ISL6308CRZ Intersil, ISL6308CRZ Datasheet - Page 20

• Manufacturer Part Number: ISL6308CRZ
• Description: IC CTRLR PWM 3PHASE BUCK 40-QFN
• Manufacturer: Intersil

Specifications of ISL6308CRZ

• Pwm Type: Voltage Mode
• Number Of Outputs: 1
• Frequency - Max: 275kHz
• Duty Cycle: 66.6%
• Voltage - Supply: 4.75 V ~ 5.25 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: 0°C ~ 70°C
• Package / Case: 40-VFQFN, 40-VFQFPN
• Frequency-max: 275kHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

dissipation in the controller itself, P
estimated as:
P
P
P
P
R
Current Balancing Component Selection
The ISL6308 senses the channel load current by sampling
the voltage across the lower MOSFET r
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 r
operating current, I
Equation 23.
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 R
phases (see “Channel Current Balance” on page 10). Choose
R
R
FIGURE 17. ISL6308 INTERNAL AND EXTERNAL CURRENT-
DR
BOOT
DR_UP
DR_LOW
EXT1
ISEN,2
ISEN
ISL6308
=
P
=
=
=
DR_UP
=
in proportion to the desired decrease in temperature
R
-----------------------
50 10
r
P
---------------------
=
DS ON
⎛
⎜
⎝
G1
Qg_Q1
--------------------------------------
R
⎛
⎜
⎝
⋅
3
SENSING CIRCUITRY
HI1
(
--------------------------------------
R
DS(ON)
+
HI2
+
–
R
-------------
R
N
6
+
)
P
GI1
HI1
R
Q1
⋅
DR_LOW
R
+
HI2
FL
I
------- -
EXT1
R
FL
N
ISEN(n)
of the lower MOSFETs; the full load
; and the number of phases, N using
EXT2
CHANNEL N
LOWER MOSFET
+
--------------------------------------- -
R
+
+
R
LO1
--------------------------------------- -
R
ISEN
20
P
R
LO2
BOOT
EXT2
R
+
LO1
R
R
+
LO2
DR
V
EXT1
R
+
=
IN
EXT2
, can be roughly
(
R
I
+
Q
CHANNEL N
UPPER MOSFET
DS(ON)
-
G2
ISEN
⎞
⎟
⎠
I L
•
•
⎞
⎟
⎠
VCC
x
+
P
---------------------
•
r DS ON
R
-------------
N
Qg_Q1
P
---------------------
I
for the affected
L
GI2
Qg_Q2
Q2
3
, as shown in
)
(
2
)
(EQ. 22)
(EQ. 23)
ISL6308
rise in order to cause proportionally less current to flow in the
hotter phase.
In Equation 24, make sure that ΔT
temperature rise above the ambient temperature, and ΔT
the measured temperature rise above the ambient
temperature. While a single adjustment according to
Equation 24 is usually sufficient, it may occasionally be
necessary to adjust R
optimal thermal balance between all channels.
Load Line Regulation Component Selection (DCR
Current Sensing)
For accurate load line regulation, the ISL6308 senses the total
output current by detecting the voltage across the output
inductor DCR of each channel (as described in “Load Line
(Droop) Regulation” on page 13). As Figure 7 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, R
chosen so that the time constant of this R
network matches the time constant of the inductor L/DCR.
Then the resistor R
sense network gain, obtaining the desired full load droop
voltage. Follow the steps below to choose the component
values for this R-C network.
Due to errors in the inductance or DCR it may be necessary
to adjust the value of R
correctly. The effects of time constant mismatch can be seen
in the form of droop overshoot or undershoot during the
initial load transient spike, as shown in Figure 18. Follow the
steps below to ensure the R-C and inductor L/DCR time
constants are matched accurately.
R
1. Choose an arbitrary value for C
2. Plug the inductor L and DCR component values, and the
3. Use the new value for R
ISEN 2 ,
value is 0.01µF.
values for C
calculate the value for R
as well as the desired full load current, I
voltage, V
calculate the value for R
R
R
COMP
S
=
=
------------------------ - R
V
R
DROOP
=
ISEN
I
FL
DROOP
---------------------------------------
DCR C
COMP
⋅
ΔT
----------
ΔT
⋅
S
⋅
L
must be chosen to set the current
, and inductor DCR in Equation 26 to
COMP
2
1
ISEN
COMP
chosen in steps 1, into Equation 25 to
COMP
COMP
two or more times to achieve
COMP
⋅
COMP
S
DCR
.
to match the time constants
and C
obtained from Equation 25,
2
COMP
.
is the desired
COMP
. The recommended
COMP
FL
, full load droop
must be
September 30, 2008
-C
COMP
(EQ. 24)
(EQ. 25)
(EQ. 26)
FN9208.4
1
is