ISL6566 datasheet(22/30 Pages) RENESAS

22 / 30 page

ISL6566

FN9178 Rev 4.00

Page 22 of 30

Mar 9, 2006

The total gate drive power losses are dissipated among the

resistive components along the transition path and in the

bootstrap diode. The portion of the total power dissipated in

the controller itself is the power dissipated in the upper drive

path resistance, PDR_UP, the lower drive path resistance,

PDR_UP, and in the boot strap diode, PBOOT. The rest of the

power will be dissipated by the external gate resistors (RG1

and RG2) and the internal gate resistors (RGI1 and RGI2) of

the MOSFETs. Figures 15 and 16 show the typical upper

and lower gate drives turn-on transition path. The total power

dissipation in the controller itself, PDR, can be roughly

estimated as:

Current Balancing Component Selection

The ISL6566 senses the channel load current by sampling

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

Figure 17. 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 channel-

current balance loop.

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 24.

In certain circumstances, it may be necessary to adjust the

value of one or more ISEN resistors. 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 Channel-Current Balance).

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 25, 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 25 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 Component Selection (DCR

Current Sensing)

For accurate load line regulation, the ISL6566 senses the

total output current by detecting the voltage across the

output inductor DCR of each channel (As described in the

Load Line Regulation

section). As Figure 18 illustrates, an

R-C network is required to accurately sense the inductor

DCR voltage and convert this information into a “droop”

voltage, which is proportional to the total output current.

Choosing the components for this current sense network is a

two step process. First, RCOMP and CCOMP must be

chosen so that the time constant of this RCOMP-CCOMP

network matches the time constant of the inductor L/DCR.

FIGURE 16. TYPICAL LOWER-GATE DRIVE TURN-ON PATH

PVCC

RGI2

RG2

RHI2

CDS

CGS

CGD

RLO2

LGATE

PDR

PDR_UP PDR_LOW PBOOT IQ VCC





(EQ. 23)

PDR_UP

RHI1

RHI1 REXT1

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

RLO1

RLO1 REXT1

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





 PQg_Q1

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



PDR_LOW

RHI2

RHI2 REXT2

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

RLO2

RLO2 REXT2

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





 PQg_Q2

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



REXT1

RG1

RGI1

NQ1

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

REXT2

RG2

RGI2

NQ2

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

PBOOT

PQg_Q1

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

FIGURE 17. ISL6566 INTERNAL AND EXTERNAL CURRENT-

SENSING CIRCUITRY

ISEN(n)

RISEN

VIN

CHANNEL N

UPPER MOSFET

CHANNEL N

LOWER MOSFET

IL rDS ON



ISL6566

RISEN

rDS ON



50 10 6

–



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

IFL

--------

(EQ. 24)

RISEN 2,

RISEN

T

----------

(EQ. 25)

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