on the relationships of f0

to the L-C pole frequency and the ESR zero frequency. For

each of the three cases which follow, there is a separate set

of equations for the compensation components.

In Equations 25, 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

resistance of the bulk output-filter capacitance; and VPP is

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

Figure 6 and Electrical Specifications.

RFB

VDROOP

70 10

–



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

(EQ. 23)

RFB

VDROOP

IFL rDS ON



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

RISEN n





(EQ. 24)

FIGURE 13. COMPENSATION CONFIGURATION FOR

LOAD-LINE REGULATED ISL6561 CIRCUIT

COMP

RFB

IDROOP

VDIFF

VDROOP

C2 (OPTIONAL)

 LC

------------------- f0

RFB

f

0Vpp LC

0.75VIN

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

0.75VIN

V

PPRFBf0

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

Case 1:

 LC

------------------- f0

C ESR



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



RFB

VPP 2

2 f

2 LC

0.75 VIN

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

0.75VIN



2 f

2 V

PPRFB LC

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

Case 2:

(EQ. 25)

C ESR



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

RFB

 f

0VppL

0.75 VIN ESR



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

0.75VIN ESR

 C

V

PPRFBf0 L

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

Case 3:

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