ISL6565BCB datasheet(20/28 Pages) RENESAS

20 / 28 page

ISL6565A, ISL6565B

FN9135 Rev 4.00

Page 20 of 28

Dec 1, 2005

the required time for this commutation is t1 and the

approximated associated power loss is PUP,1.

At turn on, the upper MOSFET begins to conduct and this

transition occurs over a time t2. In Equation 23, the

approximate power loss is PUP,2.

A third component involves the lower MOSFET’s reverse-

recovery charge, Qrr. Since the inductor current has fully

commutated to the upper MOSFET before the lower-

MOSFET’s body diode can recover all of Qrr, it is conducted

through the upper MOSFET across VIN. The power

dissipated as a result is PUP,3.

Finally, the resistive part of the upper MOSFETs is given in

Equation 25 as PUP,4.

The total power dissipated by the upper MOSFET at full load

can now be approximated as the summation of the results

from Equations 22, 23, 24 and 25. Since the power

equations depend on MOSFET parameters, choosing the

correct MOSFETs can be an iterative process involving

repetitive solutions to the loss equations for different

MOSFETs and different switching frequencies.

Current Sensing Component Selection

The ISL6565A supports MOSFET rDS(ON) current sensing,

while the ISL6565B uses inductor DCR current sensing. The

procedures for choosing the components for each method of

current sensing are very different and are described in the

next two sections.

MOSFET rDS(ON) SENSING (ISL6565A ONLY)

The ISL6565A senses the channel load current by sampling

the voltage across the lower MOSFET rDS(ON), as shown in

Figure 15. The ISEN pins are denoted ISEN1, ISEN2, and

ISEN3. The resistors connected between these pins and the

respective phase nodes determine the gains in the load-line

regulation loop and the channel-current balance loop as well

as setting the overcurrent trip point.

Select values for these resistors based on the room

temperature rDS(ON) of the lower MOSFETs; the full-load

operating current, IFL; and the number of phases, N using

Equation 26.

In certain circumstances, it may be necessary to adjust the

value of one or more ISEN resistor. When the components of

one or more channels are inhibited from effectively dissipating

their heat so that the affected channels run hotter than

desired, choose new, smaller values of RISEN for the affected

phases (see the section entitled Voltage Regulation). Choose

RISEN,2 in proportion to the desired decrease in temperature

rise in order to cause proportionally less current to flow in the

hotter phase.

In Equation 27, 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 27 is usually

sufficient, it may occasionally be necessary to adjust RISEN

two or more times to achieve optimal thermal balance

between all channels.

INDUCTOR DCR SENSING (ISL6565B ONLY)

The ISL6565B senses the channel load current by sampling

the voltage across the output inductor DCR, as described in

the Current Sensing section. As Figure 16 illustrates, an R-C

network across the inductor is required to sense the channel

current accurately.

PUP 1, VIN

------

IPP

---------



 t1

----









(EQ. 22)

PUP 2

VIN

------

IPP

---------

–



 t2

----









(EQ. 23)

PUP 2

VIN

------

IPP

---------

–



 t2

----









PUP 3,

VIN Qrr fS

(EQ. 24)

PUP 4, rDS ON



------





 2

IPP

----------



(EQ. 25)

FIGURE 15. ISL6565A INTERNAL AND EXTERNAL CURRENT-

SENSING CIRCUITRY

ISEN(n)

RISEN

VIN

CHANNEL N

UPPER MOSFET

CHANNEL N

LOWER MOSFET

IL rDS ON



ISL6565A

RISEN

rDS ON



70 10 6

–



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

IFL

--------

(EQ. 26)

RISEN 2,

RISEN

T

----------

(EQ. 27)

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