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ISL6566 Fiches technique(PDF) 21 Page - Intersil Corporation

No de pièce ISL6566
Description  Three-Phase Buck PWM Controller with Integrated MOSFET Drivers for VRM9, VRM10, and AMD Hammer Applications
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Fabricant  INTERSIL [Intersil Corporation]
Site Internet  http://www.intersil.com/cda/home
Logo INTERSIL - Intersil Corporation

ISL6566 Fiches technique(HTML) 21 Page - Intersil Corporation

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21
FN9178.3
July 25, 2005
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
Q2
D
S
G
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
3
---------------------
=
PDR_LOW
RHI2
RHI2 REXT2
+
--------------------------------------
RLO2
RLO2 REXT2
+
----------------------------------------
+


 PQg_Q2
2
---------------------
=
REXT1
RG1
RGI1
NQ1
-------------
+
=
REXT2
RG2
RGI2
NQ2
-------------
+
=
PBOOT
PQg_Q1
3
---------------------
=
FIGURE 17. ISL6566 INTERNAL AND EXTERNAL CURRENT-
SENSING CIRCUITRY
ISEN(n)
RISEN
VIN
CHANNEL N
UPPER MOSFET
CHANNEL N
LOWER MOSFET
-
+
IL rDS ON
()
IL
ISL6566
RISEN
rDS ON
()
50 10 6
×
-----------------------
IFL
N
--------
=
(EQ. 24)
RISEN 2,
RISEN
∆T
2
∆T
1
----------
=
(EQ. 25)
ISL6566


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