ISL6565BCB datasheet(22/28 Pages) RENESAS

22 / 28 page

ISL6565A, ISL6565B

FN9135 Rev 4.00

Page 22 of 28

Dec 1, 2005

important to note that when using Equations 33 and 34 the

resistor divider ratio of the corresponding phase RC network

is being changed. In the phase being adjusted, this new ratio,

Knew (described in Equation 35), can not exceed 1.0.

If this occurs, the current in the hot phase cannot be reduced

any more. Instead of decreasing the current in the hot phase,

the current must be increased in the colder phases. To

accomplish this, use Equations 33 and 34 to get the desired

temperature rise in the cold phases.

While a single adjustment, according to Equations 33 and 34,

is usually sufficient, it may occasionally be necessary to adjust

R1 and R2 in the corresponding channels two or more times

to achieve optimal thermal balance between all phases.

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). Once the ISEN resistor has been

chosen, the load-line regulation resistor can be calculated

using Equation 36.

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

balance, as in Equation 26, the load-line regulation resistor

should be selected according to Equation 37 where IFL is the

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

connected to the nth ISEN pin.

Temperature Compensation Resistor

By combining Equations 17 and 18 found in the Temperature

Compensation

section, the value of the TCOMP resistor can

be determined using Equation 38.

In Equation 38, KT is the temperature coupling coefficient

between the ISL6565A and the closest lower MOSFET, or

the ISL6565B and the output inductor. It represents how

closely the controller temperature tracks the lower MOSFET

or inductor temperature. The value of KT is typically between

75% and 100%. KTC is the temperature dependant

transconductance of the internal compensation circuit. Its

value is designed as 2

A/V/°C. The temperature coefficient

of MOSFET rDS(ON) or inductor DCR is given by . This is

the ratio of the change in resistance to the change in

temperature. Resistance is normalized to the value at 25°C

and the value of

is typically between 0.35%/°C and

0.50%/°C.

According to Equation 38, a voltage regulator with 80%

thermal coupling coefficient between the controller and lower

MOSFET and 0.4%/°C temperature coefficient of MOSFET

rDS(ON) requires a 2.5k TCOMP resistor.

If the exact value for KT and  are not known, Equation 38

can give an incorrect value for RTCOMP. If this is the case,

follow the steps below to obtain an accurate value for

RTCOMP. This procedure works by making two output

voltage measurements. The first is made by using too much

temperature compensation, and the second with too little.

Each of the measurements produces an error and a linear

interpolation is used to find a TCOMP resistor value to

produce zero error. Make all measurements using a digital

multimeter accurate to 100

V or better.

1. Install a 5k

 resistor (R1) for RTCOMP.

2. Start the regulator at room temperature and apply full

load current. Record the output voltage, V1, immediately

after loading the regulator.

3. Allow the board to heat until the output voltage stabilizes

(usually several minutes). Record the output voltage, V2.

4. Install a 1k

 resistor (R2) for RTCOMP.

5. Start the regulator at room temperature and apply full

load current. Record the output voltage, V3, immediately

after loading the regulator.

6. Allow the board to heat until the output voltage stabilizes

(usually several minutes). Record the output voltage, V4.

7. Calculate the correct value for RTCOMP using

Equation 39.

Compensation

The two opposing goals of compensating the voltage

regulator are stability and speed.

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.

Knew

T

----------

(EQ. 35)

RFB

VDROOP

70 10

–



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

(EQ. 36)

RFB

VDROOP

IFL rDS ON



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

RISEN n





(EQ. 37)

(EQ. 38)

RTCOMP



KT KTC

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



(EQ. 39)

RTCOMP

R1 R1 R2

–



V2 V1

–



V2 V1

–



V3 V4

–



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